PIC18F2420/2520/4420/4520
16.4.9
SETUP FOR PWM OPERATION
16.4.10 OPERATION IN POWER-MANAGED
MODES
The following steps should be taken when configuring
the ECCP module for PWM operation:
In Sleep mode, all clock sources are disabled. Timer2
will not increment and the state of the module will not
change. If the ECCP pin is driving a value, it will con-
tinue to drive that value. When the device wakes up, it
will continue from this state. If Two-Speed Start-ups are
enabled, the initial start-up frequency from INTOSC
and the postscaler may not be stable immediately.
1. Configure the PWM pins, P1A and P1B (and
P1C and P1D, if used), as inputs by setting the
corresponding TRIS bits.
2. Set the PWM period by loading the PR2 register.
3. If auto-shutdown is required:
• Disable auto-shutdown (ECCPASE = 0)
In PRI_IDLE mode, the primary clock will continue to
clock the ECCP module without change. In all other
power-managed modes, the selected power-managed
mode clock will clock Timer2. Other power-managed
mode clocks will most likely be different than the
primary clock frequency.
• Configure source (FLT0, Comparator 1 or
Comparator 2)
• Wait for non-shutdown condition
4. Configure the ECCP module for the desired
PWM mode and configuration by loading the
CCP1CON register with the appropriate values:
16.4.10.1 Operation with Fail-Safe
Clock Monitor
• Select one of the available output
configurations and direction with the
P1M<1:0> bits.
If the Fail-Safe Clock Monitor is enabled, a clock failure
will force the device into the power-managed RC_RUN
mode and the OSCFIF bit (PIR2<7>) will be set. The
ECCP will then be clocked from the internal oscillator
clock source, which may have a different clock
frequency than the primary clock.
• Select the polarities of the PWM output
signals with the CCP1M<3:0> bits.
5. Set the PWM duty cycle by loading the CCPR1L
register and CCP1CON<5:4> bits.
6. For Half-Bridge Output mode, set the dead-
band delay by loading PWM1CON<6:0> with
the appropriate value.
See the previous section for additional details.
16.4.11 EFFECTS OF A RESET
7. If auto-shutdown operation is required, load the
ECCP1AS register:
Both Power-on Reset and subsequent Resets will force
all ports to Input mode and the CCP registers to their
Reset states.
• Select the auto-shutdown sources using the
ECCPAS<2:0> bits.
• Select the shutdown states of the PWM
output pins using the PSSAC<1:0> and
PSSBD<1:0> bits.
This forces the Enhanced CCP module to reset to a
state compatible with the standard CCP module.
• Set the ECCPASE bit (ECCP1AS<7>).
• Configure the comparators using the CMCON
register.
• Configure the comparator inputs as analog
inputs.
8. If auto-restart operation is required, set the
PRSEN bit (PWM1CON<7>).
9. Configure and start TMR2:
• Clear the TMR2 interrupt flag bit by clearing
the TMR2IF bit (PIR1<1>).
• Set the TMR2 prescale value by loading the
T2CKPS bits (T2CON<1:0>).
• Enable Timer2 by setting the TMR2ON bit
(T2CON<2>).
10. Enable PWM outputs after a new PWM cycle
has started:
• Wait until TMRx overflows (TMRxIF bit is set).
• Enable the CCP1/P1A, P1B, P1C and/or P1D
pin outputs by clearing the respective TRIS
bits.
• Clear the ECCPASE bit (ECCP1AS<7>).
© 2008 Microchip Technology Inc.
DS39631E-page 159
MICROCHIP [ MICROCHIP ]
