pi-MCP4725

A Python module for the Raspberry Pi to interface with the MCP4725 I²C Digital-to-Analogue Converter.

Installation

Compatibility

Tested on Python 3.9 on a Raspberry Pi Zero W v1.1, but should be compatible with most Raspberry Pi models running the latest version of Python.

Module works with Microchip's MCP4725 DAC.

Pre-requisites

• smbus2 library

  The MCP4725 module uses the Raspberry Pi's built-in I²C driver to communicate with the chip. This is done on Python using the SMBus protocol. However, due to SMBus incorporating only a subset of I²C features, it is not fully compatible with I²C commands. This is fixed by using smbus2, a drop-in replacement of the smbus module with extra functionality permitting the extra I²C features. It can be imported as smbus to be backward compatible. See below for more details.

  The module must be imported within the main code (it is not imported within the module).

Install

The MCP4725 module can be installed by cloning the files into your project folder.

TO-DO: Add the module to PiPy for easier installation with pip.

Usage Example

Initialisation

The library and its pre-requisites must be imported first, and an SMBus object is created:

import MCP4725
import smbus2 as smbus

BUS_No = 1

bus = smbus.SMBus(BUS_No)

A DAC can now be initialised using the MCP4725 class:

# Default address for the MCP4725. Change as needed accordingly.
ADDRESS = 0X67

# Initialise MCP4725 object, providing the bus instance, device address and smbus module
dac = MCP4725.MCP4725(bus, ADDRESS, smbus)

(Note that the SMBus module is sent to the init. function as one of its structure classes is needed by the module)

Setting the output voltage

To set the output voltage:

V_DD = 5 		# Supply voltage of the chip

v_out = 2.5		# Desired output voltage (must be between 0 and V_DD)

dac_value = 4095 * v_out/V_DD	# The value must be between 0 and 4095 (i.e. 12 bits)

MCP4725.write(dac_value)		# Set the DAC output voltage

Saving value to EEPROM

The DAC features an integrated EEPROM so that the requested voltage output persists through power cycles. To write to EEPROM:

MCP4725.write(dac_value, True)

Power down mode

The MCP4725 also features a power-down mode, which will disable the output voltage and instead connect the output to ground through a pre-set resistance. The available resistances are 1kOhm, 100kOhm and 500kOhm (see datasheet). To power up the device, simple write a voltage to it.

To power down the device:

RESISTANCE = 100	# Set power down resistance to 100kOhm

# Power down the device
MCP4725.write(power_down=RESISTANCE)

# Power down device, and retain powered-down state through power cycles:
MCP4725.write(write_to_EEPROM=True, power_down=RESISTANCE)

Note that in both cases, a voltage value can be sent, but will have no effect given that the device will be powered down.

To-do:

• Add module to PyPi
• Add logging to the library
• Add exception catching and set function to return a success flag
• Document I2C interface details