Max number encoder pulses through interrupt change on PorB

Question

I am using a 16F877A pic with 20MHz crystal and a change interruption on portB, pin 6-7 connected to an encoder. I'm using the encoder to calculate the velocity of a wheel and I have a doubt about the maximum ppr that I can use to avoid the program to stop or freeze? Thanks

Answer 1

I watched a student have this problem in a lab next to me. Without interrupt shadow registers, you'll find the maximum quadrature decoding rate probably slower than you want. IIRC under 100000pps

You can measure it easily by running your wheel backwards and forwards with a motor and going faster until the counts for forward and reverse passes no longer line up.

Microchip recommend using the PIC16F18877 in new designs, which has automatic register shadowing on interrupt. All the 18 series PIC have this feature too and it raises the rate significantly to IIRC over 200000pps.

I'm sorry I can't give hard numbers, the exact figures are at an earlier employer.