avr-libc_ Frequently Asked Questions
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1. Back to FAQ Index I get undefined reference to for functions like sin In order to access the mathematical functions that are declared in lt math h gt 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 1m at the end That is the initial ib and the filename suffix from the library are written immediately after a flag So for a libfoo a library 1f00 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 1ibm 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 libraries 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 17 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 compi2. 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 reset include lt avr wdt h gt Function Pototype void wdt_init void _ attribute __ naked _ attribute section init3 Function Implementation void wdt_init void MCUSR 0 wdt_disable return 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 code 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 16 of 19 avr libc Frequently Asked Questions Attention to re entera
3. 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 0 txt echo Converting lt cp lt tmp echo n 0 tr 0 000 gt gt tmp S OBJCOPY I binary O elf32 avr rename section data progmem data 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 gt h echo extern const char end PROGMEM gt gt h echo extern const char size _sym gt gt h echo define size int size sym gt gt h rm tmp OBJDIR 0 bin echo Converting lt S OBJCOPY I binary O elf32 avr rename section data progmem data contents alloc load readonly data redefine sym _binary _bin_start redefine sym binary bin _end end redefine sym _binary _bin_size _size_ sym lt echo extern const char PROGMEM gt h echo extern const char end PROGMEM gt gt h echo extern const char size _sym gt gt h echo define
4. 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
5. Which one of the five linker script files is actually used depends on command line options given to Id 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 1d 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 data progmem data contents alloc load readonly data I binary O elf32 avr foo bin foo o
6. 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 pointer 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 zero 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 1 of 19 avr libc Frequently Asked Questions 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
7. 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 details 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 nec
8. r24 0x21 p33 80 08 d0 reall LG 0x92 This code reads the pointer to the desired string from the ROM table array into a register pair http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 9 of 19 avr libc Frequently Asked Questions The value of i in r22 123 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 Oxffda 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 strepy _P buf p 82 69 al ldd r22 Y 33 7 0x21 84 7a al ldd r23 Y 34 0x22 86 ce 01 movw r24 r28 88 01 96 adiw r24 0x01 1 8a Oc do reall 24 Oxa4 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 strepy_pP This will copy the string from ROM to buf 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
9. 4 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 e Call used registers 118 127 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 e 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 e Fixed registers r0 r1 Never allocated by gcc for local data but often used for fixed purposes 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 rl assumed to be always zero in any C code may be used to r
10. 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 TCNT the input capture register ICR 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 ISRQ 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 use
11. ands for bit value in case someone might ask you 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 Pp7 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 e Obviously none of the C related standard functions classes and template classes are available e 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 e Some of the supplied include files are not C safe i e they need to be wrapped into extern C ua This could certainly be fixed too e 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 t
12. art 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 cleared Back to FAQ Index What is all this _BVQ 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 place 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 gt 1 lt lt 3 gt 0x08 However using the macro often makes the program better readable BV st
13. ation levels and added about 2 5 KB to the code Optimization flags Size of text Time for test 1 Time for test 2 03 6898 903 Aps 19 7 ms oz fesse 972 As 20 1 ms Os IZE 955 Aus 1 20 1 ms Os mcall prologues 6474 972 Aus 201 ms _ The difference between 955 As and 972 Ays 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 03 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 W1 compiler option W1 section start bootloader 0x1E000 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 nonsens
14. ault 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 gece 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 a
15. avr libc Frequently Asked Questions AVR Libe eee Home Page ae User Library Alphabetical Example Man bage Manual Reference Index Projects Frequently Asked Questions FAQ Index My program doesn t recognize a variable updated within an interrupt routine I get undefined reference to for functions like sinQ How to permanently bind a variable to a register How to modify MCUCR or WDTCR early What is all this BV stuff about Can I use C on the AVR Shouldn t I initialize all my variables Why do some 16 bit timer registers sometimes get trashed How do I use a define d constant in an asm statement 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 foo bar 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
