On Tue, Nov 29, 2011 at 11:46 AM, Yigit Turgut wrote: > However, I can measure very precisely between 200Hz - 60Khz, there is > no accuracy below 200Hz. I had read somewhere about this very case of > low frequency inaccuracy when using Capture module, but can't seem to > find it now. Are you checking to see if your timer overflowed? If your incoming period is larger than timer_max*prescaler_period you will overflow. Assuming you are using a 16 bit counter the maximum value is 65535 and will overflow back to 0. When reading the captured timer value you need to check if the timer has overflowed, either by interrupt or by reading the flag. You an avoid overflows by assigning a larger prescaler to the timer at the cost of resolution. You may be able to get away with storing an overflow count during a timer interrupt service to add back in later when you calculate the frequency. You will be off by some cycles (unless you keep careful track of the instructions used during the ISR / or adjust the timer accordingly) but you'll be helped by the fact the incoming signal is lower frequency. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .