Start 2 timers in 1 instructions? (nRF52)

[Cedric Honnet]

Hi all!

TL;DR

Is there a way to start 2 timers in 1 instructions? (or to synchronize them afterward?)

- - -

Background

I need to take 8 timestamps of GPIO edges using PPI events/tasks (more explanations here).

This needs 2 timers as they don't have more than 4 (or 6) capture registers each.

To have coherent timestamps, the timers should start simultaneously, and I'm looking for a good way to do it.

- - -

1st idea

It seems unlikely that a simultaneous start is possible as they have different register addresses, ex:

TIMER1: 0x40009000 

TIMER2: 0x4000A000

But as they are running at 16MHz and the core runs at 64MHz, it might not be impossible that the 2 start request happen simultaneously:

nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_START);
nrf_timer_task_trigger(NRF_TIMER2, NRF_TIMER_TASK_START);

Should I use a PPI event to reset both timers in the same time? (it seems complicated but maybe the most appropriate)

- - -

2nd idea

Maybe there's a reliable way to check that the 2 timers are synchronized, and if not, copy the value of one in the other?

- - -

Does anyone have any suggestion?

Thanks a lot!

Cedric ;)

[Rune Holmgren]

Hi,

The first idea using PPI seems like the best way to do this. PPI supports a feature called "FORK" which enable you to cause a single event to start two timers. This will allow you to start both of the timers at the same time and not end up with a one CPU clock cycle offset.

While PPI may seem a bit complicated it often turns out to be the far simplest solution when you need precise concurrency. (It's also very low power compared to using the CPU)

Best regards,
Rune Holmgren

[Cedric Honnet]

Thanks a lot, Rune!

I've been playing a bit with PPI and the fork feature was actually my next exploration plan.

My current test uses PPI in a fairly simple way:

https://github.com/HiveTracker/ppi_test/blob/master/PPI.cpp#L61-L86

...and it's using Nordic's lib, so I guess I could use the fork wrapper too:

https://github.com/HiveTracker/ppi_test/blob/master/nRF_SDK/nrf_ppi.h#L234-L265

Is there any easy example that you would recommend to understand its use better?

Thanks again ;)

[Rune Holmgren]

In the nRF5 SDK you can find a reasonably simple PPI example in the folder "nRF5_SDK_15.0.0_a53641a/examples/peripheral/ppi". That is a good starting point to see an implementation utilizing PPI if you haven't seen it already. Other than that you can search in the SDK and find other examples using PPI to see "real" applications using it.

Hoping that may be of help.

Best regards,
Rune Holmgren

[Cedric Honnet]

Sorry, PPI is OK, I was asking if you had FORK examples ;)

Thanks again!

[Rune Holmgren]

Sorry, it doesn't seem like we have any examples which use FORK. You will have to make due with the PPI documentation, but as far as I can tell FORK is pretty straightforward so I think you will be fine.

Best regards,
Rune Holmgren