Support for ADC multichannel continuous conversion?

[yjh0502]

Hi, I'm trying to run a ADC multichannel (scan mode) continuous conversion. I dig into the code to check current status:

Continuous conversion: set cont() bit on start() https://github.com/stm32-rs/stm32f1xx-hal/blob/89600ec0708cc9cb66c72c1b2c4b6b3a4827c7cc/src/adc.rs#L558-L568

Scan conversion: not setting cont() bit on start() clear cont() explicitly on initialization. It is definitely not intended for continuous conversion. https://github.com/stm32-rs/stm32f1xx-hal/blob/89600ec0708cc9cb66c72c1b2c4b6b3a4827c7cc/src/adc.rs#L608-L617

I think there are several ways to support continuous scan conversion mode:

• I could add quick and dirty fix by introducing ContinuousScan struct (or struct Scan { continuous: bool } if it's okay to break API), but I'm not sure if it's a right way.
• I could think of generalized struct/trait which covers single/multichannel one-shot/continuous conversion scenario. Single-channel conversion could be treated as a special case of multichannel conversion anyway.

[TheZoq2]

I'm not quite sure what you're asking, as far as I remember, the DMA should do continuous conversion, or does it just do multiple conversions on the same channel?

I think breaking the API is perfectly fine, we do that quite often.

[yjh0502]

https://www.st.com/resource/en/application_note/cd00258017-stm32s-adc-modes-and-their-applications-stmicroelectronics.pdf

I mean Scan mode on stm32f1xx-hal API with continuous conversion.

On current API, we should choose between

• single-channel single-shot conversion (OneShot::read API)
• single-channel continuous conversion with DMA (Adc::with_dma, which returns Continuous mode) or,
• multi-channel single-shot conversion with DMA (Adc::with_scan_dma, which returns Scan mode, scanning multiple channels by order, but only once)

We could run multi-channel single-shot conversion with DMA by Adc::with_scan_dma API, but it does not support continuous conversion. For example, with given scan sequence CH0 -> CH1 -> CH2 -> CH3 and continuous mode (with cont() bit), it will run CH0 -> CH1 -> CH2 -> CH3 -> CH0 -> CH1 -> ... indefinitely and continuously.

I'll try to prepare a POC PR :)