On Wed, Apr 14, 2021 at 09:21:31PM +0200, Uwe Kleine-König wrote: > On Wed, Apr 14, 2021 at 02:09:14PM +0200, Clemens Gruber wrote: > > Hi Uwe, > > > > On Tue, Apr 13, 2021 at 09:38:18PM +0200, Uwe Kleine-König wrote: > > > Hello Clemens, > > > > > > On Tue, Apr 13, 2021 at 02:11:38PM +0200, Clemens Gruber wrote: > > > > On Mon, Apr 12, 2021 at 10:10:19PM +0200, Uwe Kleine-König wrote: > > > > > On Mon, Apr 12, 2021 at 06:39:28PM +0200, Clemens Gruber wrote: > > > > > > With your suggested round-down, the example with frequency of 200 Hz > > > > > > would no longer result in 30 but 29 and that contradicts the datasheet. > > > > > > > > > > Well, with PRESCALE = 30 we get a frequency of 196.88 Hz and with > > > > > PRESCALE = 29 we get a frequency of 203.45 Hz. So no matter if you pick > > > > > 29 or 30, you don't get 200 Hz. And which of the two possible values is > > > > > the better one depends on the consumer, no matter what rounding > > > > > algorithm the data sheet suggests. Also note that the math here contains > > > > > surprises you don't expect at first. For example, what PRESCALE value > > > > > would you pick to get 284 Hz? [If my mail was a video, I'd suggest to > > > > > press Space now to pause and let you think first :-)] The data sheet's > > > > > formula suggests: > > > > > > > > > > round(25 MHz / (4096 * 284)) - 1 = 20 > > > > > > > > > > The resulting frequency when picking PRESCALE = 20 is 290.644 Hz (so an > > > > > error of 6.644 Hz). If instead you pick PRESCALE = 21 you get 277.433 Hz > > > > > (error = 6.567 Hz), so 21 is the better choice. > > > > > > > > > > Exercise for the reader: > > > > > What is the correct formula to really determine the PRESCALE value that > > > > > yields the best approximation (i.e. minimizing > > > > > abs(real_freq - target_freq)) for a given target_freq? > > > > > > I wonder if you tried this. > > > > We could calculate both round-up and round-down and decide which one is > > closer to "real freq" (even though that is not the actual frequency but > > just our backwards-calculated frequency). > > Yeah, the backwards-calculated frequency is the best assumption we > have. > > > But I can't give you a formula with minimized abs(real_freq-target_freq) > > Is it a different round point than 0.5 and maybe relative to f ? > > > > Please enlighten us :-) > > Sorry, I cannot. I spend ~20 min today after lunch with pencil and > paper, but without success. I was aware that it isn't trivial and this > is the main reason I established round-down as default for new drivers > instead of round-nearest. Oh, I thought you already solved it. I tried too for a while but was unsuccessful. Not trivial indeed! But regarding you establishing round-down: Wouldn't it be even better if the driver did what I suggested above, namely calculate backwards from both the rounded-up as well as the rounded-down prescale value and then write the one with the smallest abs(f_target - f_real) to the register? Clemens
