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Confidential - Advance Information
Date: February 8, 2002
Technical Note: 26029 0.2
CPG Technical Marketing

FSB_Sense Circuitry
Abstract: This technical note is intended for AMD motherboard partners
developing products based on mobile AMD AthlonTM and
AMD DuronTM processors model 8 and higher. This document
describes the motherboard circuitry required to automatically
determine the Front Side Bus (FSB) speed of the processor.

This information will be incorporated into the next revision of
the AMD AthlonTM Processor-Based Motherboard Design Guide,
order# 24363.

Problem: Currently mobile motherboards are unable to automatically
distinguish between processors that have 100-MHz FSB and
those that have a 133-MHz FSB.

Solution: Mobile Model 8 and higher processors have a pin named
FSB_Sense that when used with the appropriate FSB_Sense
circuitry allows the motherboard to distinguish between
processors that have a 100-MHz FSB and those that have a
133-MHz FSB.
Note: For the exact location of the FSB_Sense Pin refer to the
appropriate processor data sheet.

Th e l og i c l eve l o f t h e F S B _ S e n s e p i n c a n b e u s e d t o
automatically configure the frequency strapping of the system
clock generator and Northbridge. Most clock generators
currently being used have the capability of selecting between
100-MHz and 133-MHz FSB frequencies by manipulating a
single Frequency Select strap that is latched by the clock
generator shortly after it powers up. For the Northbridge
frequency strapping, there may be one or two straps that must
be manipulated as it powers up. The logic level of the
FSB_Sense signal from the CPU will typically need to be
inverted to match the logic level of the clock generator and
Northbridge strap inputs.

The FSB_Sense pin will have an open circuit connection for
mobile processors with a 100-MHz FSB. The FSB_Sense pin will
be connected to the VSS (GND) plane of the processor through