Datasheet

PIC24FJ64GA004 FAMILY
DS39881E-page 136  2010-2013 Microchip Technology Inc.
14.3 Pulse-Width Modulation Mode
The following steps should be taken when configuring
the output compare module for PWM operation:
1. Set the PWM period by writing to the selected
Timery Period register (PRy).
2. Set the PWM duty cycle by writing to the OCxRS
register.
3. Write the OCxR register with the initial duty cycle.
4. Enable interrupts, if required, for the timer and
output compare modules. The output compare
interrupt is required for PWM Fault pin utilization.
5. Configure the output compare module for one
of two PWM Operation modes by writing to the
Output Compare Mode bits, OCM<2:0>
(OCxCON<2:0>).
6. Set the TMRy prescale value and enable the time
base by setting TON (TyCON<15>) = 1.
14.3.1 PWM PERIOD
The PWM period is specified by writing to PRy, the
Timery Period register. The PWM period can be
calculated using Equation 14-1.
EQUATION 14-1: CALCULATING THE PWM
PERIOD
(1)
14.3.2 PWM DUTY CYCLE
The PWM duty cycle is specified by writing to the
OCxRS register. The OCxRS register can be written to
at any time, but the duty cycle value is not latched into
OCxR until a match between PRy and TMRy occurs
(i.e., the period is complete). This provides a double
buffer for the PWM duty cycle and is essential for glitch-
less PWM operation. In the PWM mode, OCxR is a
read-only register.
Some important boundary parameters of the PWM duty
cycle include:
• If the Output Compare x register, OCxR, is loaded
with 0000h, the OCx pin will remain low (0% duty
cycle).
• If OCxR is greater than PRy (Timery Period
register), the pin will remain high (100% duty
cycle).
• If OCxR is equal to PRy, the OCx pin will be low
for one time base count value and high for all
other count values.
See Example 14-1 for PWM mode timing details.
Table 14-1 and Table 14-2 show example PWM
frequencies and resolutions for a device operating at
4 and 16 MIPS.
EQUATION 14-2: CALCULATION FOR MAXIMUM PWM RESOLUTION
(1)
Note: This peripheral contains input and output
functions that may need to be configured
by the Peripheral Pin Select. See
Section 10.4 “Peripheral Pin Select
(PPS)” for more information.
Note: The OCxR register should be initialized
before the output compare module is first
enabled. The OCxR register becomes a
read-only Duty Cycle register when the
module is operated in the PWM modes.
The value held in OCxR will become the
PWM duty cycle for the first PWM period.
The contents of the Output Compare x
Secondary register, OCxRS, will not be
transferred into OCxR until a time base
period match occurs.
Note: A PRy value of N will produce a PWM
period of N + 1 time base count cycles. For
example, a value of 7 written into the PRy
register will yield a period consisting of
8 time base cycles.
PWM Period = [(PRy) + 1] • TCY • (Timer Prescale Value)
PWM Frequency = 1/[PWM Period]
Where:
Note 1: Based on T
CY = 2 * TOSC; Doze mode
and PLL are disabled.
(
)
Maximum PWM Resolution (bits) =
F
CY
FPWM • (Timer Prescale Value)
log
10
log
10
(2)
bits
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.