AM7968-125JC datasheet(68/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

threshold level of the receiver’s differential amplifier. Once the Receiver recognizes the

state change, variations in the falling edge are not significant.

To avoid edge rate variations due to driver turn-off, we must equate the voltage to which

the driver is taken at turn-off with a point in the logic swing which will guarantee that the

Receiver changes state. Since PECL logic swings are 800 mV, we may safely choose a

500 mV change at the driver (100 mV past the midpoint) as a guaranteed state change

at the Receiver. If the driver turns off instantly, we require the voltage divider formed by

RE and Z0 to produce a 500 mV change from VOH. We can write:

VOH RE/(RE + Z0) = VOH – 0.5

4.1 x RE/(RE + Z0) = 3.6

RE = 7.33 Z0

As a general rule, we may then say that:

186 < RE < 7.33 Z0

Figure 5-3

Pull-Down and Termination for AC Coupled Link

12330E-10

VCC

SERIN

TAXI RX

SEROUT

TAXI TX

Pseudo-ECL

Driver

Copper Media

Figure 5-4

Serial Link with Output Driver Model

12330E-11

VCC

SERIN

TAXI RX

SWI

TAXI TX

Pseudo-ECL

Driver (Model)

Copper Media

VOH

5.4 Baseline Wander and the AC Coupling Capacitor

The 4B/5B and 5B/6B data encoding schemes which are used by the TAXIchip set are

run-length limited to a maximum of 3 consecutive LOW states (non-transitions in NRZI).

This type of encoding ensures that on average there will be less than

±10% variation in

the DC component of the encoded data.

When the encoded data is passed through an AC coupled link, the high-pass filtering of

the AC coupling will introduce jitter because of the fluctuating threshold caused by the

variation in DC component. This undesired side-effect of AC coupling is often described

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