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13. CLK Required for UP# to be Driven
PROBLEM: The Upgrade Present (UP#) output pin is intended to be driven
low after power-up to indicate that an upgrade processor is present in
the system. The UP# pin on the Pentium OverDrive processor requires that
the CLK input toggle while the system is starting to insure that RESET
reaches the UP# circuitry. If CLK does not toggle, UP# may or may not be
driven low, depending on the initial state of the UP# circuitry.
IMPLICATION: If an Intel486 processor system is dependent on having UP#
driven low before driving the CLK input on the Pentium OverDrive
processor, it may never boot since CLK may never be driven to the
processor if UP# remains high. This is generally only a potential issue
in systems with two processor sites, such as those with a surface-mount Intel486 processor and a PGA upgrade processor site.
WORKAROUND: Most two site systems have the ability to disable the
processor in the surface-mount location so that a different Intel486 (non-upgrade) processor may be used in the PGA socket location. If the
system has a jumper or switch that is documented to specifically disable
the surface mount processor (or the original processor in a two socket
system), use it to disable the second processor and thereby route the
CLK signal to the upgrade PGA socket location.
STATUS: This erratum has been fixed in the C-0 stepping.
Louis Ohland wrote:
https://ardent-tool.com/CPU/docs/Intel/Pentium/spec_update/242480-041.pdf
page 87 physical
2. Removal of VCC Pin A4
To resolve compatibility concerns with a limited number of incorrectly designed motherboards, pin A4 (VCC) will be removed from the Pentium OverDrive processors and will no longer be present on the package. The removal of this VCC pin will not adversely affect the operation of the processor due to the large number of VSS
and VCC pins remaining in the outer row.
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