Specifications

PENTIUM® PRO PROCESSOR AT 150, 166, 180, and 200 MHz E
40
Table 20. Specifications for Signal Quality
Parameter Description Specification
Maximum Signal
Overshoot/Undershoot
Maximum Absolute voltage a signal extends above V
TT
or below
V
SS
(simulated w/o protection diodes).
0.3 V
(guideline)
Settling Limit The maximum amount of ringing, at the receiving chip pad, a
signal must be limited to before its next transition. This signal
should be within 10% of the signal swing to its final value, when
either in its high state or low state.
±10% of
(V
OH
-V
OL
)
(guideline)
Maximum Signal
Ringback (Nominal)
The maximum amount of ringing allowed for a signal at a
receiving chip pad within the receiving chips setup and hold time
window before the next clock. This value is dependent upon the
specific receiver design. (Normally ringing within the setup and
hold windows must not come within 200 mV of V
REF
although
specific devices may allow more ringing and loosen this
specification. See Section 4.1.3.1 for more details.)
V
REF
±200 mV
The overshoot/undershoot guideline is provided to
limit signals transitioning beyond V
CC
or V
SS
due to
fast signal edge rates. Violating the
overshoot/undershoot guideline is acceptable, but
since excessive ringback is the harmful effect
associated with overshoot/undershoot it will make
satisfying the ringback specification very difficult.
Violations of the Settling Limit guideline are
acceptable if simulations of 5 to 10 successive
transitions do not show the amplitude of the ringing
increasing in the subsequent transitions. If a signal
has not settled close to its final value before the next
logic transition, then the timing delay to V
REF
of the
succeeding transition may vary slightly due to the
stored reactive energy in the net inherited from the
previous transition. This is akin to "eye" patterns in
communication systems caused by inter-symbol
interference. The resulting effect is a slight variation
in flight time.
4.1.3.1. Ringback Tolerance
The nominal maximum ringback tolerated by GTL+
receivers is stated in Table 20, namely: no closer to
V
REF
than a ±200 mV overdrive zone. This
requirement is usually necessary to guarantee that a
receiver meets its specified minimum setup time
(T
SU
), since set-up time usually degrades as the
magnitude of overdrive beyond the switching
threshold (V
REF
) is reduced.
Exceptions to the nominal overdrive requirement can
be made when it is known that a particular receiver’s
setup time (as specified by its manufacturer) is
relatively insensitive (less than 0.05 ns impact) to
well-controlled ringing into the overdrive zone or even
to brief re-crossing of the switching threshold, V
REF
.
Such “ringback-tolerant” receivers give the system
designer more design freedom, and, if not exploited,
at least help maintain high system reliability.
To characterize ringback tolerance, employ the
idealized Lo-to-Hi input signal shown in Figure 34.
The corresponding waveform for a Hi-to-Lo transition
is shown in Figure 35. The object of ringback
characterization is to determine the range of values
for the different parameters shown on the diagram,
which would maintain receiver setup time and correct
logic functionality.
These parameters are defined as follows:
τ is the minimum time that the input must spend, after
crossing V
REF
at the high level, before it can ring
back, having overshot V
IN_HIGH_MIN
by at least α,
while ρ, δ, and φ (defined below) are at some preset
values, all without increasing T
SU
by more than
0.05 ns. Analogously for Hi-to-Lo transitions.