Three Phase inverter that has a fixed output frequency

I am also making a three phase inverter using arduino as the microcontroller. But I'd like to ask, how do you generate a fixed output of 50 Hz? I have fairly understood that part on pwm, but I wish to have an output with only one frequency. What part of the code do i change? i am hoping for a reply. thank you

What code?

I think that makes four times you've asked the same question, mostly in inappropriate sections of the forum.