PIC18F2480/2580/4480/4580
17.4.2
PWM DUTY CYCLE
EQUATION 17-3:
The PWM duty cycle is specified by writing to the
ECCPR1L register and to the ECCP1CON<5:4> bits.
Up to 10-bit resolution is available. The ECCPR1L
contains the eight MSbs and the ECCP1CON<5:4> bits
contain the two LSbs. This 10-bit value is represented
by ECCPR1L:ECCP1CON<5:4>. The PWM duty cycle
is calculated by the following equation.
FOSC
log
(FPWM)
bits
PWM Resolution (max) =
log(2)
Note:
If the PWM duty cycle value is longer than
the PWM period, the CCP1 pin will not be
cleared.
EQUATION 17-2:
PWM Duty Cycle = (ECCPR1L:ECCP1CON<5:4> •
TOSC • (TMR2 Prescale Value)
17.4.3
PWM OUTPUT CONFIGURATIONS
The EPWM1M<1:0> bits in the ECCP1CON register
allow one of four configurations:
ECCPR1L and ECCP1CON<5:4> can be written to at
any time, but the duty cycle value is not copied into
ECCPR1H until a match between PR2 and TMR2
occurs (i.e., the period is complete). In PWM mode,
ECCPR1H is a read-only register.
• Single Output
• Half-Bridge Output
• Full-Bridge Output, Forward mode
• Full-Bridge Output, Reverse mode
The ECCPR1H 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 ECCPR1H and 2-bit latch match TMR2,
concatenated with an internal 2-bit Q clock or two bits
of the TMR2 prescaler, the ECCP1 pin is cleared. The
maximum PWM resolution (bits) for a given PWM
frequency is given by the following equation.
The Single Output mode is the standard PWM mode
discussed in Section 17.4 “Enhanced PWM Mode”.
The Half-Bridge and Full-Bridge Output modes are
covered in detail in the sections that follow.
The general relationship of the outputs in all
configurations is summarized in Figure 17-2.
TABLE 17-2: 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)
DS39637D-page 180
© 2009 Microchip Technology Inc.
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