Adding a multiplexer on SAADC Example - nRF52840 SDK16

[Nikosant03]

Hi everyone,

I was looking at the SAADC example and I need to modify it in order to fit my design. I want to perform analog readings from eight sensors using an eight-channel multiplexer.

I will keep the timer/PPI arrangement and I want after each successful sample event to change MUX's channel in order to read the next sensor. What is the most efficient way of doing this?

I mean, is it possible to fill in the buffer with all sensors' values before saadc_callback(). If yes, how could I switch the MUX channel after each sampling event (I'm looking to sampling at 100Hz)?

One solution I can figure out is to define SAMPLES_IN_BUFFER 1 (to read only one sensor) and change the MUX's channel each time inside the saads_callback()..but.. it sounds like a dodgy business

Any suggestions?

Thanks in advance

Nick

[haakonsh]

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

[Nikosant03]

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...

[haakonsh]

How fast can the mux switch?

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

[Nikosant03]

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

[haakonsh]

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. 

[Nikosant03]

Thanks for the guidance, I have some questions

1. How do I enable the DONE event interrupt?

2. Where do I find the ISR?

haakonsh said:

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. 

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

3.2 Why should I avoid PPI?

[haakonsh]

1. nrf_saadc_int_enable(NRF_SAADC_INT_DONE);
   
   
2. 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.
   
   
3.  
   
   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.