PIC16F631/677/685/687/689/690
11.3.1
PWM PERIOD
EQUATION 11-2: PULSE WIDTH
The PWM period is specified by the PR2 register of
Timer2. The PWM period can be calculated using the
formula of Equation 11-1.
Pulse Width = (CCPR1L:CCP1CON<5:4>) •
TOSC • (TMR2 Prescale Value)
EQUATION 11-1: PWM PERIOD
EQUATION 11-3: DUTY CYCLE RATIO
PWM Period = [(PR2) + 1] • 4 • TOSC •
(TMR2 Prescale Value)
(CCPR1L:CCP1CON<5:4>)
Duty Cycle Ratio = -----------------------------------------------------------------------
4(PR2 + 1)
When TMR2 is equal to PR2, the following three events
occur on the next increment cycle:
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.
• TMR2 is cleared
• The CCP1 pin is set. (Exception: If the PWM duty
cycle = 0%, the pin will not be set.)
The 8-bit timer TMR2 register is concatenated with
either the 2-bit internal system clock (FOSC), or 2 bits of
the prescaler, to create the 10-bit time base. The system
clock is used if the Timer2 prescaler is set to 1:1.
• The PWM duty cycle is latched from CCPR1L into
CCPR1H.
Note:
The Timer2 postscaler (see Section 7.1
“Timer2 Operation”) is not used in the
determination of the PWM frequency.
When the 10-bit time base matches the CCPR1H and
2-bit latch, then the CCP1 pin is cleared (see
Figure 11-3).
11.3.2
PWM DUTY CYCLE
11.3.3
PWM RESOLUTION
The PWM duty cycle is specified by writing a 10-bit
value to multiple registers: CCPR1L register and
DC1B<1:0> bits of the CCP1CON register. The
CCPR1L contains the eight MSbs and the DC1B<1:0>
bits of the CCP1CON register contain the two LSbs.
CCPR1L and DC1B<1:0> bits of the CCP1CON
register can be written to at any time. The duty cycle
value is not latched into CCPR1H until after the period
completes (i.e., a match between PR2 and TMR2
registers occurs). While using the PWM, the CCPR1H
register is read-only.
The resolution determines the number of available duty
cycles for a given period. For example, a 10-bit resolution
will result in 1024 discrete duty cycles, whereas an 8-bit
resolution will result in 256 discrete duty cycles.
The maximum PWM resolution is 10 bits when PR2 is
255. The resolution is a function of the PR2 register
value as shown by Equation 11-4.
EQUATION 11-4: PWM RESOLUTION
log[4(PR2 + 1)]
Equation 11-2 is used to calculate the PWM pulse
width.
Resolution = ----------------------------------------- bits
log(2)
Equation 11-3 is used to calculate the PWM duty cycle
ratio.
Note:
If the pulse width value is greater than the
period the assigned PWM pin(s) will
remain unchanged.
TABLE 11-2: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS (FOSC = 20 MHz)
PWM Frequency
1.22 kHz
4.88 kHz
19.53 kHz
78.12 kHz
156.3 kHz
208.3 kHz
Timer Prescale (1, 4, 16)
PR2 Value
16
0xFF
10
4
1
1
0x3F
8
1
0x1F
7
1
0xFF
10
0xFF
10
0x17
6.6
Maximum Resolution (bits)
TABLE 11-3: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS (FOSC = 8 MHz)
PWM Frequency
1.22 kHz
4.90 kHz
19.61 kHz
76.92 kHz
153.85 kHz 200.0 kHz
Timer Prescale (1, 4, 16)
PR2 Value
16
0x65
8
4
0x65
8
1
0x65
8
1
0x19
6
1
0x0C
5
1
0x09
5
Maximum Resolution (bits)
© 2007 Microchip Technology Inc.
DS41262D-page 131
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