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       FAQ - .TH "FAQ" 3 "Thu Aug 12 2010" "Version 1.6.8" "avr-libc"


       FAQ - .SH "FAQ Index"

       1.  My program doesn't recognize a variable updated within an interrupt

       2.  I get 'undefined reference to...' for functions like 'sin()'

       3.  How to permanently bind a variable to a register?

       4.  How to modify MCUCR or WDTCR early?

       5.  What is all this _BV() stuff about?

       6.  Can I use C++ on the AVR?

       7.  Shouldn't I initialize all my variables?

       8.  Why do some 16-bit timer registers sometimes get trashed?

       9.  How do I use a #define'd constant in an asm statement?

       10. Why does the PC randomly jump around when single-stepping through
           my program in avr-gdb?

       11. How do I trace an assembler file in avr-gdb?

       12. How do I pass an IO port as a parameter to a function?

       13. What registers are used by the C compiler?

       14. How do I put an array of strings completely in ROM?

       15. How to use external RAM?

       16. Which -O flag to use?

       17. How do I relocate code to a fixed address?

       18. My UART is generating nonsense! My ATmega128 keeps crashing! Port F
           is completely broken!

       19. Why do all my '' strings eat up the SRAM?

       20. Why does the compiler compile an 8-bit operation that uses bitwise
           operators into a 16-bit operation in assembly?

       21. How to detect RAM memory and variable overlap problems?

       22. Is it really impossible to program the ATtinyXX in C?

       23. What is this 'clock skew detected' message?

       24. Why are (many) interrupt flags cleared by writing a logical 1?

       25. Why have 'programmed' fuses the bit value 0?

       26. Which AVR-specific assembler operators are available?

       27. Why are interrupts re-enabled in the middle of writing the stack

       28. Why are there five different linker scripts?

       29. How to add a raw binary image to linker output?

       30. How do I perform a software reset of the AVR?

       31. I am using floating point math. Why is the compiled code so big?
           Why does my code not work?

       32. What pitfalls exist when writing reentrant code?

       33. Why are some addresses of the EEPROM corrupted (usually address

       34. Why is my baud rate wrong?

My program doesn't recognize a variable updated within an interrupt routine

       When using the optimizer, in a loop like the following one:

       uint8_t flag;
       ISR(SOME_vect) {
         flag = 1;

               while (flag == 0) {

       the compiler will typically access flag only once, and optimize further
       accesses completely away, since its code path analysis shows that
       nothing inside the loop could change the value of flag anyway. To tell
       the compiler that this variable could be changed outside the scope of
       its code path analysis (e. g. from within an interrupt routine), the
       variable needs to be declared like:

       volatile uint8_t flag;

       Back to FAQ Index.

I get 'undefined reference to...' for functions like 'sin()'

       In order to access the mathematical functions that are declared in
       <math.h>, the linker needs to be told to also link the mathematical
       library, libm.a.

       Typically, system libraries like libm.a are given to the final C
       compiler command line that performs the linking step by adding a flag
       -lm at the end. (That is, the initial lib and the filename suffix from
       the library are written immediately after a -l flag. So for a libfoo.a
       library, -lfoo needs to be provided.) This will make the linker search
       the library in a path known to the system.

       An alternative would be to specify the full path to the libm.a file at
       the same place on the command line, i. e. after all the object files
       (*.o). However, since this requires knowledge of where the build system
       will exactly find those library files, this is deprecated for system

       Back to FAQ Index.

How to permanently bind a variable to a register?

       This can be done with

       register unsigned char counter asm('r3');

       Typically, it should be safe to use r2 through r7 that way.

       Registers r8 through r15 can be used for argument passing by the
       compiler in case many or long arguments are being passed to callees. If
       this is not the case throughout the entire application, these registers
       could be used for register variables as well.

       Extreme care should be taken that the entire application is compiled
       with a consistent set of register-allocated variables, including
       possibly used library functions.

       See C Names Used in Assembler Code for more details.

       Back to FAQ Index.

How to modify MCUCR or WDTCR early?

       The method of early initialization (MCUCR, WDTCR or anything else) is
       different (and more flexible) in the current version. Basically, write
       a small assembler file which looks like this:

       ;; begin xram.S

       #include <avr/io.h>

               .section .init1,'ax',@progbits

               ldi r16,_BV(SRE) | _BV(SRW)
               out _SFR_IO_ADDR(MCUCR),r16

       ;; end xram.S

       Assemble it, link the resulting xram.o with other files in your
       program, and this piece of code will be inserted in initialization
       code, which is run right after reset. See the linker script for
       comments about the new .initN sections (which one to use, etc.).

       The advantage of this method is that you can insert any initialization
       code you want (just remember that this is very early startup -- no
       stack and no __zero_reg__ yet), and no program memory space is wasted
       if this feature is not used.

       There should be no need to modify linker scripts anymore, except for
       some very special cases. It is best to leave __stack at its default
       value (end of internal SRAM -- faster, and required on some devices
       like ATmega161 because of errata), and add -Wl,-Tdata,0x801100 to start
       the data section above the stack.

       For more information on using sections, see Memory Sections. There is
       also an example for Using Sections in C Code. Note that in C code, any
       such function would preferably be placed into section .init3 as the
       code in .init2 ensures the internal register __zero_reg__ is already

       Back to FAQ Index.

What is all this _BV() stuff about?

       When performing low-level output work, which is a very central point in
       microcontroller programming, it is quite common that a particular bit
       needs to be set or cleared in some IO register. While the device
       documentation provides mnemonic names for the various bits in the IO
       registers, and the AVR device-specific IO definitions reflect these
       names in definitions for numerical constants, a way is needed to
       convert a bit number (usually within a byte register) into a byte value
       that can be assigned directly to the register. However, sometimes the
       direct bit numbers are needed as well (e. g. in an SBI() instruction),
       so the definitions cannot usefully be made as byte values in the first

       So in order to access a particular bit number as a byte value, use the
       _BV() macro. Of course, the implementation of this macro is just the
       usual bit shift (which is done by the compiler anyway, thus doesn't
       impose any run-time penalty), so the following applies:

       _BV(3) => 1 << 3 => 0x08

       However, using the macro often makes the program better readable.

       Example: clock timer 2 with full IO clock (CS2x = 0b001), toggle OC2
       output on compare match (COM2x = 0b01), and clear timer on compare
       match (CTC2 = 1). Make OC2 (PD7) an output.

               TCCR2 = _BV(COM20)|_BV(CTC2)|_BV(CS20);
               DDRD = _BV(PD7);

       Back to FAQ Index.

