Datasheet

 2010-2012 Microchip Technology Inc. DS41412F-page 197
PIC18(L)F2X/4XK22
FIGURE 14-13: EXAMPLE OF PWM DIRECTION CHANGE AT NEAR 100% DUTY CYCLE
14.4.3 ENHANCED PWM AUTO-
SHUTDOWN MODE
The PWM mode supports an Auto-Shutdown mode that
will disable the PWM outputs when an external
shutdown event occurs. Auto-Shutdown mode places
the PWM output pins into a predetermined state. This
mode is used to help prevent the PWM from damaging
the application.
The auto-shutdown sources are selected using the
CCPxAS<2:0> bits of the ECCPxAS register. A
shutdown event may be generated by:
•A logic ‘0’ on the INT pin
• Comparator Cx (async_CxOUT)
• Setting the CCPxASE bit in firmware
A shutdown condition is indicated by the CCPxASE
(Auto-Shutdown Event Status) bit of the ECCPxAS
register. If the bit is a ‘0’, the PWM pins are operating
normally. If the bit is a ‘1’, the PWM outputs are in the
shutdown state.
When a shutdown event occurs, two things happen:
The CCPxASE bit is set to ‘1’. The CCPxASE will
remain set until cleared in firmware or an auto-restart
occurs (see Section 14.4.4 “Auto-Restart Mode”).
The enabled PWM pins are asynchronously placed in
their shutdown states. The PWM output pins are
grouped into pairs [PxA/PxC] and [PxB/PxD]. The state
of each pin pair is determined by the PSSxAC<1:0> and
PSSxBD<1:0> bits of the ECCPxAS register. Each pin
pair may be placed into one of three states:
• Drive logic ‘1’
• Drive logic ‘0’
• Tri-state (high-impedance)
Forward Period Reverse Period
PxA
TON
TOFF
T = TOFF – TON
PxB
PxC
PxD
External Switch D
Potential
Shoot-Through Current
Note 1: All signals are shown as active-high.
2: T
ON is the turn-on delay of power switch QC and its driver.
3: T
OFF is the turn-off delay of power switch QD and its driver.
External Switch C
t1
PW
PW
Note 1: The auto-shutdown condition is a level-
based signal, not an edge-based signal.
As long as the level is present, the auto-
shutdown will persist.
2: Writing to the CCPxASE bit is disabled
while an auto-shutdown condition
persists.
3: Once the auto-shutdown condition has
been removed and the PWM restarted
(either through firmware or auto-restart),
the PWM signal will always restart at the
beginning of the next PWM period.