Using a wheel-mouse sensor as a motor speed sensor.

[lsteele]

Is it possible you are trying to read more pulses than the PIC can keep up with?

I think you may be right. Here's a listing of my polling version of the code:

Device 16F876a
Config WDT_OFF,CPD_OFF,CP_OFF, LVP_OFF, BODEN_OFF, HS_OSC
Xtal = 20

HSERIAL_BAUD = 57600							   ' Set baud rate to 9600
HSERIAL_RCSTA = %10010000       ' Enable serial port and continuous receive
HSERIAL_TXSTA = %00100100       ' Enable transmit and asynchronous mode 
HSERIAL_CLEAR = ON		' Enable Error clearing on received characters


dim lastsens1   as byte
dim lastsens2   as byte
dim sens1Tcks   as word
dim sens2Tcks   as word
dim i           as word

'CMCON = 7
TRISB = %11111111
trisc = %00000000
gosub flashled
hserout [$0D, $0A, "Polling speed test"] 
sens1Tcks=0
sens2Tcks=0
lastsens1=0
lastsens2=0
i=0

while 1=1
	if portb.4 <> lastsens1 then
	    inc sens1Tcks
	    lastsens1=portb.4
	endif
	if portb.5 <> lastsens2 then
	   inc sens2Tcks
	   lastsens2=portb.5
	endif
	inc i
	if i = 10000 then
	   hserout [$0D, $0A, #sens1Tcks, "    ", #sens2Tcks] 
	   sens1Tcks=0
	   sens2Tcks=0
	   i=0
	endif 
wend

flashled:

	dim lp
	lp=0
	for lp = 0 to 10 step 1
		portc.5=1
		delayms 30
		portc.5=0
		delayms 30
	next
	return

I ran this code for 4s and counted 69 outputs on my terminal program. So I reckon that works out

10000 x 69 /4 cycles per second = 172500 cycles per second

The motor specs say the output does 221 rpm at 7.2V through a 75:1 gearbox. I'm giving it 4.8V right now, so that's

221x75x4.8/7.2 rpm = 11050rpm (motor shaft) *MADE A MISTAKE HERE - SEE POST BELOW*

~40 spokes on encoder wheel. Two ‘transitions’ per spoke, so 80 changes in state per revolution. So

80 x 11050 = 884000 events per second (per sensor)!

Which if I've done my sums right (admittedly, a big assumption) is considerably more that I can measure.

Yes we want to see everything. Code, schematic etc etc etc

I'm going to draw up a schematic of the hardware setup and post it shortly.