Step detection circuit and the non-linearity of accelerometer output

so here's a more standard circuit diagram for the step detection circuit. of course it won't work without some code for the pic chip..



i'm still stuck on the weird output from the accelerometer. to calculate steps per minute, i'm looking at the number of samples that are read between each step that is detected. the idea is that at 60 steps per minute, the pic chip would read a number of samples from the accelerometer at the sample rate (say, 100hz). then, at 120 steps per minute, the pic chip would be reading about half as many samples between each step (if the sample rate is 100hz, for 120 steps per minute we'd be seeing 50 samples between each step). problem is, i mapped out BPM versus the number of samples that were read from the accelerometer, and it's not a linear curve. that is, at 60 BPM i get a consistent 110 samples read, but at 120 BPM i get around 45. huh?! it's also surprisingly a consistent curve.

need to talk to tom on this one..