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2037-07 9/23/99
SMH4042
CompactPCI
Applications Aid
I/O Pre-charge
The CompactPCI specs require the add-in board to pre-charge the board
’
s I/Os before making contact with bus pins
and the pre-charge voltage is 1V ±0.1V. The SMH4042 provides an accurate 1volt reference output that is accurate
and stable prior to the medium length pins making contact. The 1Vref output should be the reference input to a unity
gain op amp circuit. Figure 8 is a typical implementation utilizing a common op amp.
Figure 8. I/O pre-charge circuit
Resistor Array
T
B
B
LMV321
+
-
early power
SMH4042
1Vref
Vcc5
2037 ILL29.0
Special Considerations
The example application shown in Figure 2 shows both of the BD_SEL inputs being used independently. These two
inputs are effectively AND
’
ed internally and they must both be low before any sequencing will proceed. In most design
cases the BD_SEL1# connection to an injector switch is redundant and realistically can be grounded.
The CompactPCI Hot Swap spec does provide a mechanism for implementing high availability systems using
“
Full
Hot Swap
”
boards. This capability entails integrating the injector/ejector handle, the blue LED and a board status
signal. Section 2.3.2 of the
CompactPCI
Hot Swap spec states the following.
“
A signal (ENUM#) is provided to notify the system host that either a board has been freshly inserted or is about
to be extracted.
”
“
A switch, actuated with the lower ejector handle of the board, is used to signal the insertion or impending
extraction of a board.
”
“
A blue LED, located on the from to the board is illuminated when it is permissible to extract a board.
”
Figure 8 illustrates a possible implementation of the circuits needed. It should be noted this will require the
implementation of a status register that works in conjunction with the switch logic to generate the ENUM# signal. Notice
the blue LED circuit and the active high reset output used to activate a current boost circuit for the LED. The sequence
of operations is as follows: