PIC18F2420/2520/4420/4520
16.4.1
PWM PERIOD
16.4 Enhanced PWM Mode
The PWM period is specified by writing to the PR2
register. The PWM period can be calculated using the
following equation.
The Enhanced PWM mode provides additional PWM
output options for a broader range of control applica-
tions. The module is a backward compatible version of
the standard CCP module and offers up to four outputs,
designated P1A through P1D. Users are also able to
select the polarity of the signal (either active-high or
active-low). The module’s output mode and polarity are
configured by setting the P1M<1:0> and CCP1M<3:0>
bits of the CCP1CON register.
EQUATION 16-1:
PWM Period = [(PR2) + 1] • 4 • TOSC •
(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:
Figure 16-1 shows a simplified block diagram of PWM
operation. All control registers are double-buffered and
are loaded at the beginning of a new PWM cycle (the
period boundary when Timer2 resets) in order to pre-
vent glitches on any of the outputs. The exception is the
PWM Dead-Band Delay register, PWM1CON, which is
loaded at either the duty cycle boundary or the period
boundary (whichever comes first). Because of the buff-
ering, the module waits until the assigned timer resets
instead of starting immediately. This means that
Enhanced PWM waveforms do not exactly match the
standard PWM waveforms, but are instead offset by
one full instruction cycle (4 TOSC).
• TMR2 is cleared
• The CCP1 pin is set (if PWM duty cycle = 0%, the
CCP1 pin will not be set)
• The PWM duty cycle is copied from CCPR1L into
CCPR1H
Note:
The Timer2 postscaler (see Section 13.0
“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.
As before, the user must manually configure the
appropriate TRIS bits for output.
FIGURE 16-1:
SIMPLIFIED BLOCK DIAGRAM OF THE ENHANCED PWM MODULE
CCP1CON<5:4>
P1M1<1:0>
CCP1M<3:0>
4
Duty Cycle Registers
2
CCPR1L
CCP1/P1A
CCP1/P1A
P1B
TRISx<x>
TRISx<x>
TRISx<x>
TRISx<x>
CCPR1H (Slave)
Comparator
P1B
Output
Controller
R
S
Q
P1C
P1C
P1D
(Note 1)
TMR2
P1D
Comparator
PR2
Clear Timer,
set CCP1 pin and
latch D.C.
PWM1CON
Note: The 8-bit TMR2 register is concatenated with the 2-bit internal Q clock, or 2 bits of the prescaler, to create the 10-bit
time base.
© 2008 Microchip Technology Inc.
DS39631E-page 149
MICROCHIP [ MICROCHIP ]
