We are working on a custom board solution and are investigating non-working CEC.
The root cause appears to be incorrect timing of bit transmission - confirmed by both an HDMI analyzer and scope - basically bits are transitioning "too fast" and very similar to issue reported here. Start and bit transitions are approximately 60% of nominal (e.g., +/-2.4ms actual versus +/-3.6ms nominal)
Our board does not populate the external 32KHz source and is therefore using the internal oscillator.
Enabling RTC and examining counter values we can see that the internal oscillator is "running fast" with the same variance as CEC bit timings.
We have some parts on order to see if external 32KHz oscillator cleans things up but was wondering:
- Has anyone else observed CEC issues with internal oscillator?
- Is there any mechanism (I can find no reference) to trim the internal oscillator for a quick test?
- Could anything else be causing this timing issue?
- The HDMI core is driving the line and no external controls.
Thanks in advance - will post our findings back to this thread.