Hi,
PTMR is the actual timer providing the timebase for the 4 PWM modules. You shouldn't need to read or write those register at all. PTPER is the period register, it is what controls the period, or frequency, of the PWM signal. Think of it this way, the timer counts from 0 and up, when it hits the value you write to PTPER it start over. Each time it start over a new cycle begins. The dutycycle is set by writing to the PDCx register for the specific channel.

There is a relationship between the PTPER and the actual dutycycle. Think of it, if the timer starts over at a value 50 then the maximum dutycycle value will be 50. If the timer starts over at 500 then the maximum dutycycle value is 500 so if you change PTPER you'll change the PWM frequency AND the dutycycle.

Bruce's example really does cover it quite well. If you can't get it going post your code and we'll take a look.

/Henrik.