MCPWM output on ESP32

[mcbp223]

Hi, I was looking for a 3-level PWM solution for filterless, and it seems that the MCPWM peripheral in ESP32 had the up-down timer necessary to synthesize the waveforms.

I have attached an implementation tested on ESP32 and ESP32-S3. The disadvantage is that it doesn't have DMA, so it needs an interrupt for every sample, and another issue, the interrupt must be in a cpp file due to a linker error when it's header only.

Some other modifications I've made in library files:

• CodecMP3Helix.h, line 124 changed addNotifyAudioChange(bi) to AudioDecoder::addNotifyAudioChange(bi) to avoid an infinite recursive call (recent bug).
• SnapClient.h line 111 moved the call to setupMDNS() to line 121, after the calls to setServerIP() and setServerPort() otherwise IP obtained by MDNS gets overridden; the issue appeared recently also.
• SnapProcessor.h line 221, added server_ip and server_port to ESP_LOGE(...), to be able to notice discrepancies like the above.

aat-mcpwm.zip

[pschatzmann]

I suggest that you submit a pull request for the Snap Client.
What is the advantage of your PWM implementation compared with PWMAudioOutput ?

Thanks for informing me about the bug in CodecMP3Helix that I have introduce with the implementation of the addNotifyAudioChange method

[pschatzmann]

I just had a quick look at the setupMDNS().
You are right, the logic has not been updated - it should actually set the class variables...

[mcbp223]

I suggest that you submit a pull request for the Snap Client.

It may take me some time to setup the repository, if it's faster for you to rectify the MDNS issue it would be preferable I think.

What is the advantage of your PWM implementation compared with PWMAudioOutput ?

I have tried it, on ESP32 it's based on the LEDC peripheral, but I wasn't able to get fluid audio with it, especially if connected to Snapclient; possibly it doesn't go up in frequency high enough. However the main characteristics I was looking for was 3-level PWM in bridge mode that is made possible with a up-down counter timer that the MCPWM peripheral has.

Thanks for informing me

Happy to contribute; your libraries are of great value to users, thank you as well.

[pschatzmann]

No worries, the snap client has been corrected already...

[pschatzmann]

Hmm, what is "3-level PWM in bridge mode" exactly and how would you use it ?

[mcbp223]

2-level is normal PWM, with one level at GND and one level at +Vcc. 3-level adds an additional level at -Vcc.

An explanation for example is in Texas Instruments' note SLAA701A - LC Filter Design, section 2.1 explains traditional (AD) PWM modulation, and section 2.2 summarizes this 3-level PWM (BD modulation), that helps reducing EMI and pretty much allows eliminating LC filters on output.