AM7968-125JC datasheet(65/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

using the Sync symbol to define byte boundaries. If the byte boundaries must be

re-aligned (on power-up or re-acquisition of signal), the logic will ensure that the CLK is

stretched (never shortened) upon re-sync to the new byte alignment. Due to this

behavior, the CLK output from the Receiver is not suitable as a direct frequency

reference for another TAXI Transmitter or Receiver. CLK is intended to be used by the

host system as a clock synchronous with the received data.

Figure 4-2

Cascaded Receiver Clock Connections

SERIN– SERIN+

12.5 MHz

Crystal

OSC

RX1

Am7969

(Primary Receiver)

VCC

CNB

CLK

IGM

DMS

RX2

Am7969

IGM

DMS

RX3

Am7969

IGM

DMS

CNB

N/C

From Serial Media

12330E-6

4.2.1 Cascade Mode Receivers (Am7969-125 Only)

When using an on-board TTL clock source, Receivers which are in Cascade mode

should have their X1 pin tied to the Crystal Oscillator and their X2 pin grounded.

Figure 4-2 shows a typical cascaded Receiver clock connection. The frequency source

for the Local mode Receiver should be either a crystal oscillator (as shown) or another

external TTL source. It should not be the CLK output of another Receiver. As discussed

above, the CLK output from the Receiver is not suitable as a frequency source for other

TAXI Receivers.

5.0 INTERFACING WITH THE SERIAL MEDIA

The Am7968/Am7969 TAXlchip set is capable of providing a high speed point-to-point

serial link over fiber-optic, coaxial, or twisted pair media. The choice of the appropriate

medium depends primarily on line length and data rate. This chapter discusses the

issues involved in media choice and the requirements for driving different types of

media.

Any TAXIchip set to media interface design must first take into account the electrical

properties of the TAXI Transmitter and TAXI Receiver. The Transmitter serial output

drivers are open emitter, emitter followers which generate pseudo-ECL (PECL) levels

when terminated by pull-down resistors to a voltage more negative than VOL. PECL is

ECL referenced to the +5 V supply, so that VOH = (5–0.8) and VOL = (5–1.8) volts. A safe

termination voltage which guarantees meeting VOL is 3 V or less. The Receiver input is a

long-tailed pair which will switch on 50 mV differential input voltage, with a large

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