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()

stm32f1xx-hal/src/adc.rs

Lines 558 to 568 in 89600ec

	impl<PINS> TransferPayload for AdcDma<PINS, Continuous> {
	fn start(&mut self) {
	self.channel.start();
	self.payload.adc.rb.cr2.modify(|_, w| w.cont().set_bit());
	self.payload.adc.rb.cr2.modify(|_, w| w.adon().set_bit());
	}
	fn stop(&mut self) {
	self.channel.stop();
	self.payload.adc.rb.cr2.modify(|_, w| w.cont().clear_bit());
	}
	}

Scan conversion: not setting cont() bit on start() clear cont() explicitly on initialization. It is definitely not intended for continuous conversion.

stm32f1xx-hal/src/adc.rs

Lines 608 to 617 in 89600ec

	self.rb.cr2.modify(|_, w| {
	w.adon()
	.clear_bit()
	.dma()
	.clear_bit()
	.cont()
	.clear_bit()
	.align()
	.bit(self.align.into())
	});

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 :)