MCP4728

Raspberry Pi library for the MCP4728 Quad DAC chip. It allows

• reading/writing the address by bitbanging at three arbitrary GPIO pins,
• changing the output voltages by kernel procedures on both i2c-0 and i2c-1 busses.

Reading and writing address for the MCP4728 chip requires an additional LDAC line and is not compliant with the I2C protocol, so bitbanging must be used. Do not forget to provide a pull-up resistor for the LDAC line!

The core of the project is an OOP (object oriented programming) compatible C library. There are two reasons for this step:

• There is no need for any other library (RPi.GPIO, pigpio...) to execute the code;
• The bitbanging part of the library is much faster and time consistent.

To create C library execute make (library in local directory) or make install (shared library).

Version 2 also offers additional improvements:

• A more precise 10kHz timing;
• The SDA line has the 7% clock period delay, mimicking Raspberry Pi's I2C kernel performance.

Running C programs is simply not as efficient as using kernel procedures, and it is still possible for the Raspberry Pi to be distracted by other operations. If less than 7% of a total of 50% of the clock period remains until the next change, the timing is reset to ensure at least 7% of the clock period.

The output voltages are changed by kernel procedures.

On this page you will find the application circuits with some additional instructions.

The library provides eight public functions: two functions for initialisation/de-initialisation, two functions for reading/writing the address and four functions for changing the output voltages. The description of the functions is given in the Python test file (MCP4728_test.py).

A simple Python script is provided as an example of how to use the library. A proper Python library could easily be created to wrap all procedures in a standard Python class. Since I assume that only advanced users will use this library, I think that the sample Python script is sufficient.

When the chip operates in internal source mode, it provides gain=1 for voltages up to 2.048 V and gain=2 for voltages up to 4.096 V or VDD, whichever is smaller. The library automatically selects gain=1 when a voltage up to 2 V is requested and gain=2 when a voltage above 2 V is requested.