frequency meter desin – 1878 days old

[djalltra]

hi guys please i want to design a frequency meter using the timer1 module in 16f877
capable of measuring frequencies of up to 20mhz but i dont have any idea on the math so anyone with relable info would be appreciated

[joantabb]

Hi. I think you are not going to be able to achieve this option easily.

the maximum clock speed of the 16F877 is 20mhz, and the micro controller takes at least 4 clock cycles to execute one instruction, so I suspect it will be difficult to capture time periods shorter than the microcontroller can run...

But I could be wrong,

Maybe you could measure the time period for a number of cycles using an external counter. however this will introduce errors if the frequency being sampled, is not stable.

regards Joan

[joantabb]

Joan, opens mouth and puts her foot in it...


I guess if you can do 60mhz frequency counter with a 16F84running at 4mhz, you should be able to do 20mhz frequency counter with a 16f877 running at 20mhz

http://www.circuitvalley.com/2011/07...r-counter.html

[John Drew]

Hi Djalltra and Joan
I reckon the basic idea of a frequency counter is to open a gate for a certain period of time (1 second if you want to measure to +- 1Hz) and count the pulses in that time.
If you are happy with 10Hz readout then open the gate for 0.1 second.
So all that is needed is a timer to measure a period depending on the accuracy you want and then count like the fellow does in your link with a simple amp that squares off the input frequency.
John

[Ioannis]

The application note AN592 from Microchip gives all the details to make it work. It is based on Timer0 specs. Very clever use of the fast timer0.

http://www.microchip.com/stellent/id...pnote=en011033

Ioannis

[Mellbreak]

Hi Djalltra,

I've actually built the frequency meter suggested by Joan in post 3. It works very well. The original circuit and code appears to be by a guy called Phil Rice (VK3BHR). If you follow this link

https://sites.google.com/site/vk3bhr/home/fm2


it will take you to the appropriate page. Links at the bottom of Phil's page will take you to the source code. The nice thing about the source code is that it allows you to select between a 16F84 and a 16F628.

Regards,

Bob