Datasheet

© 2010 Microchip Technology Inc. DS70135G-page 87
dsPIC30F4011/4012
13.4.2 PWM PERIOD
The PWM period is specified by writing to the PRx
register. The PWM period can be calculated using
Equation 13-1.
EQUATION 13-1: PWM PERIOD
PWM frequency is defined as 1/[PWM period].
When the selected TMRx is equal to its respective
Period register, PRx, the following four events occur on
the next increment cycle:
• TMRx is cleared
• The OCx pin is set
- Exception 1: If PWM duty cycle is 0x0000,
the OCx pin will remain low.
- Exception 2: If duty cycle is greater than PRx,
the pin will remain high.
• The PWM duty cycle is latched from OCxRS into
OCxR
• The corresponding timer interrupt flag is set
See Figure 13-1 for key PWM period comparisons.
Timer3 is referred to in the figure for clarity.
FIGURE 13-1: PWM OUTPUT TIMING
PWM period = [(PRx) + 1] • 4 • TOSC •
(TMRX prescale value)
Period
Duty Cycle
TMR3 = Duty Cycle (OCxR)
TMR3 = Duty Cycle (OCxR)
TMR3 = PR3
T3IF = 1
(Interrupt Flag)
OCxR = OCxRS
TMR3 = PR3
(Interrupt Flag)
OCxR = OCxRS
T3IF = 1