I have a couple of questions about frequency measurement in Kinetis MCUs (K22).
First of all, I would like to measure some fast signal (a clock) with a frequency of 1/10th of core clock( let's say 48MHz of core clock, 5 mhz max of input frequency). According to the datasheet, my max input frequency shall be core clock /4.
Is there a way to calculate what will my accuracy be? Ok, I suppose it will depend from the "architecture" I am going to use to perform measurements. And so, here comes the first "true" question.
What's the best approach to measure a fast signal like this one?
Three possibile approaches:
- Using FTM on CHn as input capture and store X values (let's say 64) of FTM_CnV. Then I will average the mean tick between the edges of the signal and computate the input frequency. Obviously, this is pretty un-accurate: +- 1 clock cycle on a 48MHZ FTM0 means that the measured frequency will vary from 5.33MHz to 4.79Hz. Not very good for my sake.
- Using FTM on CHn and count the number of edges in a bigger period. For example, FTM0 as input capture, FTM1 as reference. When a TOF IRQ is triggered on FTM1 I can estimate the frequency doing a simple divison: Freq = (NumberOfEdges / MeasurementTime).
- Anyway, for a fast clock measurement in my sysyem this method seems to have a serious problem: nested IRQs. So I switched to FTM1 to FTM0 and check for the asserted flag into IRQ.
- Using FTM0 as an external clocked timer, set as upcounter and FTM1 as reference for a bigger period. BUT I can set up prescalers on FTM0 in order to slowdown the clock and prevent the nested IRQs trouble.