Can I use C++ on the AVR?

       Basically yes, C++ is supported (assuming your compiler has been
       configured and compiled to support it, of course). Source files ending
       in .cc, .cpp or .C will automatically cause the compiler frontend to
       invoke the C++ compiler. Alternatively, the C++ compiler could be
       explicitly called by the name avr-c++.

       However, there's currently no support for libstdc++, the standard
       support library needed for a complete C++ implementation. This imposes
       a number of restrictions on the C++ programs that can be compiled.
       Among them are:

       o Obviously, none of the C++ related standard functions, classes, and
         template classes are available.

       o The operators new and delete are not implemented, attempting to use
         them will cause the linker to complain about undefined external
         references. (This could perhaps be fixed.)

       o Some of the supplied include files are not C++ safe, i. e. they need
         to be wrapped into

        extern 'C' { . . . }

        (This could certainly be fixed, too.)

       o Exceptions are not supported. Since exceptions are enabled by default
         in the C++ frontend, they explicitly need to be turned off using
         -fno-exceptions in the compiler options. Failing this, the linker
         will complain about an undefined external reference to

       Constructors and destructors are supported though, including global

       When programming C++ in space- and runtime-sensitive environments like
       microcontrollers, extra care should be taken to avoid unwanted side
       effects of the C++ calling conventions like implied copy constructors
       that could be called upon function invocation etc. These things could
       easily add up into a considerable amount of time and program memory
       wasted. Thus, casual inspection of the generated assembler code (using
       the -S compiler option) seems to be warranted.

       Back to FAQ Index.

Shouldn't I initialize all my variables?

       Global and static variables are guaranteed to be initialized to 0 by
       the C standard. avr-gcc does this by placing the appropriate code into
       section .init4 (see The .initN Sections). With respect to the standard,
       this sentence is somewhat simplified (because the standard allows for
       machines where the actual bit pattern used differs from all bits being
       0), but for the AVR target, in general, all integer-type variables are
       set to 0, all pointers to a NULL pointer, and all floating-point
       variables to 0.0.

       As long as these variables are not initialized (i. e. they don't have
       an equal sign and an initialization expression to the right within the
       definition of the variable), they go into the .bss section of the file.
       This section simply records the size of the variable, but otherwise
       doesn't consume space, neither within the object file nor within flash
       memory. (Of course, being a variable, it will consume space in the
       target's SRAM.)

       In contrast, global and static variables that have an initializer go
       into the .data section of the file. This will cause them to consume
       space in the object file (in order to record the initializing value),
       and in the flash ROM of the target device. The latter is needed since
       the flash ROM is the only way that the compiler can tell the target
       device the value this variable is going to be initialized to.

       Now if some programmer 'wants to make doubly sure' their variables
       really get a 0 at program startup, and adds an initializer just
       containing 0 on the right-hand side, they waste space. While this waste
       of space applies to virtually any platform C is implemented on, it's
       usually not noticeable on larger machines like PCs, while the waste of
       flash ROM storage can be very painful on a small microcontroller like
       the AVR.

       So in general, variables should only be explicitly initialized if the
       initial value is non-zero.

           Recent versions of GCC are now smart enough to detect this
           situation, and revert variables that are explicitly initialized to
           0 to the .bss section. Still, other compilers might not do that
           optimization, and as the C standard guarantees the initialization,
           it is safe to rely on it.

       Back to FAQ Index.

Why do some 16-bit timer registers sometimes get trashed?

       Some of the timer-related 16-bit IO registers use a temporary register
       (called TEMP in the Atmel datasheet) to guarantee an atomic access to
       the register despite the fact that two separate 8-bit IO transfers are
       required to actually move the data. Typically, this includes access to
       the current timer/counter value register (TCNTn), the input capture
       register (ICRn), and write access to the output compare registers
       (OCRnM). Refer to the actual datasheet for each device's set of
       registers that involves the TEMP register.

       When accessing one of the registers that use TEMP from the main
       application, and possibly any other one from within an interrupt
       routine, care must be taken that no access from within an interrupt
       context could clobber the TEMP register data of an in-progress
       transaction that has just started elsewhere.

       To protect interrupt routines against other interrupt routines, it's
       usually best to use the ISR() macro when declaring the interrupt
       function, and to ensure that interrupts are still disabled when
       accessing those 16-bit timer registers.

       Within the main program, access to those registers could be
       encapsulated in calls to the cli() and sei() macros. If the status of
       the global interrupt flag before accessing one of those registers is
       uncertain, something like the following example code can be used.

               uint8_t sreg;
               uint16_t val;

               sreg = SREG;
               val = TCNT1;
               SREG = sreg;

               return val;

       Back to FAQ Index.

How do I use a #define'd constant in an asm statement?

       So you tried this:

       asm volatile('sbi 0x18,0x07;');

       Which works. When you do the same thing but replace the address of the
       port by its macro name, like this:

       asm volatile('sbi PORTB,0x07;');

       you get a compilation error: 'Error: constant value required'.

       PORTB is a precompiler definition included in the processor specific
       file included in avr/io.h. As you may know, the precompiler will not
       touch strings and PORTB, instead of 0x18, gets passed to the assembler.
       One way to avoid this problem is:

       asm volatile('sbi %0, 0x07' : 'I' (_SFR_IO_ADDR(PORTB)):);

           For C programs, rather use the standard C bit operators instead, so
           the above would be expressed as PORTB |= (1 << 7). The optimizer
           will take care to transform this into a single SBI instruction,
           assuming the operands allow for this.

       Back to FAQ Index.

Why does the PC randomly jump around when single-stepping through my program

       in avr-gdb?
       When compiling a program with both optimization (-O) and debug
       information (-g) which is fortunately possible in avr-gcc, the code
       watched in the debugger is optimized code. While it is not guaranteed,
       very often this code runs with the exact same optimizations as it would
       run without the -g switch.

       This can have unwanted side effects. Since the compiler is free to
       reorder code execution as long as the semantics do not change, code is
       often rearranged in order to make it possible to use a single branch
       instruction for conditional operations. Branch instructions can only
       cover a short range for the target PC (-63 through +64 words from the
       current PC). If a branch instruction cannot be used directly, the
       compiler needs to work around it by combining a skip instruction
       together with a relative jump (rjmp) instruction, which will need one
       additional word of ROM.

