PIC18F2X1X Datasheet, PDF (142/380 Pages) Microchip Technology – 28/40/44-PIN FLASH MICROCONTROLLERS WITH 10-BIT A/D AND NANO WATT TECHNOLOGY

PIC18F2X1X/4X1X
15.4.2 PWM DUTY CYCLE
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 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.
EQUATION 15-2:
PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>) •
TOSC • (TMR2 Prescale Value)
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 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
operation. 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:
( ) ^{log FOSC}
PWM Resolution (max) =
FPWM _bits
log(2)
Note:
If the PWM duty cycle value is longer than
the PWM period, the CCP1 pin will not be
cleared.
15.4.3 PWM OUTPUT CONFIGURATIONS
The P1M1:P1M0 bits in the CCP1CON register allow
one of four configurations:
• Single Output
• Half-Bridge Output
• Full-Bridge Output, Forward mode
• Full-Bridge Output, Reverse mode
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.
The general relationship of the outputs in all
configurations is summarized in Figure 15-2.
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)
16
4
1
1
1
1
PR2 Value
FFh
FFh
FFh
3Fh
1Fh
17h
Maximum Resolution (bits)
10
10
10
8
7
6.58
DS39636A-page 140
Preliminary
 2004 Microchip Technology Inc.