Datasheet

PIC16C712/716
DS41106C-page 42  1999-2013 Microchip Technology Inc.
7.3 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 PORTB data latch,
the TRISCCP<2> bit must be cleared to make the
CCP1 pin an output.
Figure 7-5 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 7.3.3 “Set-
Up for PWM Operation”.
FIGURE 7-5: SIMPLIFIED PWM BLOCK
DIAGRAM
A PWM output (Figure 7-6) 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 7-6: PWM OUTPUT
7.3.1 PWM PERIOD
The PWM period is specified by writing to the PR2
register. The PWM period can be calculated using the
following formula:
PWM period = [(PR2) + 1] • 4 • T
OSC •
(TMR2 prescale value)
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)
• The PWM duty cycle is latched from CCPR1L into
CCPR1H
7.3.2 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:
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.
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
concatenated with an internal 2-bit Q clock or 2 bits of
the TMR2 prescaler, the CCP1 pin is cleared.
Maximum PWM resolution (bits) for a given PWM
frequency:
For an example PWM period and duty cycle calcula-
tion, see the PIC
®
Mid-Range Reference Manual,
(DS33023).
Note: Clearing the CCP1CON register will force
the CCP1 PWM output latch to the default
low level. This is neither the PORTB I/O
data latch nor the DATACCP latch.
CCPR1L
CCPR1H (Slave)
Comparator
TMR2
Comparator
PR2
(Note 1)
R
Q
S
Duty cycle registers
CCP1CON<5:4>
Clear Timer,
CCP1 pin and
latch D.C.
TRISCCP<2>
RB3/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.
Period = PR2+1
Duty Cycle
TMR2 = PR2
TMR2 = Duty Cycle (CCPR1H)
TMR2 = PR2
Note: The Timer2 postscaler (see Section 6.0
“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.
Note: If the PWM duty cycle value is longer than
the PWM period the CCP1 pin will not be
cleared.
log(
F
PWM
log(2)
F
OSC
)
bits
=