Using Stereo Soundcard with the Compute Module, I2C Problems

[oweno]

I've been noticing a strange problem trying to use the Stereo Soundcard (v1.5) with the Raspberry Pi Compute Module 3.

The card does not configure properly and I tracked it down to problems on the I2C bus. Using a logic analyzer, I can see many NAK transmissions on the bus. The WM8731 is not responding with ACK, and this is causing configuration to fail. About 20% of transmissions end in NAK.

I double checked all connections and tried different pull up values. Lowering the value helps, e.g. from 4k7 to 1k, but either of those should be acceptable values. and even with the 1k, there are still NAKs. The signals look good on the scope, right levels, sharp transitions.

I thought it was an issue with the compute module itself, perhaps a pin config issue, but I was also able to reproduce the problem using a custom WM8731 board and a PI 3B. (I also posted about this here https://www.raspberrypi.org/forums/view ... 8&t=222602)

I'm wondering if anyone here has noticed this behavior?