Specifications

PENTIUM® PRO PROCESSOR AT 150, 166, 180, and 200 MHz E
44
If either the rising or falling edge is slower than
0.3V/ns through the overdrive region beyond V
REF
,
(i.e., does not always stay ahead of an 0.3 V/ns line),
then the flight time for a rising edge is determined by
extrapolating back from the signal crossing of V
REF
+200 mV to V
REF
using an 0.3 V/ns slope as
indicated in Figure 27.
Figure 27. Flight Time of a Rising Edge Slower Than 0.3V/ns
If the signal is not monotonic while traversing the
overdrive region (V
REF
to V
REF
+200 mV rising, or
V
REF
to V
REF
-200 mV falling), or rings back into the
overdrive region after crossing V
REF
, then flight time
is determined by extrapolating back from the last
crossing of V
REF
± 200 mV using a line with a slope
of 0.8 V/ns
(the maximum allowed rising edge rate).
This yields a new V
REF
crossing point to be used for
the flight time calculation. Figure 28 represents the
situation where the signal is non-monotonic after
crossing V
REF
on the rising edge.
Figure 29 shows a falling edge that rings back into
the overdrive region after crossing V
REF
, and the
0.8V/ns line used to extrapolate flight time. Since
strict adherence to the edge rate specification is not
required for Hi-to-Lo transitions, and some drivers’
falling edges are substantially faster than 0.8V/ns—at
both the fast and slow corners—care should be
taken when using the 0.8 V/ns extrapolation. The
extrapolation is invalid whenever it yields a V
REF
crossing that occurs earlier than when the signal’s
actual edge crosses V
REF
. In that case, flight time is
defined to be the longer of: the time when the input at
the receiver crosses V
REF
initially, or when the line
extrapolated (at 0.8 V/ns) crosses V
REF
. Figure 29
illustrates the situation where the extrapolated value
would be used.