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Project Notes

#261 DigitalPots/MCP4017

Control the MCP4017 single 7-Bit I²C digital potentiometer with an Arduino.



I’m trying out some digital potentiometers for an application. This is a test of the MCP4017T-104E/LT (100kΩ rheostat version).

The MCP4017 acts as a rheostat - it sets the resistance between the Wiper and B pins, independent of VDD/VSS (i.e. the potentioneter is not grounded or tied to power).

Conclusion? Probably not the chip I’d choose for the job in mind. Key observations:

  • does not support hardware I²C address bits (and so far I think there’s no way set the address over I²C)
  • the 2.5mA current rating means it can probably only be used connected to a high impedance load, else it requires some form of buffer circuit.
  • SC70 - dang that’s a small package! Managed to hand solder to a 2.54mm DIP adapter without too much trouble (with the aid of a magnifying glass)

I²C Protocol Support

The MCP4017/18/19 come with a fixed bus address 0b0101111. So only one can be controlled on the same bus at the same time.

It has a very basic read/write I²C implementation, with no commands defined. Just read and write the wiper value.

Wiper Setting Properties
07Fh Full Scale (W = A)
07Eh - 040h W = N
03Fh W = N (Mid Scale)
03Eh - 001h W = N
000h Zero Scale (W = B)

Test Script and Performance

Measuring the actual resistance I see a full scale of 0Ω - 81.3kΩ (for the ‘100kΩ’’ chip). That is within spec - note the resistance rating of the chip is ±20%.

The MCP4017.ino sketch is a simple exercise of the chip. It steps from 0Ω to full scale with a 2µs delay between steps.

Note the trace looks logarithmic, but this is expected for a linear change in resistance, as I am measuring the resistance in a voltage divider - x/(x+10kΩ). The voltage divider is in the test circuit to avoid pumping more than the rated ±2.5 mA through the chip.






Credits and References

About LEAP#261 ArduinoDigital PotMCP4017
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