Adding a multiplexer on SAADC Example - nRF52840 SDK16

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

haakonsh — Tue, 21 Apr 2020 13:36:23 GMT

Yeah, I had to edit that post 6 times to make as "easy" to read as it is now. It's a bit abstract, but it is a good solution, especially at higher sample rates. Even at 100sps you will get a more reliable adc.

The RTC will have a lot of lower current consumption than a TIMER, but it has a minimum time resolution of 30.51µs. If you can use a multiple of 30.51µs then the RTC is the best option.

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

Nikosant03 — Tue, 21 Apr 2020 13:17:57 GMT

Woo, too much information but it looks like a robust solution... I will try it and return with feedback.

Also, I need a TIMER or RTC to trigger the SAADC's SAMPLE task, isn't it?

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

haakonsh — Tue, 21 Apr 2020 12:49:33 GMT

I just realized that we can control the MUX with a TIMER and 3 GPIOTE channels.

You'll need a TIMER set up as a counter, and have the SAADC's DONE event trigger the TIMER's COUNT task. This will increment the TIMER once for each of the SAADC's DONE events.

The GPIOTE peripheral can toggle the state of a pin with an OUT task. We can then use the TIMER/COUNTER and GPIOTE to iterate through the MUX's truth table without any CPU activity at all.

COMPARE event on COUNT = 1, Toggle ADD0
COMPARE event on COUNT = 2, Toggle ADD0 and ADD1
COMPARE event on COUNT = 3, Toggle ADD0
COMPARE event on COUNT = 4, Toggle ADD0, ADD1, ADD2, and reset the COUNT to 0 by CLEARing the TIMER(this can be achieved through a SHORT where the last COMPARE event is coupled directly to the CLEAR task internally in the TIMER).

This will create a truth table equal to the one in the MUX's datasheet.

The PPI peripheral can fork any EVENT to one other TASK. For the fourth COMPARE event we'll need to set up somthing different to be able have the last COMPARE EVENT trigger the 3 GPIOTE TASKs.
For that we need to use the Event Generator Unit. It's a simple peripheral where TASK[x] will trigger EVENT[x]. We can use EVENTs from the EGU to fork into other TASKs.

F.ex: The TIMER's last COMPARE event triggers ADD0 toggle TASK, and through the forked event, trigger EGU TASK[0]. Then we fork(again) the EGU EVENT[0] to trigger the ADD1 and ADD2 toggle TASKs. This way the last COMPARE EVENT can trigger 3 TASKS.

This will toggle MUX GPIOs at most 187.5ns after the SAADC's DONE event, with the MUX's switching time of 24ns you'll use at most ~210ns to switch the MUX, and you can therefore most likely, run the SAADC at 200ksps without issues.

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

haakonsh — Tue, 21 Apr 2020 10:50:44 GMT

nrf_saadc_int_enable(NRF_SAADC_INT_DONE);
nrfx_saadc_irq_handler() is found on line 102 in nrfx_saadc.c on nRF5 SDK 16.0.

Inject an if (nrf_saadc_event_check(NRF_SAADC_EVENT_DONE)){} on line 183.
1. Either a TIMER or an RTC, I'd choose an RTC with the RTC driver, f.ex RTC2.
2. Because the SoftDevice will block the SAADC ISR from executing during timing-critical radio events. If you do not get to execute the MUX change before the next SAMPLE task is triggered you've got at least one corrupt sample. Therefore you should call nrfx_saadc_sample() in the RTC drivers callback (or the app_timer callback). Since the SAADC ISR has execution priority over any application callback you're guaranteed to set the proper MUX settings before the SAMPLE task is triggered.

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

Nikosant03 — Tue, 21 Apr 2020 09:47:24 GMT

Thanks for the guidance, I have some questions

1. How do I enable the DONE event interrupt?

2. Where do I find the ISR?

[quote userid="13562" url="~/f/nordic-q-a/60381/adding-a-multiplexer-on-saadc-example---nrf52840-sdk16/245761"]You can then use a TIMER to manually trigger(not PPI) the SAADC's SAMPLE task to get the correct sample rate, and set the buffer to as many samples as you want. [/quote]

3.1 Do you mean the app timer (RTC1) or the timer driver?

3.2 Why should I avoid PPI?

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

haakonsh — Tue, 21 Apr 2020 08:30:15 GMT

You can modify the SAADC driver's ISR to check for the SAADC's DONE event, and then control the MUX's GPIOs accordingly. The DONE event is set after each sample, remember to enable the DONE event interrupt.

-edit:
You can then use a TIMER to manually trigger(not PPI) the SAADC's SAMPLE task to get the correct sample rate, and set the buffer to as many samples as you want.

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

Nikosant03 — Tue, 21 Apr 2020 06:20:11 GMT

Hi haakonsh,

THIS is the datasheet of the MUX. Probably I will work on 1.8V, so the switching is 24ns for tON and 16ns for tOFF

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

haakonsh — Mon, 20 Apr 2020 15:40:43 GMT

How fast can the mux switch?

What are the signals used to switch the 8/16-channel mux?

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

Nikosant03 — Mon, 20 Apr 2020 12:23:24 GMT

Hi haakonsh,

Thanks for your reply. Yes, this could be an option but I would like to use the MUX as I want to have available the rest of the analog inputs for other tasks. Also, in the future is may be required to use a 16-channel MUX, so I have to work around with this solution...

RE: Adding a multiplexer on SAADC Example - nRF52840 SDK16

haakonsh — Mon, 20 Apr 2020 08:24:26 GMT

Why do you need to use an external mux? Can't you use the 8 analog input pins of the nRF52840?