Browse Prior Art Database

Isolation Circuitry

IP.com Disclosure Number: IPCOM000116965D
Original Publication Date: 1995-Dec-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 61K

Publishing Venue

IBM

Related People

Appel, WD: AUTHOR [+6]

Abstract

Disclosed is an enhancement for devices with pinnout limitations and inadequate drive capability. One such device is the Power Management (PM) controller used in the Desktop PowerPC* design: a Signetics 87C750 which has very few pins and relatively weak high-drivers. This design includes requirements for a pull-up/pull-down input and a drive high/drive low output. Since these requirements are needed in mutually exclusive system states, and since the Signetics has a very limited number of pins, it is desirable to combine these two functions onto one pin. However, if a pull-down resistor is connected to one of the Signetics' pins to accomplish a given function, the controller cannot drive a full logic '1' on that pin, making the pin unusable for any output function that may require a high logic level.

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This is the abbreviated version, containing approximately 52% of the total text.

Isolation Circuitry

      Disclosed is an enhancement for devices with pinnout
limitations and inadequate drive capability.  One such device is the
Power Management (PM) controller used in the Desktop PowerPC* design:
a Signetics 87C750 which has very few pins and relatively weak
high-drivers.  This design includes requirements for a
pull-up/pull-down input and a drive high/drive low output.  Since
these requirements are needed in mutually exclusive system states,
and since the Signetics has a very limited number of pins, it is
desirable to combine these two functions onto one pin.  However, if a
pull-down resistor is connected to one of the Signetics' pins to
accomplish a given function, the controller cannot drive a full logic
'1' on that pin, making the pin unusable for any output function that
may require a high logic level.  This design provides a very simple
means to disconnect one set of circuitry from a given pin of a device
to allow that pin to be used for an additional function.

      A device's pin limitation and weak high-driving capabilities
can be overcome by isolating the pulldown circuitry from the device
once that circuitry is no longer needed; thus allowing the same pin
to have a different function for two different system states.  In the
PM design of the Desktop PowerPC, a specific pull-down functionality
is needed when only the PM circuitry is on; that pull-down circuit is
isolated from the device once the rest of the system is powered on....