Well, it is fact that the PIC can count the low freq, lets say
around 10MHz, and also uses a multiplexor to select 1 of 8
inputs, but I am a little bit confused. I am quite new to PIC,
even that I know other controllers, but reading the PIC 30430c.pdf
file, page 83, I can see the TIMER0 Clock Timings, as said that
as RA4/T0CKI, and the Tt0H (high level pulse width) to be 20ns +
0.5Tcy (same for the low level). It means that running the unit
with a cycle time of 50ns (20MHz), it will be able to sample
external clocks (not using the prescaler) up to (25+20ns) per
level, what takes 90ns for a complete cycle = 11MHz.  So, one
can say a PIC can count (no prescaler) frequencies up to 55% of
its osc clock, when running at 20MHz. This percentage increases
reducing the osc clock, for 10MHz osc, it goes to 71%.

So, it means, according to what I can understand, that the unit
can't just count pulses, it depends on the internal clock to sample
the T0CKI, as many other controllers do, including the 8051 family.
In that particular case, the 8051 is terrible since it requires
12 clock cycles for one instruction cycle, and so the maximum
external frequency to be counted can not be higher than osc/12.

So, or I understood it wrong, or some previous post is not correct,
when somebody said the PIC could read external frequencies 8 times
its own clock.

Now, using prescaler, the min time for a complete external cycle
at that pin is 60ns (for VDD between 3 and 6 Volts), it also means
a maximum external frequency of 16MHz.

Again, somebody also said about 50MHz to be easy to read.

Well, I can be wrong, I don't know all the PIC available literature
and models.  If somebody know some model or some other information,
please let me know about it.

Thank you for your cooperation.
--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:   http:/www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm