PIC16F87X
A PWM output (Figure 8-4) has a time-base (period)
and a time that the output stays high (duty cycle). The
frequency of the PWM is the inverse of the period
(1/period).
The CCPR1H 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.
When the CCPR1H and 2-bit latch match TMR2 con-
catenated with an internal 2-bit Q clock or 2 bits of the
TMR2 prescaler, the CCP1 pin is cleared.
FIGURE 8-4: PWM OUTPUT
Maximum PWM resolution (bits) for a given PWM
frequency:
Period
FOSC
log( )
FPWM
Resolution
bits
=
Duty Cycle
TMR2 = PR2
log(2)
Note: If the PWM duty cycle value is longer than
the PWM period, the CCP1 pin will not be
cleared.
TMR2 = Duty Cycle
TMR2 = PR2
8.3.3
SET-UP FOR PWM OPERATION
8.3.1
PWM PERIOD
The following steps should be taken when configuring
the CCP module for PWM operation:
The PWM period is specified by writing to the PR2 reg-
ister. The PWM period can be calculated using the fol-
lowing formula:
1. Set the PWM period by writing to the PR2 register.
2. Set the PWM duty cycle by writing to the
CCPR1L register and CCP1CON<5:4> bits.
PWM period = [(PR2) + 1] • 4 • TOSC •
(TMR2 prescale value)
3. Make the CCP1 pin an output by clearing the
TRISC<2> bit.
PWM frequency is defined as 1 / [PWM period].
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
4. Set the TMR2 prescale value and enable Timer2
by writing to T2CON.
5. Configure the CCP1 module for PWM operation.
• TMR2 is cleared
• The CCP1 pin is set (exception: if PWM duty
cycle = 0%, the CCP1 pin will not be set)
• The PWM duty cycle is latched from CCPR1L into
CCPR1H
Note: The Timer2 postscaler (see Section 8.1) is
not used in the determination of the PWM
frequency. The postscaler could be used to
have a servo update rate at a different fre-
quency than the PWM output.
8.3.2
PWM DUTY CYCLE
The PWM duty cycle is specified by writing to the
CCPR1L register and to the CCP1CON<5:4> bits. Up
to 10-bit resolution is available. The CCPR1L contains
the eight MSbs and the CCP1CON<5:4> contains the
two LSbs. This 10-bit value is represented by
CCPR1L:CCP1CON<5:4>. The following equation is
used to calculate the PWM duty cycle in time:
PWM duty cycle = (CCPR1L:CCP1CON<5:4>) •
Tosc • (TMR2 prescale value)
CCPR1L and CCP1CON<5:4> can be written to at any
time, but the duty cycle value is not latched into
CCPR1H until after a match between PR2 and TMR2
occurs (i.e., the period is complete). In PWM mode,
CCPR1H is a read-only register.
1999 Microchip Technology Inc.
DS30292B-page 61
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