134kHz sine wave w/DMA, DAC and PDB on Teensy 3.6

sin134-pdb-dma-dac.c

// Generating a 134kHz sine wave using Teensy 3.6 with virtually
// 0 cpu usage by filling the DAC up via DMA triggered by PDB
// -samy kamkar

#include <DMAChannel.h>
#include "pdb.h"

DMAChannel dma(false);

static volatile uint16_t sinetable[] = {
  2048,2277,2503,2724,2936,3137,3324,3495,3648,3781,3892,3980,4044,4082,4095,4082,4044,3980,3892,3781,3648,3495,3324,3137,2936,2724,2503,2277,2048,1818,1592,1371,1159,958,771,600,447,314,203,115,51,13,0,13,51,115,203,314,447,600,771,958,1159,1371,1592,1818
};

void setup()
{
  dma.begin(true); // allocate the DMA channel first
  
  SIM_SCGC2 |= SIM_SCGC2_DAC0; // enable DAC clock
  DAC0_C0 = DAC_C0_DACEN | DAC_C0_DACRFS; // enable the DAC module, 3.3V reference

  // slowly ramp up to DC voltage, approx 1/4 second
  for (int16_t i=0; i<2048; i+=8)
  {
    *(volatile int16_t *)&(DAC0_DAT0L) = i;
    delay(1);
  }
  
  // set the programmable delay block to trigger DMA requests
  SIM_SCGC6 |= SIM_SCGC6_PDB; // enable PDB clock
  PDB0_IDLY = 0; // interrupt delay register
  PDB0_MOD = 0; //PDB_PERIOD; // modulus register, sets period
  
  PDB0_SC = PDB_CONFIG | PDB_SC_LDOK; // load registers from buffers
  PDB0_SC = PDB_CONFIG | PDB_SC_SWTRIG; // reset and restart
  PDB0_CH0C1 = 0x0101; // channel n control register?
  
  dma.sourceBuffer(sinetable, sizeof(sinetable));
  dma.destination(*(volatile uint16_t *)&(DAC0_DAT0L));
  dma.triggerAtHardwareEvent(DMAMUX_SOURCE_PDB);
  dma.enable();
}

void loop() {
}

Nice! I'm surprised you're saying the max freq is 15kHz? I know I was able to generate either 125kHz or 134kHz on a Teensy 3.1 and 3.2 as well (those were specific frequencies I wanted, not maximums). I'm sure I adjusted the sinetable and perhaps something else.

I also was doing it in the main loop with a separate test, but I'm guessing you want to keep your main loop unoccupied.

The frequency I was looking for was 15kHz so I've updated the register. It is not the max frequency that we can see on the scope before setting PDB0_MOD register.

However, if a better method exist, it welcome.

I will use 2 interrupts to calculate the phase difference between reference frequency (fixed at 15kHz) and a 2nd incoming signal.

The loop will populated with some functions like a OLED display and capacitive touch.