I do not know if this is the correct solution.
Are you using the uC just for PWM, then clock down the chip to below 1MHz and try getting a 50Hz square wave with 50% duty cycle. Id this is not possible, the use clock dividers to reduce the clock.
Use a Bandpass Filter (I would go with a second order band pass filter with peak at 50Hz)
But there will be lots of noise if you use PWM and Filters if the filter is not good enough.
Try using a 741 for sine-wave generation and then controlling it with a DAC and microcontroller.