       Another side effect of optimization is that variable usage is
       restricted to the area of code where it is actually used. So if a
       variable was placed in a register at the beginning of some function,
       this same register can be re-used later on if the compiler notices that
       the first variable is no longer used inside that function, even though
       the variable is still in lexical scope. When trying to examine the
       variable in avr-gdb, the displayed result will then look garbled.

       So in order to avoid these side effects, optimization can be turned off
       while debugging. However, some of these optimizations might also have
       the side effect of uncovering bugs that would otherwise not be obvious,
       so it must be noted that turning off optimization can easily change the
       bug pattern. In most cases, you are better off leaving optimizations
       enabled while debugging.

       Back to FAQ Index.

How do I trace an assembler file in avr-gdb?

       When using the -g compiler option, avr-gcc only generates line number
       and other debug information for C (and C++) files that pass the
       compiler. Functions that don't have line number information will be
       completely skipped by a single step command in gdb. This includes
       functions linked from a standard library, but by default also functions
       defined in an assembler source file, since the -g compiler switch does
       not apply to the assembler.

       So in order to debug an assembler input file (possibly one that has to
       be passed through the C preprocessor), it's the assembler that needs to
       be told to include line-number information into the output file. (Other
       debug information like data types and variable allocation cannot be
       generated, since unlike a compiler, the assembler basically doesn't
       know about this.) This is done using the (GNU) assembler option


         $ avr-as -mmcu=atmega128 --gstabs -o foo.o foo.s

       When the assembler is not called directly but through the C compiler
       frontend (either implicitly by passing a source file ending in .S, or
       explicitly using -x assembler-with-cpp), the compiler frontend needs to
       be told to pass the --gstabs option down to the assembler. This is done
       using -Wa,--gstabs. Please take care to only pass this option when
       compiling an assembler input file. Otherwise, the assembler code that
       results from the C compilation stage will also get line number
       information, which confuses the debugger.

           You can also use -Wa,-gstabs since the compiler will add the extra
           '-' for you.


         $ EXTRA_OPTS="-Wall -mmcu=atmega128 -x assembler-with-cpp"
         $ avr-gcc -Wa,--gstabs ${EXTRA_OPTS} -c -o foo.o foo.S

       Also note that the debugger might get confused when entering a piece of
       code that has a non-local label before, since it then takes this label
       as the name of a new function that appears to have been entered. Thus,
       the best practice to avoid this confusion is to only use non-local
       labels when declaring a new function, and restrict anything else to
       local labels. Local labels consist just of a number only. References to
       these labels consist of the number, followed by the letter b for a
       backward reference, or f for a forward reference. These local labels
       may be re-used within the source file, references will pick the closest
       label with the same number and given direction.


       myfunc: push    r16
               push    r17
               push    r18
               push    YL
               push    YH
               eor     r16, r16        ; start loop
               ldi     YL, lo8(sometable)
               ldi     YH, hi8(sometable)
               rjmp    2f              ; jump to loop test at end
       1:      ld      r17, Y+         ; loop continues here
               breq    1f              ; return from myfunc prematurely
               inc     r16
       2:      cmp     r16, r18
               brlo    1b              ; jump back to top of loop

       1:      pop     YH
               pop     YL
               pop     r18
               pop     r17
               pop     r16

       Back to FAQ Index.

How do I pass an IO port as a parameter to a function?

       Consider this example code:

       #include <inttypes.h>
       #include <avr/io.h>

       set_bits_func_wrong (volatile uint8_t port, uint8_t mask)
           port |= mask;

       set_bits_func_correct (volatile uint8_t *port, uint8_t mask)
           *port |= mask;

       #define set_bits_macro(port,mask) ((port) |= (mask))

       int main (void)
           set_bits_func_wrong (PORTB, 0xaa);
           set_bits_func_correct (&PORTB, 0x55);
           set_bits_macro (PORTB, 0xf0);

           return (0);

       The first function will generate object code which is not even close to
       what is intended. The major problem arises when the function is called.
       When the compiler sees this call, it will actually pass the value of
       the PORTB register (using an IN instruction), instead of passing the
       address of PORTB (e.g. memory mapped io addr of 0x38, io port 0x18 for
       the mega128). This is seen clearly when looking at the disassembly of
       the call:

           set_bits_func_wrong (PORTB, 0xaa);
        10a:   6a ea           ldi     r22, 0xAA       ; 170
        10c:   88 b3           in      r24, 0x18       ; 24
        10e:   0e 94 65 00     call    0xca

       So, the function, once called, only sees the value of the port register
       and knows nothing about which port it came from. At this point,
       whatever object code is generated for the function by the compiler is
       irrelevant. The interested reader can examine the full disassembly to
       see that the function's body is completely fubar.

       The second function shows how to pass (by reference) the memory mapped
       address of the io port to the function so that you can read and write
       to it in the function. Here's the object code generated for the
       function call:

           set_bits_func_correct (&PORTB, 0x55);
        112:   65 e5           ldi     r22, 0x55       ; 85
        114:   88 e3           ldi     r24, 0x38       ; 56
        116:   90 e0           ldi     r25, 0x00       ; 0
        118:   0e 94 7c 00     call    0xf8

       You can clearly see that 0x0038 is correctly passed for the address of
       the io port. Looking at the disassembled object code for the body of
       the function, we can see that the function is indeed performing the
       operation we intended:

       set_bits_func_correct (volatile uint8_t *port, uint8_t mask)
         f8:   fc 01           movw    r30, r24
           *port |= mask;
         fa:   80 81           ld      r24, Z
         fc:   86 2b           or      r24, r22
         fe:   80 83           st      Z, r24
        100:   08 95           ret

       Notice that we are accessing the io port via the LD and ST

       The port parameter must be volatile to avoid a compiler warning.

           Because of the nature of the IN and OUT assembly instructions, they
           can not be used inside the function when passing the port in this
           way. Readers interested in the details should consult the
           Instruction Set datasheet.

       Finally we come to the macro version of the operation. In this
       contrived example, the macro is the most efficient method with respect
       to both execution speed and code size:

           set_bits_macro (PORTB, 0xf0);
        11c:   88 b3           in      r24, 0x18       ; 24
        11e:   80 6f           ori     r24, 0xF0       ; 240
        120:   88 bb           out     0x18, r24       ; 24

       Of course, in a real application, you might be doing a lot more in your
       function which uses a passed by reference io port address and thus the
       use of a function over a macro could save you some code space, but
       still at a cost of execution speed.

