Datasheet

© 2007 Microchip Technology Inc. DS39629C-page 153
PIC18F6390/6490/8390/8490
14.4 PWM Mode
In Pulse-Width Modulation (PWM) mode, the CCP2 pin
produces up to a 10-bit resolution PWM output. Since
the CCP2 pin is multiplexed with a PORTC or PORTE
data latch, the appropriate TRIS bit must be cleared to
make the CCP2 pin an output.
Figure 14-4 shows a simplified block diagram of the
CCP2 module in PWM mode.
For a step-by-step procedure on how to set up the CCP
module for PWM operation, see Section 14.4.3
“Setup for PWM Operation”.
FIGURE 14-4: SIMPLIFIED PWM BLOCK
DIAGRAM
A PWM output (Figure 14-5) 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 14-5: PWM OUTPUT
14.4.1 PWM PERIOD
The PWM period is specified by writing to the PR2
register. The PWM period can be calculated using the
following formula:
EQUATION 14-1:
PWM frequency is defined as 1/[PWM period].
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
•TMR2 is cleared
• The CCP2 pin is set (exception: if PWM duty
cycle = 0%, the CCP2 pin will not be set)
• The PWM duty cycle is latched from CCPR2L into
CCPR2H
Note: Clearing the CCP2CON register will force
the RC1 or RE7 output latch (depending
on device configuration) to the default low
level. This is not the PORTC or PORTE
I/O data latch.
CCPR2L
CCPR2H (Slave)
Comparator
TMR2
Comparator
PR2
(Note 1)
R
Q
S
Duty Cycle Registers
CCP2CON<5:4>
Clear Timer,
CCP2 pin and
latch D.C.
TRISC<2>
CCP2
Note 1: The 8-bit TMR2 value is concatenated with the 2-bit
internal Q clock, or 2 bits of the prescaler, to create the
10-bit time base.
Note: The Timer2 postscalers (see Section 12.0
“Timer2 Module”) are 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.
Period
Duty Cycle
TMR2 = PR2
TMR2 = Duty Cycle
TMR2 = PR2
PWM Period = (PR2) + 1] • 4 • TOSC •
(TMR2 Prescale Value)