PIC24FJ256DA210 Datasheet, PDF (205/408 Pages) Microchip Technology – 64/100-Pin, 16-Bit Flash Microcontrollers with Graphics Controller and USB On-The-Go (OTG)

PIC24FJ256DA210 FAMILY
14.3.2 PWM DUTY CYCLE
The PWM duty cycle is specified by writing to the
OCxRS and OCxR registers. The OCxRS and OCxR
registers can be written to at any time, but the duty
cycle value is not latched 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.
Some important boundary parameters of the PWM duty
cycle include:
• If OCxR, OCxRS, and PRy are all loaded with
0000h, the OCx pin will remain low (0% duty
cycle).
• If OCxRS is greater than PRy, the pin will remain
high (100% duty cycle).
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 MIPS and 10 MIPS, respectively.
EQUATION 14-2: CALCULATION FOR MAXIMUM PWM RESOLUTION⁽¹⁾
( ) ^log10
Maximum PWM Resolution (bits) =
FCY
FPWM • (Timer Prescale Value)
log₁₀⁽²⁾
bits
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.
EXAMPLE 14-1: PWM PERIOD AND DUTY CYCLE CALCULATIONS⁽¹⁾
1. Find the Timer Period register value for a desired PWM frequency of 52.08 kHz, where FOSC = 8 MHz with PLL
(32 MHz device clock rate) and a Timer2 prescaler setting of 1:1.
TCY = 2 * TOSC = 62.5 ns
PWM Period = 1/PWM Frequency = 1/52.08 kHz = 19.2 ms
PWM Period = (PR2 + 1) • TCY • (Timer2 Prescale Value)
19.2 ms = 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
Note 1: Based on TCY = 2 * TOSC; Doze mode and PLL are disabled.
TABLE 14-1: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS AT 4 MIPS (FCY = 4 MHz)⁽¹⁾
PWM Frequency
7.6 Hz
61 Hz
122 Hz 977 Hz 3.9 kHz 31.3 kHz 125 kHz
Timer Prescaler Ratio
8
1
1
1
1
1
1
Period Register Value
FFFFh FFFFh
7FFFh
0FFFh
03FFh
007Fh
001Fh
Resolution (bits)
16
16
15
12
10
7
5
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.
TABLE 14-2: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS AT 16 MIPS (FCY = 16 MHz)⁽¹⁾
PWM Frequency
30.5 Hz 244 Hz 488 Hz 3.9 kHz 15.6 kHz 125 kHz 500 kHz
Timer Prescaler Ratio
8
1
1
1
1
1
1
Period Register Value
FFFFh FFFFh
7FFFh
0FFFh
03FFh
007Fh
001Fh
Resolution (bits)
16
16
15
12
10
7
5
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.
 2010 Microchip Technology Inc.
DS39969B-page 205