       Care should be taken when such an indirect port access is going to one
       of the 16-bit IO registers where the order of write access is critical
       (like some timer registers). All versions of avr-gcc up to 3.3 will
       generate instructions that use the wrong access order in this situation
       (since with normal memory operands where the order doesn't matter, this
       sometimes yields shorter code).

       dev/2003-01/msg000.html for a possible workaround.

       avr-gcc versions after 3.3 have been fixed in a way where this
       optimization will be disabled if the respective pointer variable is
       declared to be volatile, so the correct behaviour for 16-bit IO ports
       can be forced that way.

       Back to FAQ Index.

What registers are used by the C compiler?

       o Data types:
          char is 8 bits, int is 16 bits, long is 32 bits, long long is 64
         bits, float and double are 32 bits (this is the only supported
         floating point format), pointers are 16 bits (function pointers are
         word addresses, to allow addressing up to 128K program memory space).
         There is a -mint8 option (see Options for the C compiler avr-gcc) to
         make int 8 bits, but that is not supported by avr-libc and violates C
         standards (int must be at least 16 bits). It may be removed in a
         future release.

       o Call-used registers (r18-r27, r30-r31):
          May be allocated by gcc for local data. You may use them freely in
         assembler subroutines. Calling C subroutines can clobber any of them
         - the caller is responsible for saving and restoring.

       o Call-saved registers (r2-r17, r28-r29):
          May be allocated by gcc for local data. Calling C subroutines leaves
         them unchanged. Assembler subroutines are responsible for saving and
         restoring these registers, if changed. r29:r28 (Y pointer) is used as
         a frame pointer (points to local data on stack) if necessary. The
         requirement for the callee to save/preserve the contents of these
         registers even applies in situations where the compiler assigns them
         for argument passing.

       o Fixed registers (r0, r1):
          Never allocated by gcc for local data, but often used for fixed

       r0 - temporary register, can be clobbered by any C code (except
       interrupt handlers which save it), may be used to remember something
       for a while within one piece of assembler code

       r1 - assumed to be always zero in any C code, may be used to remember
       something for a while within one piece of assembler code, but must then
       be cleared after use (clr r1). This includes any use of the
       [f]mul[s[u]] instructions, which return their result in r1:r0.
       Interrupt handlers save and clear r1 on entry, and restore r1 on exit
       (in case it was non-zero).

       o Function call conventions:
          Arguments - allocated left to right, r25 to r8. All arguments are
         aligned to start in even-numbered registers (odd-sized arguments,
         including char, have one free register above them). This allows
         making better use of the movw instruction on the enhanced core.

       If too many, those that don't fit are passed on the stack.

       Return values: 8-bit in r24 (not r25!), 16-bit in r25:r24, up to 32
       bits in r22-r25, up to 64 bits in r18-r25. 8-bit return values are
       zero/sign-extended to 16 bits by the called function (unsigned char is
       more efficient than signed char - just clr r25). Arguments to functions
       with variable argument lists (printf etc.) are all passed on stack, and
       char is extended to int.

           There was no such alignment before 2000-07-01, including the old
           patches for gcc-2.95.2. Check your old assembler subroutines, and
           adjust them accordingly.

       Back to FAQ Index.

How do I put an array of strings completely in ROM?

       There are times when you may need an array of strings which will never
       be modified. In this case, you don't want to waste ram storing the
       constant strings. The most obvious (and incorrect) thing to do is this:

       #include <avr/pgmspace.h>

       PGM_P array[2] PROGMEM = {

       int main (void)
           char buf[32];
           strcpy_P (buf, array[1]);
           return 0;

       The result is not what you want though. What you end up with is the
       array stored in ROM, while the individual strings end up in RAM (in the
       .data section).

       To work around this, you need to do something like this:

       #include <avr/pgmspace.h>

       const char foo[] PROGMEM = 'Foo';
       const char bar[] PROGMEM = 'Bar';

       PGM_P array[2] PROGMEM = {

       int main (void)
           char buf[32];
           PGM_P p;
           int i;

           memcpy_P(&p, &array[i], sizeof(PGM_P));
           strcpy_P(buf, p);
           return 0;

       Looking at the disassembly of the resulting object file we see that
       array is in flash as such:

       00000026 <array>:
         26:   2e 00           .word   0x002e  ; ????
         28:   2a 00           .word   0x002a  ; ????

       0000002a <bar>:
         2a:   42 61 72 00                                         Bar.

       0000002e <foo>:
         2e:   46 6f 6f 00                                         Foo.

       foo is at addr 0x002e.
        bar is at addr 0x002a.
        array is at addr 0x0026.

       Then in main we see this:

           memcpy_P(&p, &array[i], sizeof(PGM_P));
         70:   66 0f           add     r22, r22
         72:   77 1f           adc     r23, r23
         74:   6a 5d           subi    r22, 0xDA       ; 218
         76:   7f 4f           sbci    r23, 0xFF       ; 255
         78:   42 e0           ldi     r20, 0x02       ; 2
         7a:   50 e0           ldi     r21, 0x00       ; 0
         7c:   ce 01           movw    r24, r28
         7e:   81 96           adiw    r24, 0x21       ; 33
         80:   08 d0           rcall   .+16            ; 0x92

       This code reads the pointer to the desired string from the ROM table
       array into a register pair.

       The value of i (in r22:r23) is doubled to accommodate for the word
       offset required to access array[], then the address of array (0x26) is
       added, by subtracting the negated address (0xffda). The address of
       variable p is computed by adding its offset within the stack frame (33)
       to the Y pointer register, and memcpy_P is called.

           strcpy_P(buf, p);
         82:   69 a1           ldd     r22, Y+33       ; 0x21
         84:   7a a1           ldd     r23, Y+34       ; 0x22
         86:   ce 01           movw    r24, r28
         88:   01 96           adiw    r24, 0x01       ; 1
         8a:   0c d0           rcall   .+24            ; 0xa4

       This will finally copy the ROM string into the local buffer buf.

       Variable p (located at Y+33) is read, and passed together with the
       address of buf (Y+1) to strcpy_P. This will copy the string from ROM to

       Note that when using a compile-time constant index, omitting the first
       step (reading the pointer from ROM via memcpy_P) usually remains
       unnoticed, since the compiler would then optimize the code for
       accessing array at compile-time.

       Back to FAQ Index.

How to use external RAM?

       Well, there is no universal answer to this question; it depends on what
       the external RAM is going to be used for.

