how may I calculate the maximum frequency

[micropad]

Dear All
Please kindly teach me how may I calculate the maximum frequency that can be count using Timer0 and Timer1 in Counter mode (Timer1 in Asynchronous Counter mode)
Any equation highly appreciated
PIC is 16f877a
Thanks in advance

 

[Nigel Goodwin]

Check the VERY old frequency counter application at MicroChip, the timer hardware can accept in excess of 50MHz.

 

[micropad]

Thanks for reply
Can I have the equation
thanks

 

[Nigel Goodwin]

Thanks for reply
Can I have the equation
thanks

What equation?, it's a hardware 'limit' of the fabrication technology.

 

[paulfjujo]

hello

I allready made some test for frequency counter.
I get a pratical limit near from 30Mhz with a PIC16F84 at 4Mhz clock.
The maximum is reached by using internal prescaler of Tmr0 with is 1/256.
theoritical limit is 10+10nS => witch gives 50Mhz
read my notes .. sorry some parts in french.

By adding special divider circuit as MB506 you can multiply by 256 this limit (in theory).
I tested it sucessfuly at 64MHz on a breadboard !!!
I have to do a clean and serious printed board to test it at 1GHz.

;Using TMR0 with External Clock
;paragraph 6.2 specsheet DS30430C page 29
;Therefore, it is necessary for T0CKI to have a
;period of at least 4Tosc (plus a small RC delay) divided
;by the prescaler value. The only requirement on T0CKI
;high and low time is that they do not violate the
;minimum pulse width requirement of 10 ns.
;
;parameter 40 High pulse width 30nS
;parameter 41 Low pulse width 20nS
;parameter 42 periode (Tcy+40)/N N=prescaler
; La valeur du quartz intervient beaucoup sur la periode mini (42)
; si Q=4Mhz avec Tcy= 4/Fosc=> 1µS
; periode mini = (1000+40)/256= ~4nS soit 250Mhz
; si Q=20Mhz periode mini (200 + 40)/256 => < 1nS ! 1Ghz!? pas glop,il y a VIOL!
; En se basant sur les limites (40 et 41)etat H=30ns + etat L=20ns => periode 50nS
; => 20Mhz ! TRES PLAUSIBLE puisque reellement mesuré (avec quartz de 4Mhz)
; En se basant sur les limites absolues (antiviol!) de 10 + 10 ns=> 20nS
; => 50Mhz, ce qui est decrit dans la note ci-dessous

; note microchip AN592.pdf
;The PIC16C5X has one 8-bit timer (Timer0), which can be used with
;an 8-bit prescaler. The prescaler runs asynchronously, hence it can
;count a very high frequency. The minimum rise and fall times of the input
;frequency are speci?ed to be 10 ns, so the fastest clock rate the TMR0
;can count is 50 MHz.