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Maximum Freq. measured by 8051


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nischay kumar

Wed Jul 30 2008, 10:49AM

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Joined: Sat Jun 07 2008, 11:09AM

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Hello Forum,

i am making a Freq.meter/counter using 8051, i have no problem in coding and the methodology but i am stuck at the thought of Max freq. that can be measured by 8051.

i know the limitation of F/24 ( crystal freq divided by 24) i am using 12 Mhz crystal and that would give me 500 Khz max freq limit. with 24Mhz Crystal the max freq. is 1 Mhz

but the Timer in counter mode or in timer mode can only count up to 65,536D (FFFFH) after which it gets reset, so how is it possible to count 500 KHz or 1Mhz.

i am using a gate interval of 1 Sec with timer1.

i hope you folks can clear my confusion

pdi33

Wed Jul 30 2008, 08:14PM

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Joined: Mon Jun 04 2007, 09:28AM

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hi nischay,
use the corresponding timer/counter overflow interrupt to increment a software counter by one each time the counter overflows. this will be ur MSB and the THX/TLX will be ur LSB. So, even if u increment a single bytein the service routine, the combination will count upto FFFFFFH which is more than sufficient for ur application.

P.S. : do not worry about losing any input pulse counts while u are incrementing the MSB in the overflow interrupt as the internal counter will continue counting even when ur service routine is being executed and will only stop when u disable it by changing the TMOD/TCON registers.

[ Edited Wed Jul 30 2008, 08:23PM ]

* inspired to develop,developing to inspire *

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nischay kumar

nischay kumar

Wed Jul 30 2008, 11:44PM

Registered Member #8311

Joined: Sat Jun 07 2008, 11:09AM

Posts: 31
Thanked 3 times in 3 posts

Thank you pdi33,

in my timer0 ISR ( set as counter) if the count reaches FFFF, i am reseting TL0 & TH0 to 00h ( i know it automatically resets to 00H).

but you are advising to INC a 8 bit register or a variable for each time the counter overflows. so total maximum count would be FF of 8 bit register and FFFF of 16 bit couner/timer.

so i need a 3 byte hex to BCD conversion routine

thanks for your suggestion