Ok I solved it, because I need to generate only 50 Hz signals, I went to wiring_analog.c, there I found
void analogWrite(uint8_t pin, int val) {
//
// duty cycle(%) = val / 255;
// Frequency of 490Hz specified by Arduino API
//
PWMWrite(pin, 255, val, 50);
}
and I just changed 490 to 50 and boom solved...
If you want different frequencies, at different pins you can use PWMWrite (pin, resolution, val, freq); function, but make sure to #include wiring_analog.c.
You can download this software from TI to look at what pins can do what, and visually plan out your pins