micropython-mcp4725

A micropython driver for the MCP4725 I²C DAC.

The MCP4725 is a digital to analog converter chip with a 12-Bit resolution on the output. The MCP4725 works with a supply voltage from 3.3V to 5V. The output range runs from 0V up to the supply voltage.

The MCP4725 can be configured to use one of two different addresses (0x62,0x63) on the I²C bus. That way it is possible to use 2 MCP4725 on any I²C bus. The device supports standard (100kbps) and fast (400kbps) and hi-speed (3.4Mbps) bus speeds. But hi-speed seems not to be supported by any of the micropython hardware boards. The fast baudrate of 400kbps works fine with the WiPy but compared with DACs that are connected to a SPI-bus updates to the output voltage on a MCP4725 are still pretty slow.

The MCP4725 supports 3 different power-down modes where the output voltage driver shuts down and the device goes to sleep to save energy. The cips wakes up from power-down mode whenever a output voltage update is send to the device.

The MCP4725 also has a small eeprom where the power-down mode and the initial output voltage can be configured that are to be used when the MCP4725 is powered up.

##Using the MCP4725 in your micropython project You need only a few lines of code to add a MCP4725 to your project. (see Issue 1 if you don't like libraries)

###Create and initialze the device on the I²C bus of your micropython board A micropython driver for an I²C device expects you to create and initialze a machine.I2C instance and pass that to the constructor of the driver code.

The arguments used in the code example below work for a WiPy board. If you try this with a different board please check the pins to use for the I²C bus.

from machine import I2C
import mcp4725

#create a I2C bus
i2c=I2C(0,I2C.MASTER,baudrate=400000,pins=('GP15','GP14')) 

#create the MCP4725 driver
dac=mcp4725.MCP4725(i2c,mcp4725.BUS_ADDRESS[0])

###Update the output on the MCP4725 The simple way to update the output on the DAC is to write a new value to the device

dac.write(1200)

The actual voltage on the output depends on the supply voltage the powers the MCP4725. If it runs on 3.3V the above command would drive the output to (3300/4096)*1200 = 996mV if the DAC is powered with 5V the output will be 1464mV. If the argument to the write(value) is negative the output will be set to 0V. If the value argument is bigger than 4095 the output will be set to the maximum output voltage of the DAC.

On power-up the MCP4725 will be initialized with the value read from the internal eeprom of the device.

###Configure the MCP4725 The power-down mode (see Datasheet) and the output value of the DAC can be configured for the active session and for future sessions by saving the settings in an internal eeprom of the DAC.

Power-down modes are selected by the keys in the POWER_DOWN_MODE dict.

POWER_DOWN_MODE = {'Off':0, '1k':1, '100k':2, '500k':3}

#configure the DAC to go into power-down mode and set the output value to maximum output.
dac.config('100k',4096)

After running the command the output value will is set to its maximum, but since the DAC is in power-down mode this will not be measurable on the output pin.

#configure the DAC to output ``Vdd/2`` on power-up or after a reset
dac.config('Off',2048, eeprom=True)

This configuration will be saved in the eeprom of the DAC and will be used everytime the DAC is powered up or reset.

###Read settings and output voltage of the MCP4725 The MCP4725 support a single read command that returns the current configuration as well as the configuration stored in the eeprom of the device.

>>>result=dac.read()
>>>print(result)
>>>(False,'Off',300,'1k',200)

The method returns a tuple with 5 items.

1. The busy-flag of the eeprom on the dac. If True the DAC is busy writing the values for power-down mode and the startup output value to its internal eeprom. If False the DAC is ready for a new config setting.
2. The current power-down configuration of the DAC. Returns a string with the active setting. (see Datasheet)
3. The current outout value
4. The power-down configuration stored in the eeprom. This setting takes effect when the DAC is reset or powered up.
5. The output value configuration stored in the eeprom. This setting takes effect when the DAC is reset or powered up.

The result shown in the code example will read as

• Write to eeprom of the chip has finshed
• The power-down mode is 'Off', the DACs output on.
• The output is set to 300/4096 of the supply voltage of the DAC.
• On powerup or after a reset the DAC will go into power-down mode '1k' (see Datasheet).
• The output voltage on startup or after a reset will be 200/4096 of the supply voltage.

##Example session on the REPL

MicroPython v1.8.3-80-g1f61fe0 on 2016-08-31; WiPy with CC3200 
Type "help()" for more information.
>>> from machine import I2C                                                                                                                                     
>>> i2c=I2C(0,I2C.MASTER,baudrate=400000,pins=('GP15','GP14'))
>>> from mcp4725 import MCP4725, BUS_ADDRESS
>>> dac=MCP4725(i2c,BUS_ADDRESS[0])
>>> dac.read()
(False, 'Off', 2048, 'Off',100)
>>> dac.write(3000)
True
>>> dac.config(power_down='Off',value=0,eeprom=True)
True                                                                                                                                                           
>>> dac.read()
(False, 'Off', 0, 'Off', 0)
>>> dac.write(2000)
True                                                                                                                                                           
>>> dac.read()
(False, 'Off', 2000, 'Off', 0)
>>>