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6.58.10 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);
}

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