PIC16(L)F722A/723A
DS41417B-page 130
2010-2012 Microchip Technology Inc.
15.3.2
PWM PERIOD
The PWM period is specified by the PR2 register of
Timer2. The PWM period can be calculated using the
formula of Equation 15-1.
EQUATION 15-1:
PWM PERIOD
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
TMR2 is cleared
The CCPx pin is set. (Exception: If the PWM duty
cycle = 0%, the pin will not be set.)
The PWM duty cycle is latched from CCPRxL into
CCPRxH.
15.3.3
PWM DUTY CYCLE
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 15-2 is used to calculate the PWM pulse
width.
Equation 15-3 is used to calculate the PWM duty cycle
ratio.
EQUATION 15-2:
PULSE WIDTH
EQUATION 15-3:
DUTY CYCLE RATIO
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 (refer to
Note:
The
Timer2
postscaler
(refer
to
used in the determination of the PWM
frequency.
PWM Period
PR2
1
+
4TOSC
=
(TMR2 Prescale Value)
Note:
TOSC = 1/FOSC
Pulse Width
CCPRxL:CCPxCON<5:4>
=
TOSC
(TMR2 Prescale Value)
Note: TOSC = 1/FOSC
Duty Cycle Ratio
CCPRxL:CCPxCON<5:4>
4PR2
1
+
-----------------------------------------------------------------------
=
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