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DS30010118D-page 177

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)

EXAMPLE 15-1:

PWM PERIOD AND DUTY CYCLE CALCULATIONS(1)

Note 1:

Based on FCY = FOSC/2; Doze mode and PLL are disabled.

Maximum PWM Resolution (bits) =

log10 (2)

FPWM • (Timer Prescale Value)

bits

FCY

()

log10

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

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

111

Period Register Value

FFFFh

7FFFh

0FFFh

03FFh

007Fh

001Fh

Resolution (bits)

Note 1:

Based on FCY = FOSC/2; Doze mode and PLL are disabled.

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

111

Period Register Value

FFFFh

7FFFh

0FFFh

03FFh

007Fh

001Fh

Resolution (bits)

Note 1:

Based on FCY = FOSC/2; Doze mode and PLL are disabled.

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