PIC18FXX20
16.4.1
PWM PERIOD
16.4 PWM Mode
The PWM period is specified by writing to the PR2
register. The PWM period can be calculated using the
following formula:
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.
PWM period = (PR2) + 1] • 4 • TOSC •
(TMR2 prescale value)
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.
PWM frequency is defined as 1 / [PWM period].
When TMR2 is equal to PR2, the following three events
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)
Figure 16-4 shows a simplified block diagram of the
CCP module in PWM mode.
For a step-by-step procedure on how to set up the CCP
module for PWM operation, see Section 16.4.3.
• The PWM duty cycle is latched from CCPR1L into
CCPR1H
Note: The Timer2 and Timer4 postscalers (see
Section 13.0) are not used in the determi-
nation of the PWM frequency. The
postscaler could be used to have a servo
update rate at a different frequency than
the PWM output.
FIGURE 16-4:
SIMPLIFIED PWM BLOCK
DIAGRAM
CCP1CON<5:4>
Duty Cycle Registers
CCPR1L
16.4.2
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:
CCPR1H (Slave)
Comparator
Q
R
S
RC2/CCP1
(Note 1)
TMR2
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.
TRISC<2>
Comparator
PR2
Clear Timer,
CCP1 pin and
latch D.C.
Note: 8-bit timer is concatenated with 2-bit internal Q clock or
2 bits of the prescaler to create 10-bit time-base.
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.
A PWM output (Figure 16-5) 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).
FIGURE 16-5:
PWM OUTPUT
The maximum PWM resolution (bits) for a given PWM
frequency is given by the equation:
Period
FOSC
FPWM
---------------
log
PWM Resolution (max)
= ----------------------------- b i t s
Duty Cycle
log(2)
TMR2 = PR2
TMR2 = Duty Cycle
Note: If the PWM duty cycle value is longer than
the PWM period, the CCP1 pin will not be
cleared.
TMR2 = PR2
DS39609A-page 154
Advance Information
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
