Microchip Technology DM164134 Data Sheet
PIC18FXX8
DS41159E-page 128
© 2006 Microchip Technology Inc.
15.4
PWM Mode
In Pulse-Width Modulation (PWM) mode, the CCP1 pin
produces up to a 10-bit resolution PWM output. Since
the CCP1 pin is multiplexed with the PORTC data latch,
the TRISC<2> bit must be cleared to make the CCP1
pin an output.
produces up to a 10-bit resolution PWM output. Since
the CCP1 pin is multiplexed with the PORTC data latch,
the TRISC<2> bit must be cleared to make the CCP1
pin an output.
Figure 15-3 shows a simplified block diagram of the
CCP module in PWM mode.
CCP module in PWM mode.
For a step-by-step procedure on how to set up the CCP
module for PWM operation, see Section 15.4.3
“Setup for PWM Operation”.
module for PWM operation, see Section 15.4.3
“Setup for PWM Operation”.
FIGURE 15-3:
SIMPLIFIED PWM BLOCK
DIAGRAM
DIAGRAM
A PWM output (Figure 15-4) 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).
and a time that the output stays high (duty cycle). The
frequency of the PWM is the inverse of the period
(1/period).
FIGURE 15-4:
PWM OUTPUT
15.4.1
PWM PERIOD
The PWM period is specified by writing to the PR2
register. The PWM period can be calculated using the
following formula.
register. The PWM period can be calculated using the
following formula.
EQUATION 15-1:
PWM frequency is defined as 1/[PWM period].
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
occur on the next increment cycle:
• 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
15.4.2
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.
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.
EQUATION 15-2:
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.
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.
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.
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,
concatenated with an internal 2-bit Q clock or 2 bits of
the TMR2 prescaler, the CCP1 pin is cleared.
concatenated with an internal 2-bit Q clock or 2 bits of
the TMR2 prescaler, the CCP1 pin is cleared.
Note:
Clearing the CCP1CON register will force
the CCP1 PWM output latch to the default
low level. This is not the PORTC I/O data
latch.
the CCP1 PWM output latch to the default
low level. This is not the PORTC I/O data
latch.
CCPR1L (Master)
CCPR1H (Slave)
Comparator
TMR2
PR2
(Note 1)
R
Q
S
Duty Cycle Registers
CCP1CON<5:4>
Clear Timer,
set CCP1 pin and
set CCP1 pin and
latch D.C.
TRISC<2>
RC2/CCP1
Note 1: 8-bit timer is concatenated with 2-bit internal Q clock,
or 2 bits of the prescaler, to create 10-bit time base.
Comparator
Period
Duty Cycle
TMR2 = PR2
TMR2 = Duty Cycle
TMR2 = PR2
Note:
The Timer2 postscaler (see Section 13.0
“Timer2 Module”) is not used in the
determination of the PWM frequency. The
postscaler could be used to have a servo
update rate at a different frequency than
the PWM output.
“Timer2 Module”) is not used in the
determination of the PWM frequency. The
postscaler could be used to have a servo
update rate at a different frequency than
the PWM output.
PWM Period = [(PR2) + 1] • 4 • T
OSC
•
(TMR2 Prescale Value)
PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>) •
T
OSC
• (TMR2 Prescale Value)