27.4.5 PWM Duty Cycle

The PWM duty cycle is specified by writing a 10-bit value to the CCPRx register. The alignment of the 10-bit value is determined by the FMT bit (see Figure 27-5). The CCPRx register can be written to at any time. However, the duty cycle value is not latched onto the 10-bit buffer until after a match between T2PR and T2TMR.

The equations below are used to calculate the PWM pulse width and the PWM duty cycle ratio.

Figure 27-5. PWM 10-Bit Alignment

Equation 27-2. Pulse Width

P u l s e W i d t h = (C C P R x H : C C P R x L r e g i s t e r v a l u e) • T_{O S C} • (T M R 2 \Pr e s c a l e V a l u e)

Equation 27-3. Duty Cycle

D u t y C y c l e R a t i o = \frac{(C C P R x H : C C P R x L r e g i s t e r v a l u e)}{4 (T 2 P R + 1)}

The CCPRx register is used to double buffer the PWM duty cycle. This double buffering is essential for glitchless PWM operation.

The 8-bit timer T2TMR register is concatenated with either the 2-bit internal system clock (F_OSC), or two bits of the prescaler, to create the 10-bit time base. The system clock is used if the Timer2 prescaler is set to 1:1.

When the 10-bit time base matches the CCPRx register, then the CCPx pin is cleared (see Figure 27-4).