       Basically, the bit SRE (SRAM enable) in the MCUCR register needs to be
       set in order to enable the external memory interface. Depending on the
       device to be used, and the application details, further registers
       affecting the external memory operation like XMCRA and XMCRB, and/or
       further bits in MCUCR might be configured. Refer to the datasheet for

       If the external RAM is going to be used to store the variables from the
       C program (i. e., the .data and/or .bss segment) in that memory area,
       it is essential to set up the external memory interface early during
       the device initialization so the initialization of these variable will
       take place. Refer to How to modify MCUCR or WDTCR early? for a
       description how to do this using few lines of assembler code, or to the
       chapter about memory sections for an example written in C.

       The explanation of malloc() contains a discussion about the use of
       internal RAM vs. external RAM in particular with respect to the various
       possible locations of the heap (area reserved for malloc()). It also
       explains the linker command-line options that are required to move the
       memory regions away from their respective standard locations in
       internal RAM.

       Finally, if the application simply wants to use the additional RAM for
       private data storage kept outside the domain of the C compiler (e. g.
       through a char * variable initialized directly to a particular
       address), it would be sufficient to defer the initialization of the
       external RAM interface to the beginning of main(), so no tweaking of
       the .init3 section is necessary. The same applies if only the heap is
       going to be located there, since the application start-up code does not
       affect the heap.

       It is not recommended to locate the stack in external RAM. In general,
       accessing external RAM is slower than internal RAM, and errata of some
       AVR devices even prevent this configuration from working properly at

       Back to FAQ Index.

Which -O flag to use?

       There's a common misconception that larger numbers behind the -O option
       might automatically cause 'better' optimization. First, there's no
       universal definition for 'better', with optimization often being a
       speed vs. code size trade off. See the detailed discussion for which
       option affects which part of the code generation.

       A test case was run on an ATmega128 to judge the effect of compiling
       the library itself using different optimization levels. The following
       table lists the results. The test case consisted of around 2 KB of
       strings to sort. Test #1 used qsort() using the standard library
       strcmp(), test #2 used a function that sorted the strings by their size
       (thus had two calls to strlen() per invocation).

       When comparing the resulting code size, it should be noted that a
       floating point version of fvprintf() was linked into the binary (in
       order to print out the time elapsed) which is entirely not affected by
       the different optimization levels, and added about 2.5 KB to the code.

       Optimization flags Size of .text Time for test #1 Time for test #2  -O3
       6898 903 s 19.7 ms  -O2 6666 972 s 20.1 ms  -Os 6618 955 s 20.1 ms  -Os
       -mcall-prologues 6474 972 s 20.1 ms

       (The difference between 955 s and 972 s was just a single timer-tick,
       so take this with a grain of salt.)

       So generally, it seems -Os -mcall-prologues is the most universal
       'best' optimization level. Only applications that need to get the last
       few percent of speed benefit from using -O3.

       Back to FAQ Index.

How do I relocate code to a fixed address?

       First, the code should be put into a new named section. This is done
       with a section attribute:

       __attribute__ ((section ('.bootloader')))

       In this example, .bootloader is the name of the new section. This
       attribute needs to be placed after the prototype of any function to
       force the function into the new section.

       void boot(void) __attribute__ ((section ('.bootloader')));

       To relocate the section to a fixed address the linker flag --section-
       start is used. This option can be passed to the linker using the -Wl
       compiler option:


       The name after section-start is the name of the section to be
       relocated. The number after the section name is the beginning address
       of the named section.

       Back to FAQ Index.

My UART is generating nonsense! My ATmega128 keeps crashing! Port F is

       completely broken!
       Well, certain odd problems arise out of the situation that the AVR
       devices as shipped by Atmel often come with a default fuse bit
       configuration that doesn't match the user's expectations. Here is a
       list of things to care for:

       o All devices that have an internal RC oscillator ship with the fuse
         enabled that causes the device to run off this oscillator, instead of
         an external crystal. This often remains unnoticed until the first
         attempt is made to use something critical in timing, like UART

       o The ATmega128 ships with the fuse enabled that turns this device into
         ATmega103 compatibility mode. This means that some ports are not
         fully usable, and in particular that the internal SRAM is located at
         lower addresses. Since by default, the stack is located at the top of
         internal SRAM, a program compiled for an ATmega128 running on such a
         device will immediately crash upon the first function call (or
         rather, upon the first function return).

       o Devices with a JTAG interface have the JTAGEN fuse programmed by
         default. This will make the respective port pins that are used for
         the JTAG interface unavailable for regular IO.

       Back to FAQ Index.

Why do all my '' strings eat up the SRAM?

       By default, all strings are handled as all other initialized variables:
       they occupy RAM (even though the compiler might warn you when it
       detects write attempts to these RAM locations), and occupy the same
       amount of flash ROM so they can be initialized to the actual string by
       startup code. The compiler can optimize multiple identical strings into
       a single one, but obviously only for one compilation unit (i. e., a
       single C source file).

       That way, any string literal will be a valid argument to any C function
       that expects a const char * argument.

       Of course, this is going to waste a lot of SRAM. In Program Space
       String Utilities, a method is described how such constant data can be
       moved out to flash ROM. However, a constant string located in flash ROM
       is no longer a valid argument to pass to a function that expects a
       const char *-type string, since the AVR processor needs the special
       instruction LPM to access these strings. Thus, separate functions are
       needed that take this into account. Many of the standard C library
       functions have equivalents available where one of the string arguments
       can be located in flash ROM. Private functions in the applications need
       to handle this, too. For example, the following can be used to
       implement simple debugging messages that will be sent through a UART:

       #include <inttypes.h>
       #include <avr/io.h>
       #include <avr/pgmspace.h>

       uart_putchar(char c)
         if (c == '0)    ');
         loop_until_bit_is_set(USR, UDRE);
         UDR = c;
         return 0; /* so it could be used for fdevopen(), too */

       debug_P(const char *addr)
         char c;

         while ((c = pgm_read_byte(addr++)))

         ioinit(); /* initialize UART, ... */
         debug_P(PSTR('foo was here0));
         return 0;

           By convention, the suffix _P to the function name is used as an
           indication that this function is going to accept a 'program-space
           string'. Note also the use of the PSTR() macro.

       Back to FAQ Index.

Why does the compiler compile an 8-bit operation that uses bitwise operators

       into a 16-bit operation in assembly?
       Bitwise operations in Standard C will automatically promote their
       operands to an int, which is (by default) 16 bits in avr-gcc.

       To work around this use typecasts on the operands, including literals,
       to declare that the values are to be 8 bit operands.

