Datasheet

SCEA019 - January 2001
Texas Instruments GTLP Frequently Asked Questions 7–89
The waveforms in Figure 9 show the effect on signal integrity in over-matched and under-
matched termination conditions. R
TT
should be less than or equal to Z
O(eff)
for optimum signal
integrity and upper noise margin.
Over-Matched Termination (R
TT
< Z
O(eff)
)
Matched Termination (R
TT
= Z
O(eff)
)
Under-Matched Termination (R
TT
> Z
)
1.5 V
GND
0.5 V
V
OL
V
OH
Noise Margin
1.0 V
Figure 9. GTLP Waveform Under Different R
TT
Conditions
The result of using an under-matched termination is that V
OL
will be closer to ground and the
upper noise margin will be less, as is clearly shown in Figure 9.
The result of a matched termination is optimal upper and lower noise margins.
The result of using an over-matched termination is that V
OL
on the card farthest from the
driver is higher and the lower noise margin is reduced.
More information on calculating distributed capacitance is in the Basic Design Considerations
for Backplanes application report, literature number SZZA016A, at
www.ti.com/sc/docs/apps/logic/appnotes.html under backplane logic.
9 What are you going to do with GTL now that you have GTLP?
TI will continue to support and expand the GTL family because these devices are well suited
for voltage-level translations, point-to-point applications, and lightly loaded bus
onboard-memory applications, e.g., Intel Pentium microprocessors operate at GTL+ signal
levels. Customers can switch between GTL and GTLP families, but must be aware of the
differences in the edge rates. GTLP devices can be used in memory applications, but, due to
the slower edge rates, the device propagation delay is longer. GTL devices should not be
used in high-frequency heavily loaded backplanes.
Intel and Pentium are trademarks of Intel Corporation.