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.
early power
Vcc5
-
Resistor Array
SMH4042
LMV321
+
1Vref
2037 ILL29.0
Figure 8. I/O pre-charge circuit
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.
• “Asignal(ENUM#)isprovidedtonotifythesystemhostthateitheraboardhasbeenfreshlyinsertedorisabout
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
implementationofastatusregisterthatworksinconjunctionwiththeswitchlogictogeneratetheENUM#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:
2037-07 9/23/99
19
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
