Simple MCP4922 SPI DAC library for Teensy 3.2 boards.

Simple MCP4922

This library provides a simple and fast way to set up a MCP4922 DAC on a Teensy 3.2 board. The code is simple and short, it was designed to fulfill an specific need for a project, disabling some of the hardware pins used by the SPI interface. But it ended up being kind of redundant since (re)setting the pin does the job in a similar way.

This library is based on Bob's Tillart MCP_DAC library while doing some of the operations in a slightly different way.

Functionalities

Constructor

• Simple_MCP4922() The constuctor intializes some of the values to the logical default ones such as gain=1, alternatePins=false etc.
• init() Function called in the constructor to intialize the values.
• begin(uint8_t cs, bool alternatePins) Recieves a pin to use as CS or Chip Select since the SPI library doesn't take care of it. Also a boolean value that controls the use of some of the altenate pins designated in the interface.

Settings

• setBuffer(bool buffered) True enables buffered outputs while False disables them.
• setGain(uint8_t gain) The attribute _gain should be either 1 or 2, usually I would have had 0 or 1 for gain 1 and 2 respectively, but the DAC sets gain to 1 when sent a high level bit, so I thought it was more convenient this way.
• end() can be used to call SPI.end() to disable the interface.

Disabling certain pins of the interface

A SPI hardware interface tipically has 4 pins: MISO for data input, MOSI for data output, CSK for the clock and CS for selecting the peripheral device.

For example, seting MISO pin as pinMode(CS, OUTPUT) will give the GPIO the control of the pin even tho the SPI interface was using it. Athough this works, I included dedicated functions to change the pin control "completely". A multiplexor sets which system controls each pin, so its possible to write to the register in order to modify which system can use it.

// PORTx_PCRn, for pin 12 (default MOSI):
PORTC_PCR7 = (PORTC_PCR7 & 0xFFFFF8FF) | (001 << 8);

Bits 8-10 in the register control the multiplexor as shown in the manual for Tensy 3.2 on page 228.

• freeMOSI() Gives GPIO the control of pin 11 or pin 7 if alternatePins is True.
• freeMISO() Gives GPIO the control of pin 12 or pin 8 if alternatePins is True.
• freeCSK() Gives GPIO the control of pin 13 or pin 14 if alternatePins is True.
• Freeing CS is not necessary since its not controlled by the library.

Ouputting data

• analogWrite(uint16_t data, uint8_t channel) Channel can be either 0 (OutputA) or 1 (OutputB).
• analogWriteFastCH1(uint16_t data) and analogWriteFastCH2(uint16_t data) ignore buffer and gain settings and use the default values (unbuffered and G=1).

Setting up the DAC

SHDN pin should be connected to high level since it enables the output. If you have spare pins a microcontroller pin can be used to switch between enabled and disabled in order to save power.

The rest of the pins are listed in the datasheet, a full list of connections for basic operation is provided below.

DAC	PIN	Connection
VDD	1	VDD
NC	2	x
!CS	3	CS: Teensy3.2 Pin 10
SCK	4	SCK: Teensy 3.2 Pin 13
SDI	5	SDI: Teensy 3.2 Pin 11
NC	6	x
NC	7	x
!LDAC	8	GND
!SHDN	9	VDD
VoutB	10	Channel 2 Ouput
VrefB	11	Channel 2 Reference (VDD)
VSS	12	GND
VrefA	13	Channel 1 Reference (VDD)
VrefB	14	Channel 1 Output

The code below shows Simple_MCP4922_example.ino. It's written to output values on channel A (or 1), connections to the DAC should be as shown in the table above.

#include "Simple_MCP4922.h"

#define T_PIN_CS 10

uint16_t data_array[6] = {0, 819, 1638, 2457, 3276, 4095};
uint8_t i = 0;

// Create a Simple_MCP4922 object.
Simple_MCP4922 DAC;

void setup() 
{    
  // Start the SPI hardware controller.
  // The Simple_MCP4922 library takes care of importing
  // the SPI and setting up the chip select pin.
  DAC.begin(T_PIN_CS, false);
}

void loop() 
{

  // Writes to channel 1
  DAC.analogWrite(data_array[i], 0);
  if(i < 5) i++;
  else i = 0;

  delay(600); //msas
  
}

Future

Possibly adding support for different Teensy boards.