In the episode, I explain how the internals of the DAC work and introduce the concept of an R-2R ladder.

PSU Project #3 – MCP4822

Sketch used in the episode (can be downloaded here):

/*

 Test sketch for the MCP3304 and MCP4822

 Both are bit-banged to get them working.

 In this demo:

 DAC channel A will count up from 0v to 4.096V

 DAC Channel B will mirror what is being read on ADC Channel 0

 Arduino   MCP3304   MCP4822

 */

 #define ADC_CS 10    // ADC chip-select

 #define DAC_CS 9    // DAC chip-select

 #define ADC_MOSI 11   // ADC_MOSI

 #define ADC_MISO 12    // ADC_MISO

 #define SPI_CLK 13  // Clock

 #define MCP4822_DAC_A_BUFFERED_GAIN_1X_NON_SHDN      0x70  // DAC A, Buffered, Gain 1x, Output power control

 #define MCP4822_DAC_A_BUFFERED_GAIN_2X_NON_SHDN      0x50  // DAC A, Buffered, Gain 2x, Output power control

 #define MCP4822_DAC_A_NON_BUFFERED_GAIN_1X_NON_SHDN  0x30  // DAC A, Non buffered, Gain 1x, Output power control

 #define MCP4822_DAC_A_NON_BUFFERED_GAIN_2X_NON_SHDN  0x10  // DAC A, Non buffered, Gain 2x, Output power control

 #define MCP4822_DAC_B_BUFFERED_GAIN_1X_NON_SHDN      0xF0  // DAC B, Buffered, Gain 1x, Output power control

 #define MCP4822_DAC_B_BUFFERED_GAIN_2X_NON_SHDN      0xD0  // DAC B, Buffered, Gain 2x, Output power control

 #define MCP4822_DAC_B_NON_BUFFERED_GAIN_1X_NON_SHDN  0xB0  // DAC B, Non buffered, Gain 1x, Output power control

 #define MCP4822_DAC_B_NON_BUFFERED_GAIN_2X_NON_SHDN  0x90  // DAC B, Non buffered, Gain 2x, Output power control

int test2=0;
 long test3=0;

void pulse_MCP_clock(void)
 {
 digitalWrite(SPI_CLK, LOW); delayMicroseconds(50);
 digitalWrite(SPI_CLK, HIGH); delayMicroseconds(50);
 }

long read_MCP(int channel)
 {
 long value=0;
 signed int x=0;
 digitalWrite(ADC_CS, LOW);
 digitalWrite(DAC_CS, HIGH);
 delayMicroseconds(50);
 digitalWrite(ADC_CS, LOW);
 delayMicroseconds(50);
 digitalWrite(ADC_MOSI, HIGH); //start pulse
 pulse_MCP_clock();
 digitalWrite(ADC_MOSI, HIGH); //single mode
 pulse_MCP_clock();
 for(x=2; x>=0; --x)
 {
 if(bitRead(channel,x))
 {
 digitalWrite(ADC_MOSI, HIGH);
 }
 else
 {
 digitalWrite(ADC_MOSI, LOW);
 }
 pulse_MCP_clock();
 }
 digitalWrite(ADC_MOSI, LOW);//put low throughout
 for(x=1;x<=3;++x)
 {
 //blank, Null, and sign bit (always 0 in single mode)
 pulse_MCP_clock();
 }
 //now get twelve bits
 for(x=11; x>=0; --x)
 {
 pulse_MCP_clock();
 if (digitalRead(ADC_MISO))
 {
 bitSet(value,x);
 }
 else
 {
 bitClear(value,x);
 }
 }
 digitalWrite(ADC_CS, HIGH);
 digitalWrite(DAC_CS, HIGH);
 return value;
 }

void init_ADC(void)
 {
 digitalWrite(SPI_CLK,HIGH); //clock
 digitalWrite(ADC_MISO, LOW); //output -- supposed to be float but not sure how to do this on Arduino
 digitalWrite(ADC_MOSI, HIGH); //input
 digitalWrite(ADC_CS, HIGH);
 delayMicroseconds(50);
 digitalWrite(ADC_CS, LOW);
 delayMicroseconds(50);
 digitalWrite(ADC_CS, HIGH);
 digitalWrite(DAC_CS, HIGH);
 }

int get_adc_avg(int channel)
 {
 long tempx = 0;
 byte ax = 0;
 for (ax=0;ax<10;ax++)
 {
 tempx+=read_MCP(channel);
 }
 tempx/=10;
 return (tempx);
 }

void mcp4822_init() {
 digitalWrite(DAC_CS,HIGH);
 digitalWrite(SPI_CLK, LOW);
 digitalWrite(ADC_CS,HIGH);
 digitalWrite(ADC_MOSI, LOW);
 }

void mcp4822_write(byte control,int data) {
 byte i;
 int tempx=0x00;
 tempx = data|(control<<8);
 digitalWrite(ADC_CS,HIGH);
 delayMicroseconds(10);
 digitalWrite(SPI_CLK,LOW);
 delayMicroseconds(10);
 digitalWrite(DAC_CS,LOW);
 delayMicroseconds(10);
 digitalWrite(ADC_MOSI,LOW);
 for(i=16;i>0;--i)
 {
 digitalWrite(SPI_CLK,LOW);
 digitalWrite(ADC_MOSI,bitRead(tempx,i-1));
 digitalWrite(SPI_CLK,HIGH);
 }
 digitalWrite(DAC_CS,HIGH);
 delayMicroseconds(10);
 }

void setup() {
 //set pin modes
 pinMode(ADC_CS, OUTPUT);
 pinMode(DAC_CS, OUTPUT);
 pinMode(ADC_MOSI, OUTPUT);
 pinMode(ADC_MISO, INPUT);
 pinMode(SPI_CLK, OUTPUT);
 init_ADC();
 mcp4822_init();
 digitalWrite(ADC_CS, HIGH);
 digitalWrite(DAC_CS, HIGH);
 Serial.begin(115200);
 Serial.print("Hello2"); // Just to show the serial is working
 }

void loop() {
 int tempx2 = 0;
 if (test3++>0xFFF) test3=0;
 tempx2 = read_MCP(0);
 mcp4822_write(MCP4822_DAC_A_NON_BUFFERED_GAIN_2X_NON_SHDN, test3);
 mcp4822_write(MCP4822_DAC_B_NON_BUFFERED_GAIN_2X_NON_SHDN, tempx2);
 delay(10);
 }