delay-avr.c

/** \file
  \brief Delay routines, AVR specific part.
*/

#if defined TEACUP_C_INCLUDE && defined __AVR__

#include <util/delay_basic.h>

#if F_CPU < 4000000UL
  #error Delay functions on AVR only work with F_CPU >= 4000000UL
#endif


/** Delay in microseconds.

  \param delay Time to wait in microseconds.

  Calibrated in SimulAVR.

  Accuracy on 20 MHz CPU clock: -1/+3 clock cycles over the whole range(!).
  Accuracy on 16 MHz CPU clock: delay is about 0.8% too short.

  Exceptions are delays of 0..2 on 20 MHz, which are all 0.75 us and delays
  of 0..3 on 16 MHz, which are all 0.93us.
*/
void delay_us(uint16_t delay) {
  // Compensate call overhead, as close as possible.
  #define OVERHEAD_CALL_CLOCKS 39 // clock cycles
  #define OVERHEAD_CALL_DIV ((OVERHEAD_CALL_CLOCKS / (F_CPU / 1000000)) + 1)
  #define OVERHEAD_CALL_REM ((OVERHEAD_CALL_DIV * (F_CPU / 1000000)) - \
                             OVERHEAD_CALL_CLOCKS)

  if (delay > OVERHEAD_CALL_DIV) {
    delay -= OVERHEAD_CALL_DIV;
    if (OVERHEAD_CALL_REM >= 2)
      _delay_loop_2((OVERHEAD_CALL_REM + 2) / 4);
  }
  else {
    return;
  }

  while (delay > (65536L / (F_CPU / 4000000L))) {
    #define OVERHEAD_LOOP_CLOCKS 13

    _delay_loop_2(65536 - (OVERHEAD_LOOP_CLOCKS + 2) / 4);
    delay -= (65536L / (F_CPU / 4000000L));
  }
  if (delay)
    _delay_loop_2(delay * (F_CPU / 4000000L));
}

#endif /* defined TEACUP_C_INCLUDE && defined __AVR__ */