This is off the top of my head. If you want more details or
it doesn't make sense email me. I'll post this reply to the
list as it may be of general interest.
There is a method which may be simpler than building a
frequency counter per se with a PIC. Briefly, don't measure
what the motor is doing - instead, tell the user  what YOU
are MAKING the motor do.

Driving a stepper motor directly from a PIC is easy. You
simply output each phase pattern in turn. For most steppers
(eg unipolar units with 2 centre tapped coils) you need 4
PIC pins and there are 4 phase sequences for full stepping
and 8 for half stepping. There are a number of circuits
around for driving bipolar steppers and unipolar steppers
are able to be driven with simple buffers directly by the
PIC.
Typical small 12 volt steppers draw around 150ma so you can
drive them with simple octal buffer ics like the ULN2803.

With 2 more pins you input up/down speed control directly to
the PIC - you vary the delay between phase changes to vary
speed. Now you don't have to MEASURE frequency of rotation
as you are generating the frequency yourself - you simply
have to convert the delay time to a frequency. This can be
by division (f = 1/t) or by lookup table. If you are using
the timer for phase change timing you will have ample time
for calculations between changes but this task could be the
main task with phase changes interrupt driven as the speed
will not vary rapidly compared to phase changes. (At 2000
RPM a 1.8 degree stepper has a new phase change every 150
microseconds). Display could be via multiplexed LEDS or any
of a number of LCD displays - there have been several
discussions on driving LCDs on the list in the recent past.

Total device is now:    PIC, driver IC, Stepper motor, LCD
display & a few buttons.
Jameco have suitable stepper motors for $5-$10 US.

    regards    Russell McMahon

-----Original Message-----
From: wayne galaugher <wayneg@ICAN.NET>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Thursday, 5 March 1998 16:26
Subject: Optical Tachometer

Most comments snipped.


>I have designed an optical tachometer using a stepper motor
with a
>slotted wheel through which one can look and stobe rotating
objects by
>adjusting the rpm of the motor.

>I need to design a 4 digit 7
>segment LED  or LCD display to read the stepper motor
pulses at 8 Hz.
>We are only measuring RPM to 2000 RPM so the frequency is
low.

>Also, it would be nice to pulse the stepper motor chip with
the PIC.