AVR® EB Family TCF NCO Frequency Waveform Generation

[Georg M. (g_m)]

Je besser die Hardware, desto kompakter der Code.

#include <avr/io.h>

int main(void)
{
  const uint32_t coefficient = 2577;                 // coefficient (@ 3.33 MHz main clock)
  uint16_t frequency = 500;                          // frequency (Hz)
  uint32_t tcfcmp = coefficient * frequency >> 8;    // compare value for TCF0.CMP

  TCF0.CTRLB = TCF_WGMODE_NCOFDC_gc;                 // NCO fixed duty-cycle
  while(TCF0.STATUS){};                              // wait if busy
  TCF0.CMP = tcfcmp;                                 // write compare value
  TCF0.CTRLC = TCF_WO0EN_bm;                         // enable TCF WO0 (PA0)
  TCF0.CTRLA = TCF_ENABLE_bm;                        // enable TCF

  TCB0.CCMP = 0x8234;                                // timeout value (20ms)
  TCB0.CTRLA = TCB_CLKSEL_DIV2_gc | TCB_ENABLE_bm;   // 1.666 MHz, enable TCB0

  while(1)
  {
    if(TCB0.INTFLAGS)
    {
      TCB0.INTFLAGS = TCB_CAPT_bm;                   // clear TCB0 interrupt flag    
      frequency += 1;
      if(frequency > 1000)
      {
        frequency = 500;
      }
      tcfcmp = coefficient * frequency >> 8;         // calculate
      while(TCF0.STATUS & TCF_CMP0BUSY_bm){};        // wait if busy
      TCF0.CMP = tcfcmp;                             // update
    }
  }
}

AVR16EB14 Data Sheet:
"This linear advantage over divide-by-n timers comes at the cost of the 
output jitter. However, the jitter that occurs periodically is always 
plus or minus one clock period, depending on the division remainder."