       This may be especially important when clearing a bit:

       var &= ~mask;  /* wrong way! */

       The bitwise 'not' operator (~) will also promote the value in mask to
       an int. To keep it an 8-bit value, typecast before the 'not' operator:

       var &= (unsigned char)~mask;

       Back to FAQ Index.

How to detect RAM memory and variable overlap problems?

       You can simply run avr-nm on your output (ELF) file. Run it with the -n
       option, and it will sort the symbols numerically (by default, they are
       sorted alphabetically).

       Look for the symbol _end, that's the first address in RAM that is not
       allocated by a variable. (avr-gcc internally adds 0x800000 to all
       data/bss variable addresses, so please ignore this offset.) Then, the
       run-time initialization code initializes the stack pointer (by default)
       to point to the last available address in (internal) SRAM. Thus, the
       region between _end and the end of SRAM is what is available for stack.
       (If your application uses malloc(), which e. g. also can happen inside
       printf(), the heap for dynamic memory is also located there. See Memory
       Areas and Using malloc().)

       The amount of stack required for your application cannot be determined
       that easily. For example, if you recursively call a function and forget
       to break that recursion, the amount of stack required is infinite. :-)
       You can look at the generated assembler code (avr-gcc ... -S), there's
       a comment in each generated assembler file that tells you the frame
       size for each generated function. That's the amount of stack required
       for this function, you have to add up that for all functions where you
       know that the calls could be nested.

       Back to FAQ Index.

Is it really impossible to program the ATtinyXX in C?

       While some small AVRs are not directly supported by the C compiler
       since they do not have a RAM-based stack (and some do not even have RAM
       at all), it is possible anyway to use the general-purpose registers as
       a RAM replacement since they are mapped into the data memory region.

       Bruce D. Lightner wrote an excellent description of how to do this, and
       offers this together with a toolkit on his web page:

       Back to FAQ Index.

What is this 'clock skew detected' message?

       It's a known problem of the MS-DOS FAT file system. Since the FAT file
       system has only a granularity of 2 seconds for maintaining a file's
       timestamp, and it seems that some MS-DOS derivative (Win9x) perhaps
       rounds up the current time to the next second when calculating the
       timestamp of an updated file in case the current time cannot be
       represented in FAT's terms, this causes a situation where make sees a
       'file coming from the future'.

       Since all make decisions are based on file timestamps, and their
       dependencies, make warns about this situation.

       Solution: don't use inferior file systems / operating systems. Neither
       Unix file systems nor HPFS (aka NTFS) do experience that problem.

       Workaround: after saving the file, wait a second before starting make.
       Or simply ignore the warning. If you are paranoid, execute a make clean
       all to make sure everything gets rebuilt.

       In networked environments where the files are accessed from a file
       server, this message can also happen if the file server's clock differs
       too much from the network client's clock. In this case, the solution is
       to use a proper time keeping protocol on both systems, like NTP. As a
       workaround, synchronize the client's clock frequently with the server's

       Back to FAQ Index.

Why are (many) interrupt flags cleared by writing a logical 1?

       Usually, each interrupt has its own interrupt flag bit in some control
       register, indicating the specified interrupt condition has been met by
       representing a logical 1 in the respective bit position. When working
       with interrupt handlers, this interrupt flag bit usually gets cleared
       automatically in the course of processing the interrupt, sometimes by
       just calling the handler at all, sometimes (e. g. for the U[S]ART) by
       reading a particular hardware register that will normally happen anyway
       when processing the interrupt.

       From the hardware's point of view, an interrupt is asserted as long as
       the respective bit is set, while global interrupts are enabled. Thus,
       it is essential to have the bit cleared before interrupts get re-
       enabled again (which usually happens when returning from an interrupt

       Only few subsystems require an explicit action to clear the interrupt
       request when using interrupt handlers. (The notable exception is the
       TWI interface, where clearing the interrupt indicates to proceed with
       the TWI bus hardware handshake, so it's never done automatically.)

       However, if no normal interrupt handlers are to be used, or in order to
       make extra sure any pending interrupt gets cleared before re-activating
       global interrupts (e. g. an external edge-triggered one), it can be
       necessary to explicitly clear the respective hardware interrupt bit by
       software. This is usually done by writing a logical 1 into this bit
       position. This seems to be illogical at first, the bit position already
       carries a logical 1 when reading it, so why does writing a logical 1 to
       it clear the interrupt bit?

       The solution is simple: writing a logical 1 to it requires only a
       single OUT instruction, and it is clear that only this single interrupt
       request bit will be cleared. There is no need to perform a read-modify-
       write cycle (like, an SBI instruction), since all bits in these control
       registers are interrupt bits, and writing a logical 0 to the remaining
       bits (as it is done by the simple OUT instruction) will not alter them,
       so there is no risk of any race condition that might accidentally clear
       another interrupt request bit. So instead of writing

       TIFR |= _BV(TOV0); /* wrong! */

       simply use

       TIFR = _BV(TOV0);

       Back to FAQ Index.

Why have 'programmed' fuses the bit value 0?

       Basically, fuses are just a bit in a special EEPROM area. For technical
       reasons, erased E[E]PROM cells have all bits set to the value 1, so
       unprogrammed fuses also have a logical 1. Conversely, programmed fuse
       cells read out as bit value 0.

       Back to FAQ Index.

Which AVR-specific assembler operators are available?

       See Pseudo-ops and operators.

       Back to FAQ Index.

Why are interrupts re-enabled in the middle of writing the stack pointer?

       When setting up space for local variables on the stack, the compiler
       generates code like this:

       /* prologue: frame size=20 */
               push r28
               push r29
               in r28,__SP_L__
               in r29,__SP_H__
               sbiw r28,20
               in __tmp_reg__,__SREG__
               out __SP_H__,r29
               out __SREG__,__tmp_reg__
               out __SP_L__,r28
       /* prologue end (size=10) */

       It reads the current stack pointer value, decrements it by the required
       amount of bytes, then disables interrupts, writes back the high part of
       the stack pointer, writes back the saved SREG (which will eventually
       re-enable interrupts if they have been enabled before), and finally
       writes the low part of the stack pointer.

       At the first glance, there's a race between restoring SREG, and writing
       SPL. However, after enabling interrupts (either explicitly by setting
       the I flag, or by restoring it as part of the entire SREG), the AVR
       hardware executes (at least) the next instruction still with interrupts
       disabled, so the write to SPL is guaranteed to be executed with
       interrupts disabled still. Thus, the emitted sequence ensures
       interrupts will be disabled only for the minimum time required to
       guarantee the integrity of this operation.

       Back to FAQ Index.

Why are there five different linker scripts?