16. bility 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 Uses global variables to tando Pandoni keep state information Use special reentrant versions rand_r random_r strtod strtol strtoul Uses the global variable Ignore errno or protect calls with cli sei or errno to return ATOMIC_BLOCK if the application can tolerate it success failure Or use sccanf or sccanf_P if possible Uses the stack pointer Protect calls with cli Q sei or ATOMIC_BLOCK O if malloc realloc and global variables to the application can tolerate it If using an OS use the calloc
17. character count in the fprintf_PQ FILE argument Assign each thread its own FILE for output Or if vfprintf oblan oan ocur ifa returned character count is unimportant do not use the vfprintf_P fputs global FILE is used _P versions fputs_PO from multiple threads Contains an embedded assert fprintf See above for See above for fprintf fprintf Alters flags in the FILE Assign each thread its own FILE for output clearerr argument Alters flags character getchar gets count and unget buffer in global FILE stdin Use only in one thread fgetc ungetc Ffaatc connfl http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 17 of 19 avr libc Frequently Asked Questions ABEOU scamy scanf_P fscanf fscanf_P vscanf vfscanf vfscanf_P fread Alters flags character Assign each thread its own FILE for input count and unget buffer Note Scanning from a string e g sscanf and in the FILE argument sscanf_P are thread safe 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
18. clock frequently with the server s clock 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 handler http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 13 of 19 avr libc Frequently Asked Questions 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 be
19. d uint16_t read_timerl void uint8_t sreg uintl6_t val sreg SREG cli val TCNT1 SREG sreg return val http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 4 of 19 avr libc Frequently Asked Questions 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 ports 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 Note For C programs rather use the standard C bit operators instead so the above would be expressed as PORTB 1 lt lt 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 0 and debug information g which is fortunately possible in avr gec the code watched in the debugger is optimized code Whi
20. d for fdevopen too void debug P const char addr char c while c pgm_read_byte addr t uart_putchar c int main void ioinit initialize UART debug_P PSTR foo was here n return 0 Note 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 amp 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 amp unsigned char mask Back to FAQ Index http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 12 of 19 avr libc Frequently Asked Questions 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 def
21. e 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 e 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 communication e The ATmegal28 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 e 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 foo bar 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
22. e 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 t28 SPE _ in r29 SPH sbiw r28 20 in tmp_reg__ __SREG li out SP H r29 out SREG_ __tmp_reg out _ SPL 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 srEc 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 1 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 14 of 19 avr libc Frequently Asked Questions 10 19 09 11 33 AM Why are there five different linker scripts From a comment in the source code
23. e 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 lt avr pgmspace h gt PGM P array 2 PROGMEM Foo Bar yi int main void char buf 32 strepy 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 lt avr pgmspace h gt const char foo PROGMEM Foo const char bar PROGMEM Bar PGM_P array 2 PROGMEM foo bar int main void char buf 32 PGM_P p int i memcpy P amp p amp 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 lt array gt 26 2e 00 word 0x002e 28 2a 00 word 0x002a gt 7 0000002a lt bar gt 2a 42 61 72 00 Bar 0000002e lt foo gt 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 amp p amp array i sizeof PGM_P 70 66 Of add E22 222 72 T ILE adc 23 23 74 6a 5d subi r22 OxDA gt 218 76 7E 4f sbci r23 OxFF 255 78 42 e0 ldi r20 0x02 2 7a 50 e0 ldi r21 0x00 gt 0 7G ce 01 movw r24 r28 7e 81 96 adiw
24. emember 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 mul s u instructions which return their result in rl r0 Interrupt handlers save and clear r1 on entry and restore rl on exit in case it was non zero e 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 Warning 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 8 of 19 avr libc Frequently Asked Questions There are times when you may need an array of strings which will never b
25. er 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 Note You can also use Wa gstabs since the compiler will add the extra for you Example EXTRA_OPTS Wall mmcu atmegal28 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 Example myfunc push ri push 1 push r18 push YL push YH eor F116 16 start loop Idi YL lo8 sometable Idi YH hi8 sometable rjmp 2E jump to loop test at end 1 ld 17 Y loop continues here breq 1f return from myfunc prematurely in 16 2 cmp Tl ELS brlo 1b jump bac
26. es 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 5 of 19 avr libc Frequently Asked Questions 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 gstabs Example avr as mmcu atmegal28 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 assembl
