On the ever busy Muffwiggler synth forum, someone asked about some test Arduino code for the Microchip MCP4922 SPI DAC.

This program outputs ever rising voltages on each of the DAC channels, until they get to the maximum level of ~5v, when it loops back to 0v again. It also prints the number sent to the DAC to the serial monitor, so you can check what it’s doing.

Like it says, it’s mostly borrowed from MrBook’s blog post on controlling a Gakken SX-150 through MIDI.

/*
MCP4922 test
most code borrowed from
http://mrbook.org/blog/2008/11/22/controlling-a-gakken-sx-150-synth-with-arduino/

connections
====================================================

+5v           > 4922 pin 1
Ard pin 10    > 4922 pin 3   (SS - slave select)
Ard pin 13    > 4922 pin 4   (SCK - clock)
Ard pin 11    > 4922 pin 5   (MOSI - data out)
Ground        > 4922 pin 8   (LDAC)
+5v           > 4922 pin 11  (voltage ref DAC B)
Ground        > 4922 pin 12
+5v           > 4922 pin 13  (voltage ref DAC A)

4922 pin 14 DAC A > 1k resistor > synth CV in

*/

// MCP4922 DAC out
#define DATAOUT 11//MOSI
#define DATAIN 12//MISO - not used, but part of builtin SPI
#define SPICLOCK  13//sck
#define SLAVESELECT0 10//ss

int i = 0;

void setup() {
  SPI_setup();
  Serial.begin(9600);
}

void loop() {
 i++;
 Serial.println(i);
 write_note(i);
 if(i>=4096) {
  i=0; 
 }
}

void write_note(int i) {
  write_valueY(i);
  write_valueX(i);
}

// **************************************************
// SPI for DAC

void SPI_setup(){
  byte clr;
  pinMode(DATAOUT, OUTPUT);
  pinMode(SPICLOCK,OUTPUT);
  pinMode(SLAVESELECT0,OUTPUT);
  digitalWrite(SLAVESELECT0,HIGH); //disable device

  SPCR = (1<<SPE)|(1<<MSTR) | (0<<SPR1) | (0<<SPR0);
  clr=SPSR;
  clr=SPDR;
  delay(10);
}

// write out through DAC A
void write_valueX(int sample)
{
  // splits int sample in to two bytes
  byte dacSPI0 = 0;
  byte dacSPI1 = 0;
  dacSPI0 = (sample >> 8) & 0x00FF; //byte0 = takes bit 15 - 12
  dacSPI0 |= (1 << 7);	  // A/B: DACa or DACb - Forces 7th bit  of    x to be 1. all other bits left alone.
  dacSPI0 |= 0x10;
  dacSPI1 = sample & 0x00FF; //byte1 = takes bit 11 - 0
  dacSPI0 |= (1<<5);  // set gain of 1
  digitalWrite(SLAVESELECT0,LOW);
  SPDR = dacSPI0;			  // Start the transmission
  while (!(SPSR & (1<<SPIF)))     // Wait the end of the transmission
  {
  };

  SPDR = dacSPI1;
  while (!(SPSR & (1<<SPIF)))     // Wait the end of the transmission
  {
  };
  digitalWrite(SLAVESELECT0,HIGH);
}

// write out through DAC B
void write_valueY(int sample)
{
  // splits int sample in to two bytes
  byte dacSPI0 = 0;
  byte dacSPI1 = 0;
  dacSPI0 = (sample >> 8) & 0x00FF; //byte0 = takes bit 15 - 12
  dacSPI0 |= 0x10;
  
  dacSPI1 = sample & 0x00FF; //byte1 = takes bit 11 - 0
  dacSPI0 |= (1<<5);  // set gain of 1
  
  digitalWrite(SLAVESELECT0,LOW);
  SPDR = dacSPI0;			  // Start the transmission
  while (!(SPSR & (1<<SPIF)))     // Wait the end of the transmission
  {
  };


  SPDR = dacSPI1;
  while (!(SPSR & (1<<SPIF)))     // Wait the end of the transmission
  {
  };
  digitalWrite(SLAVESELECT0,HIGH);
}

One thing I’d point out that I’ve not seen much elsewhere is that by default the MCP4922 seems to be set to output with a gain of 2. So as we’ve got it setup, rather than the number 4095 representing the maximum voltage that can be output, 2047 will actually make us hit ~5v . Setting the output gain select bit on output will change the gain from 2x to 1x.

dacSPI0 |= (1<<5);  // set gain of 1