I'm a novice programmer trying to develop a racing ignition system but I'm somewhat hooped at my first hurdle, measuring the period. I do this on my spare time which is somewhat removed from my regular work. I'm writing the code in Dunfield C with some assembler routines for the interrupt handling. The problem is the period, for the most part is no way near the frequency I'm simulating. I've also made a Shmidtt trigger out of an op amp with ~1.5v hysteresis which didn't help. I'm using a P89C51RC2HBP @ 18.432MHz. Essentially the formula for frequency is 1/[(1/(18.432MHz/6)) * (starting Capture value - ending Capture value)].I do some other stuff as the Capture registers can't be reset to zero. I'm at a loss at this point and I don't want to implement this in the PCA counters. Could someone review my code to see what I'm missing?