Messages in this thread | | | Subject | Re: [PATCH 0/3] clk: divider: three exactness fixes (and a rant) | From | Philipp Zabel <> | Date | Fri, 13 Mar 2015 09:13:52 +0100 |
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Am Freitag, den 13.03.2015, 08:50 +0100 schrieb Uwe Kleine-König: > On Thu, Mar 12, 2015 at 09:57:53AM +0100, Philipp Zabel wrote: > > Am Mittwoch, den 11.03.2015, 18:21 -0700 schrieb Stephen Boyd: > > [...] > > > Why does Philipp like 110Hz the most? Where is the desire for that rate > > > coming from? > > > > > > > And the lower > > > > abs(1 / 110 - 1 / r) the better. > > > > > > Similarly, where is this requirement coming from? Some datasheet? Or is > > > it just some arbitrary decision we've made that may not hold true for > > > all consumers? > It's not comming from a datasheet. But that's what I guess is the right > metric for quite some cases. E.g. an UART sample rate and I also > wouldn't be surprised if Philipp's panel example would call for this > metric, too. > > For an UART running with say 38400 Bd you want to sample with a freqency > of 38400 Hz (not considering oversampling, but that is only a factor > that doesn't makes my reasoning wrong). If you now consider 38401 Hz and > 38399 Hz the respective deltas are 1 Hz. But if you look at the time > between two samples we have: > > 38401 Hz -> 26.04098852 us -> delta: 0.6781507 ns > 38400 Hz -> 26.04166667 us > 38399 Hz -> 26.04234485 us -> delta: 0.6781861 ns > > So with 38401 it takes a little longer until the slightly deviating rate > results in sampling the wrong bit. > > > In this use case, the driver doesn't want the pixel clock to stay below > > a hard frequency limit, but to get as close as possible to the target > > frequency, either above or below, so the relative error to the nominal > > panel refresh rate stays as small as possible. Thus for a fictional > > target rate of 110 Hz, I'd like to minimize abs((round_rate / 110) - 1). > Note that minimizing > > abs((round_rate / 110) - 1) > > is equivalent to minimizing > > abs(round_rate - 110)
Of course, and you're right, I should want to minimize the delta of the interval time, not of the rate so that if playing back a video stream at exactly the nominal frequency, it takes as long as possible until I have to drop or duplicate a frame to stay in sync.
regards Philipp
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