Maximum frequency for Timer1 counter for 18F series???

[hello188]

Hi.
I am using counter to measure frequency which reflects some other analog quantity. Because I am using counter, if I make the input frequency range greater, a single bit error caused by the nature of counting pulses will be of less percentage of the whole range.

So, what I would like to know is, where in the datasheet I can find the maximum frequency that the timer1 module can accurately count from T1CKI pin? I looked at the timer1 section and DC characteristics section but could not find it.

Thank you.

[Mike Walne]

I found this in the 18Fxx2 data sheet, (electrical characteristics).

[gpsmikey]

Unless the input clock circuit is somehow dependent on the system clock for example to sample the input signal. I have run into things like that in the past where the "equivalent circuit" in the data sheet skips some of the important details. If the input to the clock is sampled along the same lines as a UART for example, then it could indeed be a low frequency. On the other hand, maybe they simply copied that part of the data sheet to the next one and nobody has noticed it (yet) :-)

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[Mike Walne]

Microchip app note AN592 describes a 50MHz frrequency counter with a PIC16C5x, by feeding the unknown signal into the TMR0 input.

Seems strange that PIC18Fxxxs are limited to 50kHz.

The CCS sample code EX_FREQC.C works into the 10's MHz region with a lowly 16F877!

Mike

[PCM programmer]

You're looking at the wrong parameter. You're looking at the Timer1
oscillator max freq. It's for a watch crystal.

For the external clock max frequency, look at "T1CKI input period".
Then convert that period to a frequency.

[gaugeguy]

You are confusing the oscillator circuit with the timer input circuit.
If you are connecting an external crystal to the timer 1 oscillator pins you are limited to 50kHz. If you are driving the Timer 1 input with an external signal you can get to 20 ns period (50MHz) with the right clock and precaler.

[Mike Walne]