On Thu, Apr 22, 2021 at 8:02 AM Sebastian Huber <sebastian.hu...@embedded-brains.de> wrote: > > On 22/04/2021 14:17, Robin Müller wrote: > > > It worked for me. Can you specify that? The overflow check would have > > to be implemented by an upper layer, right? > > I can adapt my former patch using the tick solution to do the > > multiplication first if this is better. > If you call HAL_GetTick() when the clock tick is 0xffffabcd and then > 0x123 and you calculate the difference of the results, the value should > be all right. I am not sure if this is the case if you calculate it by > rtems_clock_get_ticks_since_boot() * 1000) / > rtems_clock_get_ticks_per_second() > It will not work properly in case of an overflow in the multiplication. A quick illustration that is (hopefully) correct:
(a*x/y)%n != (a*x % n) / y In case of n a power of 2, the division of the overflow does not divide out the value of 2^n, so you end up with an error of 2^(n-1)/y in your result. Example in 8-bits, with x = 8 and y = 2: start = 7 ticks stop = 9 ticks The right answer should be (9-7)*8/2 = 8 With (a*x)/y: 7*8/2 = 28 9*8/2 = 4 4 - 56 = 232 (or so) With a*(x/y) 7*(8/2) = 28 9*(8/2) = 36 36 - 28 = 8 (hooray) when a*(x/y) overflows this will also work as usual for wrapped overflow arithmetic, example: start = 63 stop = 65 expected (65-63)*8/2 = 8 63*(8/2) = 252 65*(8/2) = 4 4 - 252 = 8 (hooray) Do the division first please. :) > -- > embedded brains GmbH > Herr Sebastian HUBER > Dornierstr. 4 > 82178 Puchheim > Germany > email: sebastian.hu...@embedded-brains.de > phone: +49-89-18 94 741 - 16 > fax: +49-89-18 94 741 - 08 > > Registergericht: Amtsgericht München > Registernummer: HRB 157899 > Vertretungsberechtigte Geschäftsführer: Peter Rasmussen, Thomas Dörfler > Unsere Datenschutzerklärung finden Sie hier: > https://embedded-brains.de/datenschutzerklaerung/ > > _______________________________________________ > devel mailing list > devel@rtems.org > http://lists.rtems.org/mailman/listinfo/devel _______________________________________________ devel mailing list devel@rtems.org http://lists.rtems.org/mailman/listinfo/devel
