PIC18F2331_07 Datasheet, PDF (191/400 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
Table 17-1 shows the minimum PWM frequencies that
can be generated with the PWM time base and the
prescaler. An operating frequency of 40 MHz
(FCYC = 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 17-1: MINIMUM PWM FREQUENCY
Minimum PWM Frequencies vs. Prescaler Value
for FCYC = 10 MIPS (PTPER = 0FFFh)
Prescale
PWM Frequency
Edge-Aligned
PWM Frequency
Center-Aligned
1:1
2441 Hz
1:4
610 Hz
1:16
153 Hz
1:64
38 Hz
1221 Hz
305 Hz
76 Hz
19 Hz
17.3.5 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.
17.4 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>).
17.4.1 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 17-5:
PWM TIME BASE INTERRUPT TIMING, FREE-RUNNING MODE
A: PRESCALER = 1:1
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
FOSC/4
PTMR
FFEh
1
FFFh
000h
001h
002h
PTMR_INT_REQ
PTIF bit
B: PRESCALER = 1:4
Q4 Q4
Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc
PTMR
PTMR_INT_REQ
FFEh
1
FFFh
000h
001h
002h
PTIF bit
Note 1: PWM Time Base Period register, PTPER, is loaded with the value FFFh for this example.
© 2007 Microchip Technology Inc.
Preliminary
DS39616C-page 189