UART, ADC, BTLE, Timer 1msec

[tommy.lane]

Currently working on an app which will utilize the ADC component of an NRF chip.

The app will use the full bluetooth stack SF132;6.0.0, adc, and will need to have ability to send uart.

First: I need to initialize the ADC for multiple channels and to not be triggered by a timer, but by the app.

Second: BLE will always be advertising/listening for a host device.

Third: UART will stream a small data packet ~10 Bytes, every second or so.

What do I need to do so that I can call the ADC read for all channels?

What would be the best way to get a 1msec tick, not a SysTick as I will need it to work even when the device is asleep.

How should the UART be initialized so that it will run even when the softdevice is handling btle packets?

Thanks.

[Einar Thorsrud]

Hi, 

The SAADC driver documentation describes how you can trigger ADC sampling. Note that you have to trigger it via PPI if you want to sample all channels in quick succession (continuous mode), but you can still sample each channel only once per time it is triggered (see Scan mode). The most sensible way to trigger sampling every 1 ms is probably using the application timer (See timer library documentation). The SDK does not have any example that shows exactly this, but Jørgen has attached a project (zip file) to this post, which regularly samples two channels to this post. Note that it is for an older SDK and id uses the RTC directly instead of via the timer library, but most of the example should be relevant.

The nRF52 only has a single CPU, so you cannot necessarily always have the UART peripheral running while the SoftDevice is handling a BLE packet. However, the UART peripheral has DMA support, so you may not need to use the CPU that much. Moreover, you should make sure to use hardware flow control if possible so that you do not risk losing data in case the nRF is not ready to receive data when the other UART device wants to transmit.

[tommy.lane]

Thanks for the info!

The nrf chip will not be receiving any UART, just sending. So I believe the DMA will work out well. Thanks for the help. I'll get into it today and see if this addresses my concerns.

[tommy.lane]

Let me clarify what I meant, we are using a library that requires a one MS interrupt. Which is what we refer to as a tick internally (pretty sure that is what it is called everywhere).

While the app timers are certainly useful they do not fit well with the libraries used on this project.

The ADC will be called by the library, and it will probably only run at the fastest of every 50ms.

[Einar Thorsrud]

I didn't quite understand your last comment, but It is probably not important. You can of course generate the tick in any way you like. (The reason I mentioned the app timer library is that it is a user friendly library that lets you reuse a RTC for multiple timers of various sorts. A repeated app timer is essentially a the same as tick - you get a regular event/interrupt at a fixed (average) interval.)

[tommy.lane]

SAADC documentation:

Example for setting up sampling by PPI:

err_code = nrf_drv_saadc_buffer_convert(buffer, length);

//Check error.

uint32_t sample_task = nrf_drv_saadc_sample_task_get();

//Set task end point in PPI using sample_task variable.

What would I use for the event end point?

Does this trigger the SAADC to sample all channels?

Thanks.

[tommy.lane]

Einar Thorsrud said:

(continuous mode)

Einar,

I'm not sure how to setup the PPI to do this.

What would I need to interconnect to sample all registered adc channels in quick succession?

And how would I trigger the event to start this off without using a task specific interrupt.

P.S. I've recently (last two hours) been informed that the ADC sample request could happen as fast as every 100 useconds.