sys/ztimer: make use of periph_timer_query_freqs #20582
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Add an API to search for the frequency supported by a timer that is closest to the given target frequency. This is in fact non-trivial to get right, as pre-scaler registers can be 16 bit or even 32 bit in size, making a naive loop over all possible pre-scalers too expensive (computationally).
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Type: new feature
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This makes use of the `periph_timer_query_freqs` feature: 1. It does choose the closest frequency supported before calling timer_init() in the ztimer_periph_timer backend. 2. It does make use of the actually chosen frequency when using `ztimer_convert_frac`. 3. It does `assert()` the frequency is within 5% of the specified when no frequency conversion is performed or `ztimer_convert_shift_up` is used.
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Contribution description
This makes use of the
periph_timer_query_freqsfeature:ztimer_convert_frac.assert()the frequency is within 5% of the specified when no frequency conversion is performed orztimer_convert_shift_upis used.Motivation
We have a number of MCUs that can run from internal oscillators (without using external crystal for accuracy):
periph_timer, so not relevant here)If those MCUs are used without external crystal, the actual CPU frequency may be off from what the the board config aims for significantly due to large production variance and temperature and supply voltage dependence. As a result, all
periph_timerinstances driven by the same clock will also be off significantly, which can hinder accuracy in time keeping significantly.This adds the support to ztimer to react on the actual frequency information provided by
periph_timer_query_freqsand adjust the time keeping accordingly.Testing procedure
As of now, only the MSP430 (and the FE310) will measure the actual CPU frequency at runtime, and only the MSP430 timer will compensate frequencies exposed via
timer_query_freqs(). So only on MSP430 this can really be tested now, e.g. withtests/sys/ztimer_msg.In
masterthis test sadly fails because oftimer_set()not yet working correctly on small timeouts (and waiting one period longer than needed).With this PR the test passes and the messages at second 30 is indeed about thirty seconds after the start message received. (It would be off by 40 % without compensation.)
Issues/PRs references
timer_get_closest_freq()#20581Fixes #8251