Hi all,

To find the vehicle speed for my project, I was going to tap into the
speedo wires coming from the gearbox, but after getting a wiring diagram
for the vehicle I changed my mind. It is 97 pages of schematics just for
a 4 cylinder car. That also took ages to get hold of, and I couldn't
believe that the manufacturer will no longer give out any info on this
type of thing. I had to get it from a friend of a friend etc. inside the
business with the assurance that I was not to disclose it.

What a drama!

Gone are the days of self maintainance.

Anyways, as simplicity if often the best solution, I decided to use a $5
opto shmitt trigger detector mounted on the inside of the rear wheel. It
"looks" between the 16 or so slots cut into the wheel rim and gives me
the signal I need.

I'm going to test drive the vehicle (which has a digital speedo) and
press a "button" on the controller when the speed is 64KPH. If I
multiply the accummulated speed pulse result by 64 (easy), I get the
speed at 1KPH. From then on, to get any speed in KPH, I just divide the
1KPH speed by the new capture value.

To do this I need to use the capture module in the 16F877. At 1KPH,
rough calculations suggest a value of C350h will end up captured when
TMR1 prescaler is set at 1:8 and capture on every falling edge.

>From what I gather, after a valid edge is detected, The TMR1 contents
are dumped into the capture registers and an interrupt can be generated.
It seems that TMR1 is not reset as a result. Is this true, or do I do
that from the interrupt routine? That seems to be a little bit
inaccurate to me, although it may not matter much. I figured on a max
frequency from the slots of only 278Hz at 110KPH and I'm only interested
in integer results.


--
Best regards

Tony

http://www.picnpoke.com
mailto:sales@picnpoke.com