PIC16F882/883/884/886/887
11.5.1
PWM PERIOD
11.5.2
PWM DUTY CYCLE
The PWM period is specified by the PR2 register of
Timer2. The PWM period can be calculated using the
formula of Equation 11-1.
The PWM duty cycle is specified by writing a 10-bit
value to multiple registers: CCPRxL register and
DCxB<1:0> bits of the CCPxCON register. The
CCPRxL contains the eight MSbs and the DCxB<1:0>
bits of the CCPxCON register contain the two LSbs.
CCPRxL and DCxB<1:0> bits of the CCPxCON
register can be written to at any time. The duty cycle
value is not latched into CCPRxH until after the period
completes (i.e., a match between PR2 and TMR2
registers occurs). While using the PWM, the CCPRxH
register is read-only.
EQUATION 11-1: PWM PERIOD
PWM Period = [(PR2) + 1] • 4 • TOSC •
(TMR2 Prescale Value)
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
Equation 11-2 is used to calculate the PWM pulse
width.
• TMR2 is cleared
• The CCPx pin is set. (Exception: If the PWM duty
cycle = 0%, the pin will not be set.)
Equation 11-3 is used to calculate the PWM duty cycle
ratio.
• The PWM duty cycle is latched from CCPRxL into
CCPRxH.
EQUATION 11-2: PULSE WIDTH
Note:
The Timer2 postscaler (see Section 7.1
“Timer2 Operation”) is not used in the
determination of the PWM frequency.
Pulse Width = (CCPRxL:CCPxCON<5:4>) •
TOSC • (TMR2 Prescale Value)
EQUATION 11-3: DUTY CYCLE RATIO
(CCPRxL:CCPxCON<5:4>)
Duty Cycle Ratio = ----------------------------------------------------------------------
4(PR2 + 1)
The CCPRxH register and a 2-bit internal latch are
used to double buffer the PWM duty cycle. This double
buffering is essential for glitchless PWM operation.
The 8-bit timer TMR2 register is concatenated with
either the 2-bit internal system clock (FOSC), or 2 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 CCPRxH and
2-bit latch, then the CCPx pin is cleared (see
Figure 11-3).
© 2007 Microchip Technology Inc.
Preliminary
DS41291D-page 129
MICROCHIP [ MICROCHIP ]
