Datasheet
SCEA019 - January 2001
7–80 Texas Instruments GTLP Frequently Asked Questions
6 What is GTL?
GTL is a reduced-voltage-swing (<1 V), open-drain/collector, differential-input JEDEC
standard that allows higher frequency operation than TTL devices in point-to-point and lightly
loaded memory-interface-bus applications. The reduced voltage swing reduces EMI and
allows higher frequencies. For best signal integrity, the open-drain/collector arrangement
facilitates matching the termination resistor (R
TT
) to the trace impedance (Z
O
). As shown in
Figure 1, the open-drain/collector pulls the signal low when the device is turned on, but when
the device is turned off, the pullup termination resistor is required to pull the signal high to the
termination voltage (V
TT
). The GTL standard is V
TT
/V
OH
= 1.2 V, V
OL
= 0.4 V, and V
IH
and
V
IL
set ±50 mV around the GTL differential-input reference voltage (V
ref
) of 0.8 V.
V
TT
= 1.2 V
R
TT
= Z
O
Line Driver
Receiver
V
ref
= 0.8 V
Figure 1. GTL Open-Drain/Collector Differential-Input Device
Open-drain and open-collector devices operate in exactly the same way to sink energy to
ground. Open drain refers to CMOS transistors, while open collector refers to bipolar
transistors. GTL devices can be manufactured using either process.
GTL+ is a variation on GTL that moves V
ref
from 0.8 V to 1.0 V so that it is farther from the
ground-bounce region. GTL+ voltage swing improves the upper and lower noise margins.
Voltage swing is from V
OL
= 0.55 V to the termination voltage of V
TT
/V
OH
= 1.5 V.
Any device that operates at GTL signal levels also can operate at GTL+ signal levels, and vice
versa, with GTL+ being the preferred signal level due to its higher noise margins. In unusually
noisy situations, V
ref
can be adjusted to other than 0.8 V or 1.0 V in either standard to
equalize, and, therefore, maximize both the upper and lower noise margins.
Open-Collector
BiCMOS/BiPolar
Open-Drain
CMOS