Datasheet

© 2009 Microchip Technology Inc. DS41288F-page 91
PIC16F610/616/16HV610/616
10.3.1 PWM PERIOD
The PWM period is specified by writing to the PR2
register of Timer2. The PWM period can be calculated
using the formula of Equation 10-1.
EQUATION 10-1: 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 the PWM duty
cycle = 0%, the pin will not be set.)
• The PWM duty cycle is latched from CCPR1L into
CCPR1H.
10.3.2 PWM DUTY CYCLE
The PWM duty cycle is specified by writing a 10-bit
value to multiple registers: CCPR1L register and
CCP1<1:0> bits of the CCP1CON register. The
CCPR1L contains the eight MSbs and the CCP1<1:0>
bits of the CCP1CON register contain the two LSbs.
CCPR1L and CCP1<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.
Equation 10-2 is used to calculate the PWM pulse
width.
Equation 10-3 is used to calculate the PWM duty cycle
ratio.
EQUATION 10-2: PULSE WIDTH
EQUATION 10-3: DUTY CYCLE RATIO
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.
The 8-bit timer TMR2 register is concatenated with
either the 2-bit internal system clock (F
OSC), 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.
When the 10-bit time base matches the CCPR1H and
2-bit latch, then the CCP1 pin is cleared (see
Figure 10-3).
10.3.3 PWM RESOLUTION
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 10-4.
EQUATION 10-4: PWM RESOLUTION
TABLE 10-4: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS (F
OSC = 20 MHz)
TABLE 10-5: EXAMPLE PWM FREQUENCIES AND RESOLUTIONS (F
OSC = 8 MHz)
Note: The Timer2 postscaler (see Section 7.1
“Timer2 Operation”) is not used in the
determination of the PWM frequency.
PWM Period PR2()1+[]4TOSC •••=
(TMR2 Prescale Value)
Note: If the pulse width value is greater than the
period the assigned PWM pin(s) will
remain unchanged.
Pulse Width CCPR1L:CCP1CON<5:4>() • =
T
OSC • (TMR2 Prescale Value)
Duty Cycle Ratio
CCPR1L:CCP1CON<5:4>()
4PR2 1+()
-----------------------------------------------------------------------=
Resolution
4PR2 1+()[]log
2()log
------------------------------------------ bits=
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) 16 4 1 1 1 1
PR2 Value 0xFF 0xFF 0xFF 0x3F 0x1F 0x17
Maximum Resolution (bits) 10 10 10 8 7 6.6
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) 16 4 1 1 1 1
PR2 Value 0x65 0x65 0x65 0x19 0x0C 0x09
Maximum Resolution (bits) 8 8 8 6 5 5