Continue to Site

# PIC MCU timer1\timer0 maximum external clocking frequency

Status
Not open for further replies.

#### D1_DRE

##### Member level 3
Hello all

Please I have the following questions about clocking timer0 or timer1 using an external clocking signal (i.e on pin T0KI or T1KI):

1. what is the maximum frequency supported,

2. Can I supply it with 5Mhz or 9Mhz xtal and it will work

2. The block diagram of the timer says no but will the frequency from the external clock source be divided by 4 also?

Thanx

real counter

This circuit is a real binary counter. The input to it can be either a logic level signal on the external pin or the internal clock divided by 4. Therefore:

1. Putting a crystal on it will do nothing useful.

2. It can count at least as fast as 1/4 of the rated external clock for your particular PIC.

Look in the PIC data sheet for the table labeled "TIMER0 AND TIMER1 EXTERNAL CLOCK REQUIREMENTS" for the details of the limits. They are very extensive on the high and low time periods under different conditions.

I see this is an old topic - but just for the record, I believe, on the 16F series, the mimimum period you can measure with the imer0 prescaler set to 256 is 40nS - makes the maximum freq. directly into input of PIC -> 25Mhz.

I am not sure if this is applicable for 18F series and higher though - maybe other can write their experience.

Re: PIC MCU timer1\timer0 maximum external clocking frequen

I see this is an old topic - but just for the record, I believe, on the 16F series, the mimimum period you can measure with the imer0 prescaler set to 256 is 40nS - makes the maximum freq. directly into input of PIC -> 25Mhz.
I am not sure if this is applicable for 18F series and higher though - maybe other can write their experience.
Sir,
I want to know how did you calculate the minimum period which can be measured with timer0 prescaler?
As i am using 16f887 timer0 to measure the time period of 10ms.
for that purpose i have omron h3cr-f8 timer but with this timer i can get the pulse minimum of 20ms.

i want to count the pulse occured in every 1/3 second using above mentioned ( 0.2 to 1.2second time range) i am getting 1:1 count with prescaler rttc_div_8. ( TRIAL AND ERROR). I have check the count using wdt prescaler but there is no 1:1 result.

may be these are basic questions but i need help .

a)I want to know how can we calculate prescaler for given pulse width?

b)As mentioned in the datasheet that

>>>"for 1:1 prescaler should be assigned to watchdog" .

>>>but there is also mentioned that "When Timer0 is in Counter mode, the synchronization
of the T0CKI input and the Timer0 register is accomplished
by sampling the prescaler output on the Q2 and
Q4 cycles of the internal phase clocks. Therefore, the
high and low periods of the external clock source must
meet the timing requirements."

I am not getting what these two statements say

for 10 ms pulsewidth what prescaler should be used ( INTERNAL OSCILLATOR 2MHZ)?