I2C Clarification

ersheff

Hello.
I am hoping to use this DMCC cape in conjunction with the Bela cape:
https://github.com/Exadler/DualMotorControlCape

According to the schematics, this cape uses the I2C bus on pins P9_19-20:
https://github.com/Exadler/DualMotorControlCape/blob/master/DualMotorCape-v7.pdf

I've confirmed that P9_19 and P9_20 are the clock and data pins respectively for I2C-2 using this spreadsheet:
https://github.com/derekmolloy/boneDeviceTree/blob/master/docs/BeagleboneBlackP9HeaderTable.pdf

My question -
The Bela pin diagram does not give any information for the pins of either I2C bus (pins P9_17-20). I realize that the Bela cape has its own dedicated I2C pins separate from the regular pin headers. Do this pins mirror either of the other I2C buses?
I've monitored pins P9_19-20 on a scope while running audio on Bela, and there did not seem to be any activity. Is it safe to use the DMCC cape on top of Bela?

Thanks!

giuliomoro

The Bela cape diagram only shows the pins that a user would want to connect to, not the full list of pins in use by the cape. This list is available here (Yes, I know, it should be in the wiki instead). So it uses pin 24 on header P8 and pins 10-15-17-18-19-20-21-22-23-24-25-26-28-29-30-31 on header P9.

P9_19-20 are in use by the SPI bus which handles the I/O for Bela's analog channels. As of a very recent patch you can safely use SPI for something else if you disable analog I/O. Note that currently this also disables the digital I/O, but with a further patch there should be a way to keep the digital channels running.

So if your application only requires the audio channel and does not require the analog (and - for now - digital) I/O, then you should be able to run the DMCC cape alongside the Bela cape, provided that there are no conflicts on the other pins (I did not check that for you).
For your reference, the Bela diagram is available online here

ersheff

We would rather use the I2C2 port instead so that we can keep using your nice custom-designed ADC and DAC. I think this is possible if we re-route those pin connections.

However, it looks like IC2C is currently connected to the Bela’s TLV320AIC3104 by default. If we just disconnect the BBB’s I2C2 pins from the TLV320AIC3104 and instead connect them to the motor driver we’d like to use, will the bela timing and software platform still work? (In other words, we just wanted to check if the other stuff that Bela needs is slaved to clock of the TLV320AIC3104 or anything like that, because in that case I guess it would be a really bad idea to disconnect the TLV320AIC3104.)

By the way, how is it working in the Capacitive touch sensing with MPR121 demo? I guess from the schematic it is sharing I2C1 (via J2) with the custom-designed Bela ADC and DAC, but somehow that makes me nervous at high clockspeeds.

giuliomoro

The audio clock is taken from the TLV320AIC3104, so disconnecting it would not allow the analogs to run, either. I guess this could be changed in the PRU code, so that it sleeps for a set amount of time instead, but I do not think we are going to add this feature ourselves, as it would make maintenance difficult later down the line.

I2C2 is used by the TLV320AIC3104 while I2C1 is made available on the extra header.
In fact, the I2C is used for the TLV320AIC3104 only at initialization time (that is: every time you start a program), but you should be able to use another device on the same bus as long as a) you do not use it at the same time and b) it has a different I2C address.

The MPR121 demo connects the MPR to I2C1 over the header.

Also note that Bela's "analog" ADC and DAC are served over the SPI bus, not I2C.