Microchip Technology DM183021 Data Sheet

 2010 Microchip Technology Inc.

DS39616D-page 181

PIC18F2331/2431/4331/4431

Table 18-1

shows the minimum PWM frequencies that

can be generated with the PWM time base and the
prescaler. An operating frequency of 40 MHz
(F

CYC

= 10 MHz) and PTPER = 0xFFF is assumed in

the table. The PWM module must be capable of gener-
ating PWM signals at the line frequency (50 Hz or
60 Hz) for certain power control applications.

TABLE 18-1:

MINIMUM PWM FREQUENCY

18.3.5

PWM TIME BASE POSTSCALER

The match output of PTMR can optionally be
postscaled through a 4-bit postscaler (which gives a
1:1 to 1:16 scaling inclusive) to generate an interrupt.
The postscaler counter is cleared when any of the
following occurs:
• Write to the PTMR register
• Write to the PTCON register
• Any device Reset
The PTMR register is not cleared when PTCON is
written.

18.4

PWM Time Base Interrupts

The PWM timer can generate interrupts based on the
modes of operation selected by the PTMOD<1:0> bits
and the postscaler bits (PTOPS<3:0>).

18.4.1

INTERRUPTS IN FREE-RUNNING
MODE

When the PWM time base is in the Free-Running mode
(PTMOD<1:0> = 00), an interrupt event is generated
each time a match with the PTPER register occurs. The
PTMR register is reset to zero in the following clock edge.
Using a postscaler selection other than 1:1 will reduce
the frequency of interrupt events.

FIGURE 18-5:

PWM TIME BASE INTERRUPT TIMING, FREE-RUNNING MODE

Minimum PWM Frequencies vs. Prescaler Value

for F

CYC

= 10 MIPS (PTPER = 0FFFh)

Prescale

PWM Frequency

Edge-Aligned

PWM Frequency

Center-Aligned

1:1

2441 Hz

1221 Hz

1:4

610 Hz

305 Hz

1:16

153 Hz

76 Hz

1:64

38 Hz

19 Hz

OSC

PTMR_INT_REQ

FFEh

FFFh

000h

001h

002h

PTIF bit

Note 1:

PWM Time Base Period register, PTPER, is loaded with the value, FFFh, for this example.