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Overshoot (or undershoot) is the absolute value of the maximum voltage above or below V

SS

. The

overshoot/undershoot specifications limit transitions beyond V

CC

 or V

SS

 due to the fast signal edge

rates. The processor can be damaged by single and/or repeated overshoot or undershoot events on
any input, output, or I/O buffer if the charge is large enough (i.e., if the over/undershoot is great
enough). Determining the impact of an overshoot/undershoot condition requires knowledge of the
magnitude, the pulse direction, and the activity factor (AF). Permanent damage to the processor is
the likely result of excessive overshoot/undershoot. 

When performing simulations to determine impact of overshoot and undershoot, ESD diodes must
be properly characterized. ESD protection diodes do not act as voltage clamps and will not provide
overshoot or undershoot protection. ESD diodes modeled within Intel’s signal integrity models do
not clamp undershoot or overshoot and will yield correct simulation results. If other signal integrity
models are being used to characterize the processor front side bus, care must be taken to ensure that
ESD models do not clamp extreme voltage levels. Intel’s signal integrity models also contain I/O
capacitance characterization. Therefore, removing the ESD diodes from a signal integrity model
will impact results and may yield excessive overshoot/undershoot. 

Magnitude describes the maximum potential difference between a signal and its voltage reference
level (V

SS

). It is important to note that overshoot and undershoot conditions are separate and their

impact must be determined independently.

Overshoot/undershoot magnitude levels must observe the absolute maximum specifications listed
in 

 through 

. These specifications must not be violated at any time regardless of

bus activity or system state. Within these specifications are threshold levels that define different
allowed pulse duration. Provided that the magnitude of the overshoot/undershoot is within the
absolute maximum specifications, the pulse magnitude, duration and activity factor must all be
used to determine if the overshoot/undershoot pulse is within specifications.

Pulse duration describes the total time an overshoot/undershoot event exceeds the overshoot/
undershoot reference voltage (V

CC

). The total time could encompass several oscillations above the

reference voltage. Multiple overshoot/undershoot pulses within a single overshoot/undershoot
event may need to be measured to determine the total pulse duration. 

Note 1: Oscillations below the reference voltage can not be subtracted from the total overshoot/
undershoot pulse duration.