Specifications

E PENTIUM® PRO PROCESSOR AT 150, 166, 180, and 200 MHz
43
4.1.4. AC Parameters: Flight Time
Signal Propagation Delay is the time between when a
signal appears at a driver pin and the time it arrives
at a receiver pin. Flight Time is often used
interchangeably with Signal Propagation Delay but it
is actually quite different. Flight time is a term in the
timing equation that includes the signal propagation
delay, any effects the system has on the T
CO
of the
driver, plus any adjustments to the signal at the
receiver needed to guarantee the T
SU
of the receiver.
More precisely, Flight Time is defined to be:
The time difference between when a signal at
the input pin of a receiving agent (adjusted to
meet the receiver manufacturer’s conditions
required for AC specifications) crosses V
REF
,
and the time that the output pin of the driving
agent crosses V
REF
were it driving the test
load used by the manufacturer to specify that
driver’s AC timings.
An example of the simplest Flight Time measurement
is shown in Figure 26. The receiver specification
assumes that the signal maintains an edge rate
greater than or equal to 0.3 V/ns at the receiver chip
pad in the OverDrive processor region from V
REF
to
V
REF
+200 mV for a rising edge and that there are no
signal quality violations after the input crosses V
REF
at the pad . The Flight Time measurement is similar
for a simple Hi-to-Lo transition. Notice that timing is
measured at the driver and receiver
pins while signal
integrity is observed at the receiver chip
pad . When
signal integrity at the pad violates the guidelines of
this specification, and adjustments need to be made
to flight time, the adjusted flight time obtained at the
chip pad can be assumed to have been obtained at
the package pin, usually with a small timing error
penalty.
The 0.3V/ns edge rate will be addressed later in this
document, since it is related to the conditions used to
specify a GTL+ receiver’s minimum set-up time.
What is meant by edge rate is neither instantaneous,
nor strictly average. Rather, it can best be described
for a rising edge—by imagining an 0.3 V/ns line
crossing V
REF
at the same moment that the signal
crosses it, and extending to V
REF
+200 mV, with the
signal staying ahead (earlier in time) of that line at all
times, until it reaches V
REF
+200 mV. Such a
requirement would always yield signals with an
average edge rate >0.3 V/ns, but which could have
instantaneous slopes that are lower or higher than
0.3V/ns, as long as they do not cause a crossing of
the inclined line.
Figure 26. Measuring Nominal Flight Time