PIC24HJ128GP206-I Datasheet, PDF (144/286 Pages) Microchip Technology – High-Performance, 16-Bit Microcontrollers

PIC24HJXXXGPX06/X08/X10
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
Timer Period register (PRy).
2. Set the PWM duty cycle by writing to the OCxRS
register.
3. Write the OxCR 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 Out-
put Compare Mode bits, OCM<2:0>
(OCxCON<2:0>).
6. Set the TMRy prescale value and enable the
time base by setting TON = 1 (TxCON<15>).
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
Secondary register, OCxRS, will not be
transferred into OCxR until a time base
period match occurs.
14.3.1 PWM PERIOD
The PWM period is specified by writing to PRy, the
Timer Period register. The PWM period can be
calculated using Equation 14-1:
EQUATION 14-1: CALCULATING THE PWM
PERIOD
PWM Period = [(PRy) + 1] • TCY • (Timer Prescale Value)
where:
PWM Frequency = 1/[PWM Period]
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
eight time base cycles.
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 glitchless 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 register, OCxR, is loaded
with 0000h, the OCx pin will remain low (0% duty
cycle).
• If OCxR is greater than PRy (Timer 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 shows example PWM frequencies and
resolutions for a device operating at 10 MIPS.
EQUATION 14-2: CALCULATION FOR MAXIMUM PWM RESOLUTION
( ) ^log10
FCY
FPWM
Maximum PWM Resolution (bits) =
bits
log₁₀(2)
EXAMPLE 14-1: PWM PERIOD AND DUTY CYCLE CALCULATIONS
1. Find the Timer Period register value for a desired PWM frequency that is 52.08 kHz, where FCY = 16 MHz and a Timer2
prescaler setting of 1:1.
TCY
= 62.5 ns
PWM Period = 1/PWM Frequency = 1/52.08 kHz = 19.2 μs
PWM Period = (PR2 + 1) • TCY • (Timer2 Prescale Value)
19.2 μs
= (PR2 + 1) • 62.5 ns • 1
PR2
= 306
2. Find the maximum resolution of the duty cycle that can be used with a 52.08 kHz frequency and a 32 MHz device clock rate:
PWM Resolution = log₁₀(FCY/FPWM)/log₁₀2) bits
= (log₁₀(16 MHz/52.08 kHz)/log₁₀2) bits
= 8.3 bits
DS70175F-page 142
© 2007 Microchip Technology Inc.