Problems using an LM324 as a comparator ?

I'm using an LM324 to drive a MOSFET from a digital output from a microcontroller.

The microcontroller runs at 3.6VDC and thus the PWM pulse train is 0 to

3.3V. This is fed into the + terminal of one of the opamps on a LM324.

I generate a reference voltage of 1.8V and feed it into the negative input of the same op amp.

The LM324 is powered by a 13VDC supply.

The output of the op amp isn't changing. Its a steady 0V.

If I change the reference voltage to 0V, the output goes high, to nearly

13V.

Why isn't the op amp outputting a PWM pulse train ?

Thanks.