Datasheet

SCEA022
7–257
Achieving Maximum Speed on Parallel Buses With Gunning Transceiver Logic (GTLP)
1 Introduction
Driver modules that are set up for a modern-wiring backplane have many complex requirements.
The drivers must switch fast, with the smallest possible propagation time and the greatest
possible number of bits in one driver, to provide the necessary data throughput over the bus.
In addition, the driver should have a sufficiently high driver capacity to generate the required
logic levels, even in the case of a full backplane with many interconnected receive and transmit
modules. Also, the drivers should have a small noise potential and a large noise margin. The
optimum driver should use little power and have the live-insertion with precharge property, which
allows exchanging modules during operation without impairing the function of the overall system.
In many electronic systems, shutdown to change a defective system part is not possible without
severe adverse consequences for the complete application. Examples are electronic
telecommunications systems, the control center in an electrical power-supply company, or
computers used by air-traffic controllers.
The GTLP bus drivers address all the above points: minimum propagation time, high-drive
capability, and the capability of being inserted into, or removed from, the system during
operation, and have been developed especially for this type of application.
This application report emphasizes system speed, which is achieved by the appropriate
architecture, using GTLP devices.
Section 2 describes physical basics of backplanes. Influences of the number of modules and the
distance between modules on the properties of transfer speed and line impedance are
explained.
Section 3 discusses two setups, both of which are based on a synchronous system design using
GTLP devices:
• Central-synchronous system clock (CSSC) – All system cards use the same clock as base
for the data.
• Source-synchronous system clock (SSSC) – The transmitter also sends the clock signal, via
an additional line, along with the data.
The theoretical considerations are confirmed by measurements using the GTLP demonstration
board.
In principle, both systems can be set up using technologies other than GTLP. However, the
combination of the advantages of high-driver capability, live-insertion capability, and flexible
edge steepness that can be modified during operation, is found only with the GTLP bus-driver
solution.