The link below can provide some more information regarding synchronous PWM.

http://electronicdesign.com/Articles/...cleID=7633

Some MCUs have PWM generator circuits that produce the basic PWM duty cycle waveform(s) where the duty cycle register specifies a count of half the "high time" and changes of the register value cause the high time to change around the center point of the high time. (I recall some of the Infineon 8051 derivative parts this capability). This scheme has its primary usage in multi-phase motor drives where a high frequency PWM waveform is modulated at a slower rate (i.e. the duty cycle register value is changed on the fly) according to some control algorithm. According to the above article such modulation scheme may improve power transfer, efficiency and/or reduce distortion.

Michael Karas