PIC24FJ64GA705 Datasheet, PDF (177/412 Pages) –

PIC24FJ256GA705 FAMILY
15.3.2 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 15-1 for PWM mode timing details.
Table 15-1 and Table 15-2 show example PWM
frequencies and resolutions for a device operating at
4 MIPS and 10 MIPS, respectively.
EQUATION 15-2: CALCULATION FOR MAXIMUM PWM RESOLUTION⁽¹⁾
FCY
( ) ^log10
Maximum PWM Resolution (bits) =
FPWM • (Timer Prescale Value)
bits
log₁₀ (2)
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.
EXAMPLE 15-1: 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 = 32 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 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
Note 1: Based on TCY = 2 * TOSC; Doze mode and PLL are disabled.
TABLE 15-1: 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
Period Register Value
FFFFh
FFFFh
7FFFh
0FFFh
Resolution (bits)
16
16
15
12
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.
1
03FFh
10
1
007Fh
7
1
001Fh
5
TABLE 15-2: 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
Period Register Value
FFFFh
FFFFh
7FFFh
0FFFh
Resolution (bits)
16
16
15
12
Note 1: Based on FCY = FOSC/2; Doze mode and PLL are disabled.
1
03FFh
10
1
007Fh
7
1
001Fh
5
 2016 Microchip Technology Inc.
DS30010118B-page 177