PIC18F87J11_12 Datasheet, PDF (231/466 Pages) Microchip Technology – 64/80-Pin, High-Performance, 1-Mbit Flash Microcontrollers

PIC18F87J11 FAMILY
19.4.6
PROGRAMMABLE DEAD-BAND
DELAY
In half-bridge applications, where all power switches
are modulated at the PWM frequency at all times, the
power switches normally require more time to turn off
than to turn on. If both the upper and lower power
switches are switched at the same time (one turned on
and the other turned off), both switches may be on for
a short period of time until one switch completely turns
off. During this brief interval, a very high current
(shoot-through current) may flow through both power
switches, shorting the bridge supply. To avoid this
potentially destructive shoot-through current from flow-
ing during switching, turning on either of the power
switches is normally delayed to allow the other switch
to completely turn off.
In the Half-Bridge Output mode, a digitally program-
mable, dead-band delay is available to avoid
shoot-through current from destroying the bridge
power switches. The delay occurs at the signal
transition from the non-active state to the active state
(see Figure 19-4 for illustration). The lower seven bits of
the ECCPxDEL register (Register 19-2) set the delay
period in terms of microcontroller instruction cycles
(TCY or 4 TOSC).
19.4.7
ENHANCED PWM
AUTO-SHUTDOWN
When the ECCP1 is programmed for any of the
Enhanced PWM modes, the active output pins may be
configured for auto-shutdown. Auto-shutdown immedi-
ately places the Enhanced PWM output pins into a
defined shutdown state when a shutdown event
occurs.
A shutdown event can be caused by either of the two
comparator modules or the FLT0 pin (or any combina-
tion of these three sources). The comparators may be
used to monitor a voltage input proportional to a current
being monitored in the bridge circuit. If the voltage
exceeds a threshold, the comparator switches state
and triggers a shutdown. Alternatively, a low-level
digital signal on the FLT0 pin can also trigger a shut-
down. The auto-shutdown feature can be disabled by
not selecting any auto-shutdown sources. The
auto-shutdown sources to be used are selected using
the ECCP1AS<2:0> bits (ECCP1AS<6:4>).
When a shutdown occurs, the output pins are
asynchronously placed in their shutdown states,
specified by the PSS1AC<1:0> and PSS1BD<1:0> bits
(ECCP1AS<3:0>). Each pin pair (P1A/P1C and P1B/
P1D) may be set to drive high, drive low or be tri-stated
(not driving). The ECCP1ASE bit (ECCP1AS<7>) is
also set to hold the Enhanced PWM outputs in their
shutdown states.
The ECCP1ASE bit is set by hardware when a
shutdown event occurs. If automatic restarts are not
enabled, the ECCP1ASE bit is cleared by firmware
when the cause of the shutdown clears. If automatic
restarts are enabled, the ECCP1ASE bit is automati-
cally cleared when the cause of the auto-shutdown has
cleared.
If the ECCP1ASE bit is set when a PWM period begins,
the PWM outputs remain in their shutdown state for that
entire PWM period. When the ECCP1ASE bit is
cleared, the PWM outputs will return to normal
operation at the beginning of the next PWM period.
Note: Writing to the ECCP1ASE bit is disabled
while a shutdown condition is active.
 2007-2012 Microchip Technology Inc.
DS39778E-page 231