Microchip Technology AC244045 Data Sheet

DS41440C-page 230

 2010-2012 Microchip Technology Inc.

24.4.3

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 CCPxAS register. A shutdown
event may be generated by:
• A  logic  ‘0’ on the FLT0 pin
• A  logic  ‘1’ on a Comparator (async_CxOUT) 

output

A shutdown condition is indicated by the CCPxASE
(Auto-Shutdown Event Status) bit of the CCPxAS
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 

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 and
PSSxBD bits of the CCPxAS register. Each pin pair may
be placed into one of three states:
• Drive  logic  ‘1’
• Drive  logic  ‘0’
• Tri-state (high-impedance)

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.

Writing to the CCPxASE bit of the
CCPxAS register is disabled while an
auto-shutdown condition persists.

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.

Prior to an auto-shutdown event caused
by a comparator output or FLT0 pin event,
a software shutdown can be triggered in
firmware by setting the CCPxASE bit of
the CCPxAS register to ‘1’. The auto-
restart feature tracks the active status of
a shutdown caused by a comparator
output or FLT0 pin event only. If it is
enabled at this time, it will immediately
clear this bit and restart the ECCP
module at the beginning of the next PWM
period.

Shutdown

PWM

CCPxASE bit

Activity

Event

Shutdown

Event Occurs

Shutdown

Event Clears

PWM
Resumes

PWM Period

Start of
PWM Period

CCPxASE

Cleared by

Firmware

Timer

Overflow

Timer

Overflow

Timer

Overflow

Timer

Overflow

Missing Pulse

(Auto-Shutdown)

Missing Pulse
(CCPxASE not clear)

Timer

Overflow