Intel Xeon Processor Multiprocessor Platform Design Guide

101
Methodology for Determining Topology and Routing Guidelines
An edge is defined to be linear if it exhibits a linear shape between V
IH
and V
IL
. Figure 9-7 depicts
a linear edge. Figure 9-9 depicts a nonlinear edge. For non-clock and non-strobe signals, the signal
seen at the receiver should be linear between V
IH
and V
IL
. On a case-by-case basis where it can be
shown that the timing can be met with no potential data corruption, it may not be necessary for the
signal to be linear between V
IH
and V
IL
.
9.2.4.2 Ringback
Ringback is defined as the amount of voltage that “rings” back towards the threshold voltage and is
measured at the receiver.
For non-clock and non-strobe signals, the signal at the receiver should not ringback into the noise
margin region unless it can be shown that the timing can be met with no potential data corruption.
9.2.5 Timing Metrics
The timing metrics consists of flight time and flight time skew. Flight time is defined as the amount
of time between the point where the signal on the unloaded driver (or loaded with a test load)
crosses a certain threshold and the time where the signal crosses the threshold at the receiver. Flight
time skew is the difference in flight times between two nets. Figure 9-7 illustrates the definition of
flight time (assuming a linear edge from V
IL
through V
IH
). The flight time should be evaluated at
V
IL
, V
threshold
, and V
IH
. The methodology is to measure flight time at these three points and record
the worst case. This is valid as long as the edge rate seen at the receiver is equal to the edge rate at
which it was characterized (the tester edge rate). If the device was characterized at a different edge
rate than the system edge rate, then the following procedures for setup and hold time calculations
should be used.
9.2.5.1 Setup Flight Time
If the edge rate seen at the receiver is faster than the specified edge rate, then the traditional method
should be used. If the edge rate seen at the receiver is slower than the specified edge rate, then the
flight time must be extrapolated from V
IL
or V
IH
to V
threshold
at the specified edge rate. See
Figure 9-8 for more details.
9.2.5.2 Calculating Flight Time for Signals with Corrupt Signal Quality
If nonlinearity or ringback occurs between V
IL
and V
IH
, then the last crossing should be
extrapolated back at the specified minimum edge rate to V
threshold
. See Figure 9-9 and Figure 9-10.