2009 Microchip Technology Inc.
DS39636D-page 137
PIC18F2X1X/4X1X
The CCPRxH 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 CCPRxH and 2-bit latch match TMR2,
concatenated with an internal 2-bit Q clock or 2 bits of
the TMR2 prescaler, the CCPx pin is cleared.
The maximum PWM resolution (bits) for a given PWM
frequency is given by the equation:
EQUATION 14-3:
TABLE 14-4:
EXAMPLE PWM FREQUENCIES AND RESOLUTIONS AT 40 MHz
14.4.3
PWM AUTO-SHUTDOWN
(CCP1 ONLY)
The PWM auto-shutdown features of the Enhanced
CCP module are also available to CCP1 in 28-pin
devices.
The
operation
of
this
feature
is
discussed in detail in Section 15.4.7 “Enhanced
Auto-shutdown features are not available for CCP2.
14.4.4
SETUP FOR PWM OPERATION
The following steps should be taken when configuring
the CCP module for PWM operation:
1.
Set the PWM period by writing to the PR2
register.
2.
Set the PWM duty cycle by writing to the
CCPRxL register and CCPxCON<5:4> bits.
3.
Make the CCPx pin an output by clearing the
appropriate TRIS bit.
4.
Set the TMR2 prescale value, then enable
Timer2 by writing to T2CON.
5.
Configure the CCPx module for PWM operation.
Note:
If the PWM duty cycle value is longer than
the PWM period, the CCP2 pin will not be
cleared.
FOSC
FPWM
---------------
log
2
()
log
-----------------------------bits
=
PWM Resolution (max)
PWM Frequency
2.44 kHz
9.77 kHz
39.06 kHz
156.25 kHz
312.50 kHz
416.67 kHz
Timer Prescaler (1, 4, 16)
16
41111
PR2 Value
FFh
3Fh
1Fh
17h
Maximum Resolution (bits)
10
8
7
6.58
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