Datasheet

SCEA019 - January 2001
7–96 Texas Instruments GTLP Frequently Asked Questions
Propagation delay is a function of the edge rate. Only high-drive GTLP devices are able to
operate at both slow and fast edge rates, with a maximum t
pd
of about 7 ns and 5.5 ns,
respectively. Medium-drive devices operate only at the slow edge rate, with a maximum t
pd
of
about 7 ns. These are design goals, and actual values may be different.
Setup and hold times and skew are inherent in the device.
Backplane flight time depends on backplane length and construction (microstrip or stripline).
Microstrip places the trace on top of the printed circuit board and does not reduce the flight
time as much as stripline, but it is more susceptible to EMI. Stripline places the trace between
printed circuit board dielectric layers and is less susceptible to EMI, but the signals are slower
and, consequently, flight time is longer. Most high-quality backplanes use stripline
construction to minimize EMI. Proper backplane layout and stub lengths of less than 1 inch
help minimize backplane capacitance.
Most backplanes use a system clock that provides an absolute clock signal to all cards at the
same time. Maximum system frequency is reduced based on backplane length, as shown in
Figure 13, to accommodate the difference between the time-of-arrival of the driver card and
receiver card clock signal (flight time), respectively. Source-synchronous operation is a
technique in which the clock is sent with the system data as a relative clock signal. This
means that there is no delay due to backplane flight time because the clock is sent along with
the data. Flight time always is zero; therefore, maximum frequency is independent of
backplane length.
Studies performed using the 19-inch 20-slot GTLP EVM with the SN74GTLPH1655 DGGR
device show that the maximum frequency is 46 MHz in system-clock mode and 120 MHz in
source-synchronous clock mode, and, for short time periods, 160 MHz in asynchronous mode.
In summary, maximum frequency (MHz) is the inverse of the maximum delay time (ns), i.e.,
10 ns = 100 MHz. Maximum delay time depends on the sum of these factors:
• Maximum propagation delay
– Fast edge (5.5 ns)
– Slow edge (7 ns)
• Setup time (2.5 ns)
• Skew time (1 ns)
• Backplane flight time
– Source synchronous - flight time is zero and is independent of backplane length
because the clock is sent with the data
– Microstrip (1 ns/6 inches)
– Stripline (~2 ns/6 inches)