Reading 3 Encoders using PPI, Timers, and GPIOTE with NRFX

RE: Reading 3 Encoders using PPI, Timers, and GPIOTE with NRFX

czuvich — Mon, 03 May 2021 23:20:13 GMT

That did the trick! Thank you!

RE: Reading 3 Encoders using PPI, Timers, and GPIOTE with NRFX

Einar Thorsrud — Mon, 03 May 2021 18:49:39 GMT

Hi,

1-4 is good.

Regarding 5, you are right that you do not need PPI. you need to trigger the capture task from SW in order to copy the counter value to a capture register so that it can be read and read it. Both these tasks are integrated in the nrfx_timer_capture() function. Regarding clearing the counter you can do that by calling nrfx_timer_clear(). However, as this does not happen at the same time as you call nrfx_timer_capture() you risk counting one more between reading out the counter and resetting it (the counting happens in HW using PPI, so critical sections are no help here). That means that unless that is not a problem or you know this will not happen, then it would probably be better to never reset the counter but instead remember the previous counter value as well, so that you can calculate the difference between each time you capture and read the value. Note that then you also need to handle when the counter overflows and wraps around.

RE: Reading 3 Encoders using PPI, Timers, and GPIOTE with NRFX

czuvich — Mon, 03 May 2021 13:26:54 GMT

Thanks for the reply! So, is my workflow correct?

1) Setup 3 separate timer instances used for counting

2) Setup a single app_timer that fires every 250ms

3) Enable GPIOTE for the pin receiving the pulse

4) Use nrfx_ppi_channel_assign to perform a NRF_TIMER_TASK_COUNT for the GPIOTE.

5) ??? How do I capture and reset the COUNTER in the app_timer interrupt? I don't need PPI for that right?

RE: Reading 3 Encoders using PPI, Timers, and GPIOTE with NRFX

Einar Thorsrud — Fri, 30 Apr 2021 11:58:43 GMT

Hi,

[quote user=""]1) Could I just use a single timer instance to capture all 3 counts separately (maybe using a different channel?)[/quote]

No, the TIMER peripherals have several capture/compare registers, but only a single counter. So you will need to use 3 timers for this. (See Figure 1: Block schematic for timer/counter for an overview).

[quote user=""]2) Could I use the app_timer to generate the software interrupt to read the hardware counters (for instance, app_timer would run every 250ms).[/quote]

Yes, the app_timer is perfect for such use cases.