Have you actually simulated 3000 RPM at your desk, with the same duty
cycle/hysterisis as you have monitored on your Honda. I would suggest
that your real problem is with the interrupt on change.

Craig

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Christian Dorner
> Sent: Friday, September 29, 2000 3:09 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: Problem with PIC Project in a Car
>
>
> Hi everyone!
>
> I'm working on a RPM-Logger (tacho) project. It works fine on my
> desk (using
> a computer with small programm to generate the ignition signal) but in my
> car it runs lousy.
>
> First some words about the project:
>
> A PIC 16F84 @ 8MHz.
> The Timer0 is running to messure the time between two ignition signals
> (TMRDiv 1:1).
> The Ignition Signal provided by the ignition-system of the car is 12V with
> falling edge to 0V when it fires. (2 times per revolution) is
> connected via
> a resistor an a zener-diode to RB7 of the pic. I use the RBIE (RB Port
> Change) Interrupt to start and stop (read out) the timer and the addtional
> counters.
> On RA3 a PC will be connected to read out the processed and calculated
> signal from the pic. ("just a wire" - serial communication)
> On a other RA a circut is connected to switch a bigger relay.
>
> Everything works fine on the desk at home. Good readings (RBIF) and the
> transmission to the computer (with Data-Logger SW) works like i want.
> The relay will be switched at the right RPM.
>
> In my car (it's a honda) it seems to work fine until the RPM goes
> up to 3000
> RPM.
> Will the RPM raise about 3000 RPM the result jumps 200 RPM up and down.
> The relay will be switch around 200 RPM below the desired position. (see a
> line befor ...)
> But the serial transmission works still fine!
>
> I've added a routine which will toggle a port when the RBIF occure and use
> my scope to see what happens. It seems like the interrupt will be
> triggered
> at the right time. But the difference must be only some 100 uS. I've no
> possibility to check this out with my old scope.
>
> The supply looks fine. (100 uF, 7805, 10 uF, 100 nF) I can't see any drops
> or noise.
> The input signal looks fine. (i've also added a 100 nF to ground. nothing
> happens)
>
> Now i don't really know what to do?
>
> I've also tried a other method to read out the signal (thank you
> Tony Nixon)
> but the result is the same and this method is to slow for my car. (It can
> rev up to 10.000 RPM)
> I think it can be a problem of the noisy enviroment in the car.
>
> Has anyone an idea to solve this problem?
>
> My next step is to try the LM 2940 Voltage regulator ...
>
> TIA
>
> Christian Dorner
> aka, Doc ...
>
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