PIC18F45J10 FAMILY
15.4.2
PWM DUTY CYCLE
Note:
If the PWM duty cycle value is longer than
the PWM period, the CCP1 pin will not be
cleared.
The PWM duty cycle is specified by writing to the
CCPR1L register and to the CCP1CON<5:4> bits. Up
to 10-bit resolution is available. The CCPR1L register
contains the eight MSbs and the CCP1CON<5:4>
contains the two LSbs. This 10-bit value is represented
by CCPR1L:CCP1CON<5:4>. The PWM duty cycle is
calculated by the following equation:
15.4.3
PWM OUTPUT CONFIGURATIONS
The P1M<1:0> bits in the CCP1CON register allow one
of four configurations:
• Single Output
EQUATION 15-2:
• Half-Bridge Output
• Full-Bridge Output, Forward mode
• Full-Bridge Output, Reverse mode
PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>) •
TOSC • (TMR2 Prescale Value)
The Single Output mode is the standard PWM mode
discussed in Section 15.4 “Enhanced PWM Mode”.
The Half-Bridge and Full-Bridge Output modes are
covered in detail in the sections that follow.
CCPR1L and CCP1CON<5:4> can be written to at any
time, but the duty cycle value is not copied into
CCPR1H until a match between PR2 and TMR2 occurs
(i.e., the period is complete). In PWM mode, CCPR1H
is a read-only register.
The general relationship of the outputs in all
configurations is summarized in Figure 15-2.
The CCPR1H register and a 2-bit internal latch are
used to double-buffer the PWM duty cycle. This
double-buffering is essential for glitchless PWM opera-
tion. When the CCPR1H and 2-bit latch match TMR2,
concatenated with an internal 2-bit Q clock or two bits
of the TMR2 prescaler, the CCP1 pin is cleared. The
maximum PWM resolution (bits) for a given PWM
frequency is given by the following equation:
EQUATION 15-3:
FOSC
log
(
)
FPWM
bits
PWM Resolution (max) =
log(2)
TABLE 15-2: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS AT 40 MHz
PWM Frequency
2.44 kHz
9.77 kHz
39.06 kHz 156.25 kHz 312.50 kHz 416.67 kHz
Timer Prescaler (1, 4, 16)
PR2 Value
16
FFh
10
4
1
1
3Fh
8
1
1Fh
7
1
FFh
10
FFh
10
17h
6.58
Maximum Resolution (bits)
DS39682E-page 138
© 2009 Microchip Technology Inc.
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