quantization test for synthesizer CV

quantize-test.c

/**
 * Quantize test: 
 * Arduino Nano pins for the MCP4822 DAC: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK).
 * potentiometer input: A0
 */
#include "SPI.h"

// pin numbers
const int ledPin = 13;
const int csPin = 10;

char buf[256];

// send a word to the DAC
void sendDacWord(uint16_t word)
{
  digitalWrite(csPin, LOW);
  SPI.transfer(word >> 8);
  SPI.transfer(word & 0xff);
  digitalWrite(csPin, HIGH);
}

void setup()
{
  // init pins
  pinMode(ledPin, OUTPUT);
  pinMode(csPin, OUTPUT);
  digitalWrite(csPin, HIGH);

  // init SPI
  SPI.beginTransaction(SPISettings(8000000, MSBFIRST, SPI_MODE0));
  SPI.begin();  // additional begin is required, otherwise the second call to SPI.transfer hangs

  // use a 2.5V external analog reference
  analogReference(EXTERNAL);

  // for debugging
  Serial.begin(115200);
}

// read an anlog input and return it as voltage, assuming an 2.5V external reference
float readVolt(uint8_t channel)
{
  uint16_t in = analogRead(channel);
  return in / 1024.0f * 2.5f;
}

void loop()
{
  // read potentiometer
  float volt = readVolt(0);

  // 12 steps per volt for quantization
  float steps = 12.0f;
  float quantized = float(int(volt * steps)) / steps;

  // convert to DAC value (internal 2.048 V reference, 12 bit)
  uint16_t out = int(quantized / 2.048f * 4096.0f);

  // limit and send to DAC
  if (out > 4095) out = 4095;
  sendDacWord(out & 0x0fff | (1 << 12) | (1 << 13));

  // debug output
  delay(1);
  // read back DAC output
  float dac = readVolt(1);
  sprintf(buf, "%i\t%i\t%i", int(volt * 1000.0f), int(quantized * 1000.0f), int(dac * 1000.0f));
  Serial.println(buf);
}