6.59.10 AVR Built-in Functions

For each built-in function for AVR, there is an equally named, uppercase built-in macro defined. That way users can easily query if or if not a specific built-in is implemented or not. For example, if __builtin_avr_nop is available the macro __BUILTIN_AVR_NOP is defined to 1 and undefined otherwise.

void __builtin_avr_nop (void)
void __builtin_avr_sei (void)
void __builtin_avr_cli (void)
void __builtin_avr_sleep (void)
void __builtin_avr_wdr (void)
unsigned char __builtin_avr_swap (unsigned char)
unsigned int __builtin_avr_fmul (unsigned char, unsigned char)
int __builtin_avr_fmuls (char, char)
int __builtin_avr_fmulsu (char, unsigned char)
These built-in functions map to the respective machine instruction, i.e. nop , sei , cli , sleep , wdr , swap , fmul , fmuls resp. fmulsu . The three fmul* built-ins are implemented as library call if no hardware multiplier is available.
void __builtin_avr_delay_cycles (unsigned long ticks)
Delay execution for ticks cycles. Note that this built-in does not take into account the effect of interrupts that might increase delay time. ticks must be a compile-time integer constant; delays with a variable number of cycles are not supported.
char __builtin_avr_flash_segment (const __memx void*)
This built-in takes a byte address to the 24-bit address space __memx and returns the number of the flash segment (the 64 KiB chunk) where the address points to. Counting starts at 0 . If the address does not point to flash memory, return -1 .
uint8_t __builtin_avr_insert_bits (uint32_t map, uint8_t bits, uint8_t val)
Insert bits from bits into val and return the resulting value. The nibbles of map determine how the insertion is performed: Let X be the n -th nibble of map
  1. If X is 0xf , then the n -th bit of val is returned unaltered.
  2. If X is in the range 0... 7, then the n -th result bit is set to the X -th bit of bits
  3. If X is in the range 8... 0xe , then the n -th result bit is undefined.

One typical use case for this built-in is adjusting input and output values to non-contiguous port layouts. Some examples:

          // same as val, bits is unused
          __builtin_avr_insert_bits (0xffffffff, bits, val)
          // same as bits, val is unused
          __builtin_avr_insert_bits (0x76543210, bits, val)
          // same as rotating bits by 4
          __builtin_avr_insert_bits (0x32107654, bits, 0)
          // high nibble of result is the high nibble of val
          // low nibble of result is the low nibble of bits
          __builtin_avr_insert_bits (0xffff3210, bits, val)
          // reverse the bit order of bits
          __builtin_avr_insert_bits (0x01234567, bits, 0)

void __builtin_avr_nops (unsigned count)
Insert count NOP instructions. The number of instructions must be a compile-time integer constant.

There are many more AVR-specific built-in functions that are used to implement the ISO/IEC TR 18037 “Embedded C” fixed-point functions of section 7.18a.6. You don't need to use these built-ins directly. Instead, use the declarations as supplied by the stdfix.h header with GNU-C99:

     #include <stdfix.h>
     // Re-interpret the bit representation of unsigned 16-bit
     // integer uval
 as Q-format 0.16 value.
     unsigned fract get_bits (uint_ur_t uval)
         return urbits (uval);