Hello everyone,
This is my first post and first crack at a PIC circuit (or any circuit for
that
matter). I apologise for the length but i would like to explain what im
trying
to achieve.

I basically want to build a device that can display some data from the engine
sensors on an LCD screen and also transmit the data out using an RS232 port.

I have already got it to display and log various voltages (using a PIC16F84
and
2 input 8 bit a-d converter) from some sensors such as Throttle Position
Sensor, air flow meter, and battery voltage (using a simple voltage divider
circuit).

Now i also want to add the functionality to display injector duty cycle, rpm,
speed,  and also have extra analog ports for other sensors in the future,
such
as a MAP sensor. Some digital inputs would be nice, and can easily be added
using unused input pins. All analog inputs will be between 0 and 5v.

I was thinking of creating the circuit with the following components.

* One PIC (F84) to display and log the analog voltages using a 8 input 8 bit
a/d converter wired in 'microwire' configuration (one i/o used between pic
and
a-d coverter)..

* 2 LM2917 freq-voltage converters to read in the speed and rpm (sending
output
to a/d converter, PIC will convert output voltage to meaningful data)

* and another PIC which is dedicated to calculating the duty cycle and
sending
the output serially to the first PIC, or even generating a PWM output which
can
be sent to the a/d converter also.


I understand that there are circuits and sample code to do all these things,
but is it possible to do all of these things (8 analog inputs through a/d
converter, bus for LCD screen, rs232 i/o, duty cycle monitor, rpm monitor,
speed monitor) all on the one PIC?

The speed sensor output varies from 4Hz upwards to around 200Hz (if that),
the
injector pulse width would be in the tens of milliseconds, and the tacho
output
would be in hundreds of Hz per second.

I imagine that this PIC would need one interrupt for each the rpm, speed and
injector pulse, and also make use of the in built timers. Either that or do
each input and frequency input in series, which means it would wait for a
pulse
and slow down the display and logger. I think this is a little too much to
ask
of one PIC, since it needs to count 3 different frequency inputs of very
different ranges. Am i wrong in thinking this?

So can it all be done using one PIC or is it better to use dedicated ICs and
let the main PIC process the incoming data and display/log it?

Are there any other recommendations to make it cheaper to build or more
accurate?

The main use for the device is to log data for tuning, diagnostics, and maybe
for a bit of circuit work.


Thanks for your time reading this.

Daren

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