       From a comment in the source code:

       Which one of the five linker script files is actually used depends on
       command line options given to ld.

       A .x script file is the default script A .xr script is for linking
       without relocation (-r flag) A .xu script is like .xr but *do* create
       constructors (-Ur flag) A .xn script is for linking with -n flag (mix
       text and data on same page). A .xbn script is for linking with -N flag
       (mix text and data on same page).

       Back to FAQ Index.

How to add a raw binary image to linker output?

       The GNU linker avr-ld cannot handle binary data directly. However,
       there's a companion tool called avr-objcopy. This is already known from
       the output side: it's used to extract the contents of the linked ELF
       file into an Intel Hex load file.

       avr-objcopy can create a relocatable object file from arbitrary binary
       input, like

       avr-objcopy -I binary -O elf32-avr foo.bin foo.o

       This will create a file named foo.o, with the contents of foo.bin. The
       contents will default to section .data, and two symbols will be created
       named _binary_foo_bin_start and _binary_foo_bin_end. These symbols can
       be referred to inside a C source to access these data.

       If the goal is to have those data go to flash ROM (similar to having
       used the PROGMEM attribute in C source code), the sections have to be
       renamed while copying, and it's also useful to set the section flags:

       avr-objcopy --rename-section,contents,alloc,load,readonly,data -I binary -O elf32-avr foo.bin foo.o

       Note that all this could be conveniently wired into a Makefile, so
       whenever foo.bin changes, it will trigger the recreation of foo.o, and
       a subsequent relink of the final ELF file.

       Below are two Makefile fragments that provide rules to convert a .txt
       file to an object file, and to convert a .bin file to an object file:

       $(OBJDIR)/%.o : %.txt
               @echo Converting $<
               @cp $(<) $(*).tmp
               @echo -n 0 | tr 0 ' 00' >> $(*).tmp
               @$(OBJCOPY) -I binary -O elf32-avr         --rename-section,contents,alloc,load,readonly,data         --redefine-sym _binary_$*_tmp_start=$*         --redefine-sym _binary_$*_tmp_end=$*_end         --redefine-sym _binary_$*_tmp_size=$*_size_sym         $(*).tmp $(@)
               @echo 'extern const char' $(*)'[] PROGMEM;' > $(*).h
               @echo 'extern const char' $(*)_end'[] PROGMEM;' >> $(*).h
               @echo 'extern const char' $(*)_size_sym'[];' >> $(*).h
               @echo '#define $(*)_size ((int)$(*)_size_sym)' >> $(*).h
               @rm $(*).tmp

       $(OBJDIR)/%.o : %.bin
               @echo Converting $<
               @$(OBJCOPY) -I binary -O elf32-avr         --rename-section,contents,alloc,load,readonly,data         --redefine-sym _binary_$*_bin_start=$*         --redefine-sym _binary_$*_bin_end=$*_end         --redefine-sym _binary_$*_bin_size=$*_size_sym         $(<) $(@)
               @echo 'extern const char' $(*)'[] PROGMEM;' > $(*).h
               @echo 'extern const char' $(*)_end'[] PROGMEM;' >> $(*).h
               @echo 'extern const char' $(*)_size_sym'[];' >> $(*).h
               @echo '#define $(*)_size ((int)$(*)_size_sym)' >> $(*).h

       Back to FAQ Index.

How do I perform a software reset of the AVR?

       The canonical way to perform a software reset of the AVR is to use the
       watchdog timer. Enable the watchdog timer to the shortest timeout
       setting, then go into an infinite, do-nothing loop. The watchdog will
       then reset the processor.

       The reason why this is preferable over jumping to the reset vector, is
       that when the watchdog resets the AVR, the registers will be reset to
       their known, default settings. Whereas jumping to the reset vector will
       leave the registers in their previous state, which is generally not a
       good idea.

       CAUTION! Older AVRs will have the watchdog timer disabled on a reset.
       For these older AVRs, doing a soft reset by enabling the watchdog is
       easy, as the watchdog will then be disabled after the reset. On newer
       AVRs, once the watchdog is enabled, then it stays enabled, even after a
       reset! For these newer AVRs a function needs to be added to the .init3
       section (i.e. during the startup code, before main()) to disable the
       watchdog early enough so it does not continually reset the AVR.

       Here is some example code that creates a macro that can be called to
       perform a soft reset:

       #include <avr/wdt.h>

       #define soft_reset()        do                          {                               wdt_enable(WDTO_15MS);      for(;;)                     {                           }                       } while(0)

       For newer AVRs (such as the ATmega1281) also add this function to your
       code to then disable the watchdog after a reset (e.g., after a soft

       #include <avr/wdt.h>

       // Function Pototype
       void wdt_init(void) __attribute__((naked)) __attribute__((section('.init3')));

       // Function Implementation
       void wdt_init(void)
           MCUSR = 0;


       Back to FAQ Index.

I am using floating point math. Why is the compiled code so big? Why does my

       code not work?
       You are not linking in the math library from AVR-LibC. GCC has a
       library that is used for floating point operations, but it is not
       optimized for the AVR, and so it generates big code, or it could be
       incorrect. This can happen even when you are not using any floating
       point math functions from the Standard C library, but you are just
       doing floating point math operations.

       When you link in the math library from AVR-LibC, those routines get
       replaced by hand-optimized AVR assembly and it produces much smaller

       See I get 'undefined reference to...' for functions like 'sin()' for
       more details on how to link in the math library.

       Back to FAQ Index.

What pitfalls exist when writing reentrant code?

       Reentrant code means the ability for a piece of code to be called
       simultaneously from two or more threads. Attention to re-enterability
       is needed when using a multi-tasking operating system, or when using
       interrupts since an interrupt is really a temporary thread.

       The code generated natively by gcc is reentrant. But, only some of the
       libraries in avr-libc are explicitly reentrant, and some are known not
       to be reentrant. In general, any library call that reads and writes
       global variables (including I/O registers) is not reentrant. This is
       because more than one thread could read or write the same storage at
       the same time, unaware that other threads are doing the same, and
       create inconsistent and/or erroneous results.

       A library call that is known not to be reentrant will work if it is
       used only within one thread and no other thread makes use of a library
       call that shares common storage with it.

       Below is a table of library calls with known issues.

