PIC17C44 datasheet(75/241 Pages) MICROCHIP

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DS30412C-page 75

PIC17C4X

12.1.3

USING PULSE WIDTH MODULATION

(PWM) OUTPUTS WITH TMR1 AND TMR2

Two high speed pulse width modulation (PWM) outputs

are provided. The PWM1 output uses Timer1 as its

time-base, while PWM2 may be software conﬁgured to

use either Timer1 or Timer2 as the time-base. The

PWM outputs are on the RB2/PWM1 and RB3/PWM2

pins.

Each PWM output has a maximum resolution of

10-bits. At 10-bit resolution, the PWM output frequency

is 24.4 kHz (@ 25 MHz clock) and at 8-bit resolution the

PWM output frequency is 97.7 kHz. The duty cycle of

the output can vary from 0% to 100%.

Figure 12-5 shows a simpliﬁed block diagram of the

PWM module. The duty cycle register is double buff-

ered for glitch free operation. Figure 12-6 shows how a

glitch could occur if the duty cycle registers were not

double buffered.

The user needs to set the PWM1ON bit (TCON2<4>)

to enable the PWM1 output. When the PWM1ON bit is

set, the RB2/PWM1 pin is conﬁgured as PWM1 output

and forced as an output irrespective of the data direc-

tion bit (DDRB<2>). When the PWM1ON bit is clear,

the pin behaves as a port pin and its direction is con-

trolled by its data direction bit (DDRB<2>). Similarly,

the PWM2ON (TCON2<5>) bit controls the conﬁgura-

tion of the RB3/PWM2 pin.

FIGURE 12-5: SIMPLIFIED PWM BLOCK

DIAGRAM

PWxDCH

Duty Cycle registers

PWxDCL<7:6>

Clear Timer,

PWMx pin and

Latch D.C.

(Slave)

Comparator

TMR2

Comparator

PRy

(Note 1)

PWMxON

RCy/PWMx

Note 1: 8-bit timer is concatenated with 2-bit internal Q clock

or 2 bits of the prescaler to create 10-bit time-base.

Read

Write

FIGURE 12-6: PWM OUTPUT

1020

3040

PWM

output

Timer

interrupt

Write new

PWM value

Timer interrupt

new PWM value

transferred to slave

Note

The dotted line shows PWM output if duty cycle registers were not double buffered.

If the new duty cycle is written after the timer has passed that value, then the PWM does

not reset at all during the current cycle causing a “glitch”.

In this example, PWM period = 50. Old duty cycle is 30. New duty cycle value is 10.

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