參數(shù)資料
型號: PIC16F818
廠商: Microchip Technology Inc.
英文描述: 18/20-Pin Enhanced FLASH Microcontrollers with nanoWatt Technology(18/20引腳,納瓦技術增強FLASH微控制器)
中文描述: 18/20-Pin增強型閃存微控制器采用納瓦技術(18/20引腳,納瓦技術增強閃存微控制器)
文件頁數(shù): 70/164頁
文件大?。?/td> 3045K
代理商: PIC16F818
PIC16F818/819
DS39598C-page 68
Preliminary
2002 Microchip Technology Inc.
9.3
PWM Mode
In Pulse Width Modulation (PWM) mode, the CCP1 pin
produces up to a 10-bit resolution PWM output. Since
the CCP1 pin is multiplexed with the PORTB data latch,
the TRISB<x> bit must be cleared to make the CCP1
pin an output.
Figure 9-3 shows a simplified block diagram of the
CCP module in PWM mode.
For a step by step procedure on how to set up the CCP
module for PWM operation, see Section 9.3.3.
FIGURE 9-3:
SIMPLIFIED PWM BLOCK
DIAGRAM
A PWM output (Figure 9-4) 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 9-4:
PWM OUTPUT
9.3.1
PWM PERIOD
The PWM period is specified by writing to the PR2
register. The PWM period can be calculated using the
following formula.
EQUATION 9-1:
PWM frequency is defined as 1 / [PWM period].
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
TMR2 is cleared
The CCP1 pin is set (exception: if PWM duty
cycle = 0%, the CCP1 pin will not be set)
The PWM duty cycle is latched from CCPR1L into
CCPR1H
9.3.2
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 following equation is
used to calculate the PWM duty cycle in time.
EQUATION 9-2:
CCPR1L and CCP1CON<5:4> can be written to at any
time, but the duty cycle value is not latched into
CCPR1H until after 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, con-
catenated with an internal 2-bit Q clock or 2 bits of the
TMR2 prescaler, the CCP1 pin is cleared.
Note:
Clearing the CCP1CON register will force
the CCP1 PWM output latch to the default
low level. This is not the PORTB I/O data
latch.
CCPR1L
CCPR1H (Slave)
Comparator
TMR2
Comparator
PR2
(
Note 1
)
R
Q
S
Duty Cycle Registers
CCP1CON<5:4>
Clear Timer,
CCP1 pin and
latch D.C.
TRISB<x>
CCP1 pin
Note 1:
8-bit timer is concatenated with 2-bit internal Q clock
or 2 bits of the prescaler to create 10-bit time-base.
Period
Duty Cycle
TMR2 = PR2
TMR2 = Duty Cycle
TMR2 = PR2
Note:
The Timer2 postscaler (see Section 8.0) 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.
PWM period = [(PR2) + 1] 4 T
OSC
(TMR2 prescale value)
PWM duty cycle = (CCPR1L:CCP1CON<5:4>)
T
OSC
(TMR2 prescale value)
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