Nano 33 BLE: setting freq (PRF) & PW using GPIO

We need get a GPIO to generate waveform with TWO selectable parameters: PRF (pulse reoccurrence freq) AND PW. We don't want to use analogWrite(). Looked all over internet, but found very little. Please point me to code (clock prescaler/min & max freqs & PWs). Thanks.

QUESTION: will the USB interface make it difficult to do this ? If yes, please point me to alternative method of board comm.

Jeff Wagner