MC9S08MP16: Accurate Frequency Measurement Using Keyboard Interrupt and ISR Counts

HI all,

Am working with an application to read the frequency based on that i need to drive DAC. To read frequency am using keyboard interrupt as a input pin (for frequency input ) in falling edge trigger. In the ISR (keyboard interrupt) placed a global variable for counting in an interval of 100ms. Actually testing with an input frequency of 50HZ for that am getting (20 to 30) counts.
why this much counts oscillating for a constant 20ms interval and also why this counts occurred because for a falling edge interrupt (100ms/20ms = 5counts) +/- 2counts instead am getting (20-30). Kindly give me suggestions to improve the application, to read the exact frequency.

Thanks in advance,
Regards,
Santhoshkumar.

Hi,


Try to decrease the 100ms count rate to some 1 ms and check. I think why results are going wrong is your count time is 100ms and interrupt time is 20ms. And also you are giving interrupt from keyboard, so could you please clear that whether you are generating interrupt manually or every 20ms you are getting interrupt.


Amit

HI AMIT KUMAR,

Thanks for the reply,,
Actually am not given any manual interrupt, keyboard interrupt is a type of external interrupt pin which is available in micro controller. I just used this pin as a input and connected with the 50Hz square pulse. So the external interrupt pin will continuously generate an interrupt of every 20ms of falling edge. This operation is separate from the 100ms timer interrupt. I just check the counts for 100ms once.

santhoshkumar.