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SCAN182245A
SCAN ABT Live Insertion and Power Cycling Characteristics
SCAN ABT is intended to serve in Live Insertion backplane
applications. It provides 2nd Level Isolation
1
which indi-
cates that while external circuitry to control the output
enable pin is unnecessary, there may be a need to imple-
ment differential length backplane connector pins for V
CC
and GND. As well, pre-bias circuitry for backplane pins
may be necessary to avoid capacitive loading effects dur-
ing live insertion.
SCAN ABT provides control of output enable pins during
power cycling via the circuit in Figure 1. It essentially con-
trols the G
n
pin until V
CC
reaches a known level.
During power-up, when V
CC
ramps through the 0.0V to
0.7V range, all internal device circuitry is inactive, leaving
output and I/O pins of the device in high impedance. From
approximately 0.8V to 1.8V V
CC
, the Power-On-Reset cir-
cuitry, (POR), in Figure 1 becomes active and maintains
device high impedance mode. The POR does this by pro-
viding a low from its output that resets the flip-flop The out-
put, Q
, of the flip-flop then goes high and disables the NOR
gate from an incidental low input on the G
n
pin. After 1.8V
V
CC
, the POR circuitry becomes inactive and ceases to
control the flip-flop. To bring the device out of high imped-
ance, the G
n
input must receive an inactive-to-active transi-
tion, a high-to-low transition on G
n
in this case to change
the state of the flip-flop. With a low on the Q
output of the
flip-flop, the NOR gate is free to allow propagation of a G
n
signal.
During power-down, the Power-On-Reset circuitry will
become active and reset the flip-flop at approximately 1.8V
V
CC
. Again, the Q output of the flip-flop returns to a high
and disables the NOR gate from inputs from the G
n
pin.
The device will then remain in high impedance for the
remaining ramp down from 1.8V to 0.0V V
CC
.
Some suggestions to help the designer with live insertion
issues:
• The G
n
pin can float during power-up until the Power-
On-Reset circuitry becomes inactive.
• The G
n
pin can float on power-down only after the
Power-On-Reset has become active.
The description of the functionality of the Power-On-Reset
circuitry can best be described in the diagram of Figure 2.
FIGURE 1.
1
Section 7, “Design Consideration for Fault Tolerant Backplanes”, Application Note AN-881.
SCAN ABT includes additional power-on reset circuitry not otherwise included in ABT devices.
FIGURE 2.
