PIC18F4539 Datasheet, PDF (125/322 Pages) Microchip Technology – Enhanced FLASH Microcontrollers with Single Phase Induction Motor Control Kernel

15.0 PULSE WIDTH MODULATION
(PWM) MODULES
PIC18FXX39 devices are equipped with two 10-bit
PWM modules. Each contains a register pair
(CCPxH:CCPxL), which operates as a Master/Slave
Duty Cycle register, and a control register (CCPxCON).
The modules use Timer2 (Section 12.0) as their time-
base reference. Figure 15-1 shows a simplified block
diagram of the module’s operation.
This section gives a brief overview of PWM operation
as controlled by the Motor Control module
(Section 14.0). Operation is described with respect to
PWM1, but is equally applicable to PWM2.
Note:
The PWM modules are used exclusively
by the Motor Control module. As such, they
are not available to users as a separate
resource. Although their locations are
shown on the device data memory maps,
users should not modify the values of
these registers.
15.1 PWM Mode
In Pulse Width Modulation, each PWM pin produces a
PWM output with a resolution of up to 10 bits.
A PWM output (Figure 15-2) has a time-base (period)
and a time that the output stays high (duty cycle). The
frequency of the PWM is the inverse of the period
(1/period).
FIGURE 15-1:
SIMPLIFIED PWM BLOCK
DIAGRAM
Duty Cycle Registers
CCPR1L
CCP1CON<5:4>
CCPR1H (Slave)
Comparator
RQ
TMR2
Comparator
PR2
(1)
S
Clear Timer,
PWM1 pin and
latch Duty Cycle
PWM1
Note 1: 8-bit timer is concatenated with 2-bit internal Q
clock, or 2 bits of the prescaler to create a
10-bit time-base.
PIC18FXX39
FIGURE 15-2:
PWM OUTPUT
Period
Duty Cycle
TMR2 = PR2
TMR2 = Duty Cycle
TMR2 = PR2
15.1.1 PWM PERIOD
The PWM period is specified when the Motor Control
module is initialized. The PWM period can be
calculated using the formula:
PWM period = [(PR2) + 1] • 4 • TOSC •
(TMR2 prescale value)
PWM frequency is defined as 1 / [PWM period].
The API method void ProMPT_Init (page 118)
sets the required PWM frequency in the application.
The parameter PWMfrequency determines the operat-
ing frequency of the module. When it is ‘0’, the PWM
frequency set in the Motor Control module is 9.75 kHz;
when it is ‘1’, the set PWM frequency is 19.53 kHz.
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
• TMR2 is cleared
• The PWM1 pin is set (exception: if PWM duty
cycle = 0%, the PWM1 pin will not be set)
• The PWM duty cycle is latched from CCPR1L into
CCPR1H
Note:
The Timer2 postscaler (see Section 12.0)
is not used in the determination of the
PWM frequency. The postscaler could be
used to have a servo update rate at a
different frequency than the PWM output.
 2002 Microchip Technology Inc.
Preliminary
DS30485A-page 123