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50Mhz frequency counter using 18F8722 possible?

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jonathanSync

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Dear all,

I would like to implement a 50Mhz frequency counter by using 18F8722, is it possible? By theory, 40MHz fc/4 = 10MHz, is unable to have a frequency counter up to 50 Mhz, is it possible to have any other solution for this?

Looking forward reply.
 

TIMER0 can be triggered by external signal sources with frequencies above osclilator clock.
See parameter Tt0P ( TIMER0 Clock Input Period ) at datasheet.

+++
 

18F8722 datasheet

Table 28-13 says that the minimum period for external clock of the timer0 is Tcy+10ns,
there is no way to use a counter with an external clock source faster than the internal clock.

Also in section 12.1 it says
"An external clock source can be used to drive Timer0;
however, it must meet certain requirements to ensure
that the external clock can be synchronized with the
internal phase clock (TOSC). There is a delay between
synchronization and the onset of incrementing the
timer/counter."

Alex
 

alexan_e


Please check where our analysis differs.
I supose is because you did not consider Prescaler but I may be wrong.

Refering to parameter #1A and #2 of table 28.6 we take :
FOSC = 25 MHz ( HS oscilator mode )
TCY = 4 / FOSC
TCY = 160 ns

And observing parameter #42 of table 28-13 we take :
T0CKI Min Period = min ( 20 , (TCY + 40)/N ) ns --> n : [ 2 , 4 , ... , 256 ]
T0CKI Min Period = 20ns
TIMER0 Max Frequency = 1/20ns = 50MHz

+++
 

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