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(__memx const void*)#
This built-in takes a byte address to the 24-bit AVR Named Address Spaces
__memx
and returns the number of the flash segment (the 64 KiB chunk) where the address points to. Counting starts at0
. 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
intoval
and return the resulting value. The nibbles ofmap
determine how the insertion is performed: LetX
be then
-th nibble ofmap
If
X
is0xf
, then then
-th bit ofval
is returned unaltered.If X is in the range 0…7, then the
n
-th result bit is set to theX
-th bit ofbits
If X is in the range 8…
0xe
, then then
-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);
}