PIC18F97J60_11 Datasheet, PDF (194/492 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1-Mbit Flash Microcontrollers with Ethernet

PIC18F97J60 FAMILY
17.4 PWM Mode
In Pulse-Width Modulation (PWM) mode, the CCPx pin
produces up to a 10-bit resolution PWM output. Since
the CCP4 and CCP5 pins are multiplexed with a
PORTG data latch, the appropriate TRISG bit must be
cleared to make the CCP4 or CCP5 pin an output.
Note:
Clearing the CCP4CON or CCP5CON
register will force the RG3 or RG4 output
latch (depending on device configuration)
to the default low level. This is not the
PORTG I/O data latch.
Figure 17-4 shows a simplified block diagram of the
CCPx module in PWM mode.
For a step-by-step procedure on how to set up a CCPx
module for PWM operation, see Section 17.4.3
“Setup for PWM Operation”.
FIGURE 17-4:
SIMPLIFIED PWM BLOCK
DIAGRAM
Duty Cycle Register
9
0
CCPRxL
CCPxCON<5:4>
Latch
Duty Cycle
CCPRxH (1)
Comparator
Reset
TMRx
TMRx = PRx
Match
Comparator
SQ
R
CCPx
Pin
2 LSbs Latched
From Q clocks
PRx
Set CCPx pin
TRIS
Output Enable
Note 1: The two LSbs of the Duty Cycle register are held by a
2-bit latch that is part of the module’s hardware. It is
physically separate from the CCPRx registers.
A PWM output (Figure 17-5) 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 17-5:
PWM OUTPUT
Period
17.4.1 PWM PERIOD
The PWM period is specified by writing to the PR2
(PR4) register. The PWM period can be calculated
using Equation 17-1:
EQUATION 17-1:
PWM Period = [(PR2) + 1] • 4 • TOSC •
(TMR2 Prescale Value)
PWM frequency is defined as 1/[PWM period].
When TMR2 (TMR4) is equal to PR2 (PR4), the
following three events occur on the next increment
cycle:
• TMR2 (TMR4) is cleared
• The CCPx pin is set (exception: if PWM duty
cycle = 0%, the CCPx pin will not be set)
• The PWM duty cycle is latched from CCPRxL into
CCPRxH
Note:
The Timer2 and Timer4 postscalers (see
Section 14.0 “Timer2 Module” and
Section 16.0 “Timer4 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.
17.4.2 PWM DUTY CYCLE
The PWM duty cycle is specified by writing to the
CCPRxL register and to the CCPxCON<5:4> bits. Up
to 10-bit resolution is available. The CCPRxL contains
the eight MSbs and the CCPxCON<5:4> contains the
two LSbs. This 10-bit value is represented by
CCPRxL:CCPxCON<5:4>. Equation 17-2 is used to
calculate the PWM duty cycle in time.
EQUATION 17-2:
PWM Duty Cycle = (CCPRXL:CCPXCON<5:4>) •
TOSC • (TMRx Prescale Value)
CCPRxL and CCPxCON<5:4> can be written to at any
time, but the duty cycle value is not latched into
CCPRxH until after a match between PR2 (PR4) and
TMR2 (TMR4) occurs (i.e., the period is complete). In
PWM mode, CCPRxH is a read-only register.
Duty Cycle
TMR2 (TMR4) = PR2 (PR4)
TMR2 (TMR4) = Duty Cycle
TMR2 (TMR4) = PR2 (TMR4)
DS39762F-page 194
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