#259 DigitalPots/M62429
Control the M62429 dual channel volume control with an Arduino.
Notes
I’m trying out some digital potentiometers for an application. This is a test of the M62429.
The M62429 is a dual channel electronic volume control:
- dual channel, with independant channel control
- Volume 0 to -83dB (1dB/step)
- Vcc = 4.5 to 5.5V
- controlled with a simple serial data protocol
The chip is primarily designed for audio applications as:
- volume levels are set on a logarithmic scale (dB)
- it centres the output around VCC/2
It does work with both AC and DC signals however.
The programming interface is a very simple two-wire (data, clock) serial protocol. But being so simple (and having a strange command length of 11 bits) it’s not really possible to use either the standard Wire or SPI libraries. Time for some bit-banging!
Protocol
- Reads data signal at the rising edge of clock signal
- Reads latch signal at the falling edge of clock
- Clock cycle time: min 4µs
- Clock pulse width: min 1.6µs
- Clock rise/fall time: max 0.4µs
- Data setup time: min 0.8µs
- Data hold time: min 0.8µs
Level Encoding
| D6 | D5 | D4 | D3 | D2 | D8 | D7 | Level |
|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | x | x | -∞dB |
| 0 | 0 | 0 | 0 | 1 | 0 | 0 | -83dB |
| 0 | 0 | 0 | 0 | 1 | 0 | 1 | -82dB |
| 0 | 0 | 0 | 0 | 1 | 1 | 0 | -81dB |
| 0 | 0 | 0 | 0 | 1 | 1 | 1 | -80dB |
| 0 | 0 | 0 | 1 | 0 | 0 | 0 | -79dB |
| . | . | . | . | . | . | . | .. |
| 1 | 0 | 1 | 0 | 0 | 1 | 1 | -4dB |
| 1 | 0 | 1 | 0 | 1 | 0 | 0 | -3dB |
| 1 | 0 | 1 | 0 | 1 | 0 | 1 | -2dB |
| 1 | 0 | 1 | 0 | 1 | 1 | 0 | -1dB |
| 1 | 0 | 1 | 0 | 1 | 1 | 1 | 0dB |
Performance
The M62429.ino sketch is a simple exercise of the chip.
- input for both channels is AC-coupled 1kHz sine wave
- left channel is set to max volume
- right channel volume is stepped up through all gradations
Here is a capture of two frames of clock/data output from the Arduino, captured with a logic analyzer:
The first sequence reads: 11011101011 + latch (data high on falling clock). That breaks down as:
112CH, channel at a time01110ATT1: -28dB10ATT2: -2dB11end bits- i.e. set channel 2 to -30dB
The first sequence reads: 11011100111 + latc:
112CH, channel at a time01110ATT1: -28dB01ATT2: -1dB11end bits- i.e. set channel 2 to -29dB
Yes, that’s as expected - the script is sequencing through level settings on channel 2.
The scope trace tells the story of what is happening on the left and right channel outputs:
- CH1 (yellow) is DC-coupled left channel
- CH2 (blue) is the AC-coupled right-channel
Next test was with a 5V DC signal on left and right channel input.
- CH1 (yellow) is DC-coupled left channel
- CH2 (blue) is the DC-coupled right-channel, offset by -2.5V
Construction
Credits and References
- M62429FP info & datasheet - parts.io
- AD5171 Digital Potentiometer - tutorial
- krupski/M62429 - M62429 support encapsulated in a library
- ..as mentioned on my blog