       Library call Reentrant Issue Workaround/Alternative  rand(), random()
       Uses global variables to keep state information. Use special reentrant
       versions: rand_r(), random_r().  strtod(), strtol(), strtoul() Uses the
       global variable errno to return success/failure. Ignore errno, or
       protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can
       tolerate it. Or use sccanf() or sccanf_P() if possible.   malloc(),
       realloc(), calloc(), free() Uses the stack pointer and global variables
       to allocate and free memory. Protect calls with cli()/sei() or
       ATOMIC_BLOCK() if the application can tolerate it. If using an OS, use
       the OS provided memory allocator since the OS is likely modifying the
       stack pointer anyway.   fdevopen(), fclose() Uses calloc() and free().
       Protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can
       tolerate it. Or use fdev_setup_stream() or FDEV_SETUP_STREAM().
        Note: fclose() will only call free() if the stream has been opened
       with fdevopen().   eeprom_*(), boot_*() Accesses I/O registers. Protect
       calls with cli()/sei(), ATOMIC_BLOCK(), or use OS locking.  pgm_*_far()
       Accesses I/O register RAMPZ. Starting with GCC 4.3, RAMPZ is
       automatically saved for ISRs, so nothing further is needed if only
       using interrupts.
       Some OSes may automatically preserve RAMPZ during context switching.
       Check the OS documentation before assuming it does.
       Otherwise, protect calls with cli()/sei(), ATOMIC_BLOCK(), or use
       explicit OS locking.   printf(), printf_P(), vprintf(), vprintf_P(),
       puts(), puts_P() Alters flags and character count in global FILE
       stdout. Use only in one thread. Or if returned character count is
       unimportant, do not use the *_P versions.
       Note: Formatting to a string output, e.g. sprintf(), sprintf_P(),
       snprintf(), snprintf_P(), vsprintf(), vsprintf_P(), vsnprintf(),
       vsnprintf_P(), is thread safe. The formatted string could then be
       followed by an fwrite() which simply calls the lower layer to send the
       string.   fprintf(), fprintf_P(), vfprintf(), vfprintf_P(), fputs(),
       fputs_P() Alters flags and character count in the FILE argument.
       Problems can occur if a global FILE is used from multiple threads.
       Assign each thread its own FILE for output. Or if returned character
       count is unimportant, do not use the *_P versions.   assert() Contains
       an embedded fprintf(). See above for fprintf(). See above for
       fprintf().  clearerr() Alters flags in the FILE argument.  Assign each
       thread its own FILE for output.

       getchar(), gets() Alters flags, character count, and unget buffer in
       global FILE stdin. Use only in one thread. ***

       fgetc(), ungetc(), fgets(), scanf(), scanf_P(), fscanf(), fscanf_P(),
       vscanf(), vfscanf(), vfscanf_P(), fread() Alters flags, character
       count, and unget buffer in the FILE argument. Assign each thread its
       own FILE for input. ***
       Note: Scanning from a string, e.g. sscanf() and sscanf_P(), are thread

       *** It's not clear one would ever want to do character input
       simultaneously from more than one thread anyway, but these entries are
       included for completeness.

       An effort will be made to keep this table up to date if any new issues
       are discovered or introduced.

       Back to FAQ Index.

Why are some addresses of the EEPROM corrupted (usually address zero)?

       The two most common reason for EEPROM corruption is either writing to
       the EEPROM beyond the datasheet endurance specification, or resetting
       the AVR while an EEPROM write is in progress.

       EEPROM writes can take up to tens of milliseconds to complete. So that
       the CPU is not tied up for that long of time, an internal state-machine
       handles EEPROM write requests. The EEPROM state-machine expects to have
       all of the EEPROM registers setup, then an EEPROM write request to
       start the process. Once the EEPROM state-machine has started, changing
       EEPROM related registers during an EEPROM write is guaranteed to
       corrupt the EEPROM write process. The datasheet always shows the proper
       way to tell when a write is in progress, so that the registers are not
       changed by the user's program. The EEPROM state-machine will always
       complete the write in progress unless power is removed from the device.

       As with all EEPROM technology, if power fails during an EEPROM write
       the state of the byte being written is undefined.

       In older generation AVRs the EEPROM Address Register (EEAR) is
       initialized to zero on reset, be it from Brown Out Detect, Watchdog or
       the Reset Pin. If an EEPROM write has just started at the time of the
       reset, the write will be completed, but now at address zero instead of
       the requested address. If the reset occurs later in the write process
       both the requested address and address zero may be corrupted.

       To distinguish which AVRs may exhibit the corrupt of address zero while
       a write is in process during a reset, look at the 'initial value'
       section for the EEPROM Address Register. If EEAR shows the initial
       value as 0x00 or 0x0000, then address zero and possibly the one being
       written will be corrupted. Newer parts show the initial value as
       'undefined', these will not corrupt address zero during a reset (unless
       it was address zero that was being written).

       EEPROMs have limited write endurance. The datasheet specifies the
       number of EEPROM writes that are guaranteed to function across the full
       temperature specification of the AVR, for a given byte. A read should
       always be performed before a write, to see if the value in the EEPROM
       actually needs to be written, so not to cause unnecessary EEPROM wear.

       AVRs use a paging mechanism for doing EEPROM writes. This is almost
       entirely transparent to the user with one exception: When a byte is
       written to the EEPROM, the entire EEPROM page is also transparently
       erased and (re)written, which will cause wear to bytes that the
       programmer did not explicitly write. If it is desired to extend EEPROM
       write lifetimes, in an attempt not to exceed the datasheet EEPROM write
       endurance specification for a given byte, then writes must be in
       multiples of the EEPROM page size, and not sequential bytes. The EEPROM
       write page size varies with the device. The EEPROM page size is found
       in the datasheet section on Memory Programming, generally before the
       Electrical Specifications near the end of the datasheet.

       The failure mechanism for an overwritten byte/page is generally one of
       'stuck' bits, i. e. a bit will stay at a one or zero state regardless
       of the byte written. Also a write followed by a read may return the
       correct data, but the data will change with the passage of time, due
       the EEPROM's inability to hold a charge from the excessive write wear.

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Why is my baud rate wrong?

       Some AVR datasheets give the following formula for calculating baud


       Unfortunately that formula does not work with all combinations of clock
       speeds and baud rates due to integer truncation during the division

       When doing integer division it is usually better to round to the
       nearest integer, rather than to the lowest. To do this add 0.5 (i. e.
       half the value of the denominator) to the numerator before the
       division, resulting in the formula:

       ((F_CPU + UART_BAUD_RATE * 8L) / (UART_BAUD_RATE * 16L) - 1)

       This is also the way it is implemented in <util/setbaud.h>: Helper
       macros for baud rate calculations.

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