Intel III Xeon 800 MHz 80526KZ800256 User Manual

 

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Overshoot/Undershoot is the absolute value of the maximum voltage differential across the input buffer relative to the 
termination voltage (V

TT

). The overshoot/undershoot guideline limits transitions beyond VTT or VSS due to the fast signal 

edge rates. The processor can be damaged by repeated Overshoot/Undershoot events on 1.5 V or 2.5 V tolerant buffers 
if the potential is large enough (i.e., if the overshoot/undershoot is great enough). Determining the impact of an 
overshoot/undershoot condition requires knowledge of the Magnitude, the Pulse Duration, and the Activity Factor. The 
Overshoot/Undershoot specifications apply to the processor, regardless of whether the processor is driving or receiving 
the signal. 

4.2.3.1 Overshoot/Undershoot Magnitude  
Overshoot magnitude describes the maximum potential difference between a signal and its reference voltage level, Vss.  
Undershoot Magnitude describes the maximum potential difference between a signal and V

TT

 (undershoot).   While 

overshoot can be measured relative to VSS using one probe (probe on signal and ground lead on VSS), Undershoot must 
be measured relative to V

TT

.  This can be accomplished by simultaneously measuring the V

TT

 plane while measuring the 

signal undershoot.  The true waveform can then be calculated by the oscilloscope itself or by the following oscilloscope 
data file analysis: 

Converted Undershoot Waveform = V

TT

 - Signal 

 
Note: This 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 conversion, the Overshoot/Undershoot Specifications can be applied to the Converted Undershoot Waveform 
using the Overshoot/Undershoot Magnitude and Pulse Duration Specifications in Tables 23, 24 and 25. 
 
Overshoot/Undershoot Magnitude levels must also observe the Absolute Maximum Specifications. 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, the impact of the Overshoot/Undershoot Magnitude may be determined based 
upon the Pulse Duration and Activity Factor.

   

4.2.3.2 Overshoot/Undershoot Pulse Duration 
Overshoot/Undershoot Pulse Duration describes the total time that an Overshoot/Undershoot event exceeds the 
Overshoot/Undershoot Reference Voltage (Vos_ref = 1.635V).  This total time could encompass several oscillations 
above the Overshoot/Undershoot Reference Voltage.  Thus, multiple Overshoot/Undershoot pulses within a single 
Overshoot/Undershoot event must be measured to determine the total Pulse Duration.   
 
Note: Oscillations below the Reference Voltage cannot be subtracted from the total Overshoot/Undershoot Pulse 
Duration. 
 
Note: Multiple Overshoot/Undershoot events occurring within the same clock cycle must be considered together as one 
event.  Using the worst-case Overshoot/Undershoot Magnitude, sum together the individual Pulse Durations to determine 
the total Overshoot/Undershoot Pulse Duration for that total event. 

4.2.3.3 Overshoot/Undershoot Activity Factor 
Activity Factor (AF) describes the frequency of Overshoot/Undershoot occurrence relative to a Clock.  Since the highest 
frequency of assertion of an AGTL+ or a CMOS signal is every other clock, an AF = 1 indicates that the specific 
Overshoot or Undershoot waveform occurs EVERY OTHER clock cycle (e.g., 1-0-1-0… system bus switching pattern).  
Thus, an AF = 0.01 indicates that the specific Overshoot or Undershoot waveform occurs 1 time in every 200 CLK cycles. 
 
The specifications provided in Tables 23, 24 and 25 show the Maximum Pulse Duration allowed for a given 
Overshoot/Undershoot Magnitude at a specific Activity Factor.  Each Table entry is independent of all others, meaning 
that the Pulse Duration reflects the existence of Overshoot/Undershoot Events of that Magnitude ONLY. A platform with 
an overshoot/undershoot that just meets the Pulse Duration for a specific Magnitude where the AF < 1, means that there 
can be NO other Overshoot/Undershoot events, even of lesser Magnitude (note that if AF = 1, then the event occurs at all 
times and no other events can occur). 
 
Note:  Activity Factor for AGTL+ signals is referenced to BCLK frequency. 
 
Note:  Activity Factor for CMOS signals is referenced to PICCLK frequency.