Reading opamp output from a GPIO

[chris43j]

This is probably just some very basic concept I am missing but it driving me up the wall. Basically what I want to do is determine the frequency of a signal using an MSP430. I have written the code and it works when I connect my oscilloscope 1kHz output to it. However, what I really want to do is to read the frequency of a sine wave I have generated using an oscillator circuit. I pass it though an opamp which goes rail-to-rail (ie from 0 to 3.5V - same as the micro) and the output to the GPIO. However, the micro doesn't seem to register anything. I thought reading a GPIO input is simply a voltage. If the voltage swing is big enough it is should read it as a 0 or 1 regardless the shape of the waveform no?

 

[KlausST]

Hi,

A GPIO is not meant to read sine waveform.
The microcontroller specifications show V_IH and V_IL.
The signal should not be inbetween both levels, best if the signal is close to either supply rails.

Use a comparator pin or use an external comparator to generate a digital signal.

Klaus

 

[chris43j]

That seemed to work. Was pretty straight forward to set up actually. I ended up using the onboard comparator which can can feed into the timer capture input. Thanks!