Table of Contents
Advertisement
GTL+ INTERFACE SPECIFICATION
The overshoot/undershoot guideline is provided to limit signals transitioning beyond V
V
due to fast signal edge rates. Violating the overshoot/undershoot guideline is acceptable, but
SS
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 tran-
sitions do not show the amplitude of the ringing increasing in the subsequent transitions. If a sig-
nal has not settled close to its final value before the next logic transition, then the timing delay
to V
of the succeeding transition may vary slightly due to the stored reactive energy in the
REF
net inherited from the previous transition. This is akin to 'eye' patterns in communication sys-
tems caused by inter-symbol interference. The resulting effect is a slight variation in flight time.
12.1.3.1.
RINGBACK TOLERANCE
The nominal maximum ringback tolerated by GTL+ receivers is stated in Table 12-3, namely:
no closer to V
than a ±200 mV overdrive zone. This requirement is usually necessary to guar-
REF
antee that a receiver meets its specified minimum setup time (T
grades as the magnitude of overdrive beyond the switching threshold (V
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
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 12-3. The corresponding waveform for a Hi-to-Lo transition is shown in Figure 12-4.
The object of ringback characterization is to determine the range of values for the different pa-
rameters 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
can ring back, having overshot V
at some preset values, all without increasing T
Lo transitions.
It is expected that the larger the overshoot α, the smaller the amount of time, τ, needed to main-
tain setup time to within +0.05 ns of the nominal value. For a given value of α, it is likely that τ
will be the longest for the slowest input edge rate of 0.3V/ns. Furthermore, there may be some
dependence between t and lower starting voltages than V
for the reason described later in the Section on receiver characterization. Analogously for Hi-Lo
transitions.
12-6
. Such "ringback-tolerant" receivers give the system designer more
by at least α, while ρ, δ, and φ (defined below) are
IN_HIGH_MIN
by more than 0.05 ns. Analogously for Hi-to-
SU
), since setup time usually de-
SU
) is reduced.
REF
at the High level, before it
REF
–0.2V (for Lo-to-Hi transitions)
REF
or
CC
Advertisement
Table of Contents
