Pentium® III Processor for the PGA370 Socket at 500 MHz to 866 MHz
3.3
AGTL+ 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 VSS. The overshoot guideline limits transitions beyond VCC or VSS due to the fast
signal edge rates. The processor can be damaged by repeated overshoot events on 1.5 V or 2.5 V
tolerant buffers if the charge is large enough (i.e., if the overshoot 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. Violating the overshoot/undershoot guideline will also make
satisfying the ringback specification difficult.
When performing simulations to determine impact of overshoot and overshoot, 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 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 III processor performance, 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.
3.3.2
Overshoot/Undershoot Magnitude
Magnitude describes the maximum potential difference between a signal and its voltage reference
level, VSS (overshoot) and VTT (undershoot). While overshoot can be measured relative to VSS
using one probe (probe to signal and GND lead to VSS), undershoot must be measured relative to
VTT. This could be acomplished by simultaneously measuring the VTT plane while measuring the
signal undershoot. Today’s oscilloscopes can easily calculate the true undershoot waveform. The
true undershoot waveform can also be obtained with the following oscilloscope data file analysis:
Converted Undershoot Waveform = VTT - Signal_measured
Note: The converted undershoot waveform appears as a positive (overshoot) signal.
Note: Overshoot (rising edge) and undershoot (falling edge) conditions are separate and their impact
must be determined independently.
After the true waveform conversion, the undershoot/overshoot specifications shown in Table 20
through Table 22 can be applied to the converted undershoot waveform using the same magnitude
and pulse duration specifications used with an overshoot waveform.
Overshoot/undershoot magnitude levels must observe the Absolute Maximum Specifications listed
in Table 20 through Table 22. 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 durations. Provided that the magnitude of the overshoot/undershoot is within the
Absolute Maximum Specifications (2.18V), the pulse magnitude, duration and activity factor must
all be used to determine if the overshoot/undershoot pulse is within specifications.
36
Datasheet
INTEL [ INTEL ]
