Intel Pentium 4 Processor in the 478-PinPackage at 1.40 GHz, 1.50 GHz, 1.60 GHz, 1.70 GHz, 1.80 GHz, 1.90 GHz, and 2GHz

System Bus Signal Quality Specifications

Datasheet 41

3.3 System Bus Signal Quality Specifications and

Measurement Guidelines

3.3.1 Overshoot/Undershoot Guidelines

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

nominal high voltage (or below V

)asshowninFigure 21. The overshoot/undershoot

guidelines limit transitions beyond V

or V

due to the fast signal edge rates. The

processor can be damaged by 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) of the incident

waveform. 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 modelled

within Intel I/O buffer models do not clamp undershoot or overshoot and will yield correct

simulation results. If other I/O buffer models are being used to characterize the Pentium 4

processor in the 478-pin package system bus, care must be taken to ensure that ESD

models do not clamp extreme voltage levels. Intel I/O buffer models also contain I/O

capacitance characterization. Therefore, removing the ESD diodes from an I/O buffer

model will impact results and may yield excessive overshoot/undershoot.

Figure 20. High-to-Low System Bus Receiver Ringback Tolerance for PWRGOOD and TAP

Buffers

0.5 * Vcc

Vt+ (min)

Vt- (max)

Vcc

Vss

Vt- (min)

ThresholdRegiontoswitch

receiver to a logic 0.

Allowable Ringback