Use slower "fast ticks" (8 Hz instead of 128 Hz) to detect button press and light duration

[alesgenova]

Every time a button is pressed, the light turns on, or a sound is playing, Movement enters a 128 Hz fast tick mode.

The cb_fast_tick() function is called 128 times per second. In this function we do among other things some checks on the time a button is held down or the amount of time left on an alarm playback.

With this PR I propose to lower the frequency of the fast tick mode to 8 Hz (the frequency is adjustable by changing FAST_TICK_FREQUENCY). This way we will reduce the invocation of cb_fast_tick() by over an order of magnitude.

We potentially lose a little bit of accuracy (up to 0.125s at 8 Hz) in the amount of time needed for a button longpress (0.5s +/- 0.125), or the light duration (1s +/- 0.125), but I think this inaccuracies are not noticeable.

Another consequence of this PR is that the 128 Hz mode is no longer reserved by Movement, and any face can now request this tick frequency.

[wryun]

👍 seems like a good thing to me.

[wryun]

I've been using this on my watch without issue.

[matheusmoreira]

Do any of the maintainers have any concerns over an 8 Hz fast tick as opposed to 128 Hz?

The code itself looks good to me. @wryun has tested this on hardware so this PR can be tagged tested-on-hw and he can be credited on the merge commit with a git trailer:

Tested-by: James Haggerty (@wryun) ...

Can also be used to credit reviewers and maintainers who signed off on it:

Reviewed-by: Matheus ...
Reviewed-by: Other reviewers ...
Signed-off-by: Maintainer ...

[matheusmoreira]

This should be surrounded in #ifndef to allow the user to override FAST_TICK_FREQUENCY. I also suggest a prefixing on the symbol to avoid namespace clashes.

[wryun]

As a continuing data point, this has been fine for me for 6 months.

[matheusmoreira]

@voloved Do you think this will impact debouncing in any way?

[voloved]

@voloved Do you think this will impact debouncing in any way?

It would. This would now make the lowest reliable debounce time be a quarter second, which is too high for debounce. Though there are other ways to debounce, such as creating a new timer each time the button is pressed and killing it once the debounce time is over.

Using 8 Hz also causes any watch face that uses 8Hz to also need to be modified to not use 8Hz. Though they can be moved to 16Hz and checked if their subec mod 8 is zero.

What is the benefit of making this change to begin with? Does it save battery? Is the battery savings noticeable on the watch's life?