I know pwm wont work as I believe the max speed is 133khz
No that is wrong it will go faster than that.
I don't need pwm, just 50% constant
The advantage of PWM is that it works without using any of the software CPU cycles.
If you follow the PWM output with a divide by two you get the square wave you are after.
Some processors have a toggle output pin on counter overflow, but not this one.
Doing it in software you have to arrange for the counter overflow ( or under flow ) to trigger an interrupt. In the ISR you simply toggle the pin.