27. essary 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 all 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 ATmegal 28 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 stremp 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 10 of 19 avr libc Frequently Asked Questions linked into the binary in order to print out the time elapsed which is entirely not affected by the different optimiz
28. 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 Note 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
29. fore 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 TOVO wrong simply use TIFR _BV TOVO 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 ar
30. free allocate and free OS provided memory allocator since the OS is likely memory modifying the stack pointer anyway Protect calls with cli sei or ATOMIC_BLOCK if the application can tolerate it Or use fdevopen fclose Uses calloc and free 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 Accesses I O register x pgm_ _far0 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 printfQ printf_PQ vprintfQ vprintf_PQ puts puts_P Alters flags and character count in global FILE stdout string 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 Q vsprintfQ vsprintf_PQ vsnprintf vsnprintf_P is thread safe The formatted string could then be followed by an fwriteQ which simply calls the lower layer to send the Alters flags and iprint
31. he compiler options Failing this the linker will complain about an undefined external reference to ___gxx_personality_sj0 Constructors and destructors are supported though including global ones 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 3 of 19 avr libc Frequently Asked Questions 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 gee 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
32. ived 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 7 of 19 avr libc Frequently Asked Questions lle 80 6f ori r24 OxFO 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 See http mail nongnu org archive html avr libc dev 2003 01 msg00044 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 e Data types char is 8 bits int is 16 bits long is 32 bits long long is 6
33. k to top of loop 13 pop YH pop YL pop r18 pop Bly pop r16 ret Back to FAQ Index How do I pass an IO port as a parameter to a function Consider this example code include lt inttypes h gt include lt avr io h gt void set_bits_func_wrong volatile uint8_t port uint8_t mask port mask http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 6 of 19 avr libc Frequently Asked Questions void 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 amp 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 megal28 This is seen clearly when looking at the disassembly of the call set_bits_func_wrong PORTB 0xaa 10a 6a ea ldi r22 OxAA gt 170 LOG 88 b3 in r24 0x18 7 24 10e Oe 94 65 00 call Oxca So the function once called only sees the value of the port register and knows nothing about which port it came from A
34. le 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 cas
35. led 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 77 begin xram s include lt avr io h gt section initl ax progbits ldi r16 BV SRE _BV SRW out _SFR_IO ADDR MCUCR r16 77 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 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 2 of 19 avr libc Frequently Asked Questions 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 ATmegal61 because of errata and add w1 Tdata 0x801100 to st
36. re 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 http lightner net avr ATtinyAvrGcc html 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
37. same amount of flash ROM so they can be initialized to the actual string by startup code The compiler can http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 11 of 19 avr libc Frequently Asked Questions 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 lt inttypes h gt include lt avr io h gt include lt avr pgmspace h gt int uart_putchar char c if c n uart_putchar r loop_until_bit_is_set USR UDRE UDR c return 0 so it could be use
38. size int size sym gt gt h Back to FAQ Index How do I perform a software reset of the AVR http www nongnu org avr libc user manual FAQ html Page 15 of 19 avr libc Frequently Asked Questions 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 lt avr wdt h gt define soft_reset do wdt_enable WDTO_15MS j for Pi a ree while 0
39. t 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 amp PORTB 0x55 112 65 e5 ldi r22 0x55 p 85 114 88 e3 ldi r24 0x38 p56 116 3 90 e0 ldi r25 0x00 0 118 0e 94 7c 00 call Oxt8 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 void set_bits_func_correct volatile uint8_t port uint8_t mask f8 foe 01 movw r30 r24 port mask fa 80 81 ld r24 Z f 86 2b or tza 222 fe 80 83 st Z r24 100 08 95 ret Notice that we are accessing the io port via the Lp and st instructions The port parameter must be volatile to avoid a compiler warning Note Because of the nature of the 1n 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 contr
40. tasheet 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 Back to FAQ Index Why is my baud rate wrong Some AVR datasheets give the following formula for calculating baud rates F_CPU UART_BAUD_RATE 16L 1 http www nongnu org avr libc user manual FAQ html 10 19 09 11 33 AM Page 18 of 19 avr libc Frequently Asked Questions 10 19 09 11 33 AM Unfortunately that formula does not work with all combinations of clock speeds and baud rates due to integer truncation during the division operator 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 lt util setbaud h gt Helper macros for baud rate calculations Back to FAQ Index Automatically generated by Doxygen 1 5 7 on 5 Mar 2009 http www nongnu org avr libc user manual FAQ html Page 19 of 19
41. 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 da
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