AM7968-125JC datasheet(77/127 Pages) AMD

77 / 127 page

AMD

TAXIchip Integrated Circuits Technical Manual

8. When terminating serial lines to or from the TAXls ensure that the Vcc rail or ground

tap is not at a noisy location. Resistors can couple noise from a power supply rail into

the Serial lines. Vcc to Ground decoupling adjacent to the resistors is recommended

when using pullup/pulldown terminating resistor setups as shown in Figure 6-4.

When using only a pulldown, do not use decoupling as this could add more Vcc noise

into the serial signals.

Figure 6-4

Decoupling Terminations

12330E-21

TAXI

+5 V

TAXI

–

+5 V

No Decoupling

Capacitor Required

Decoupling

Capacitor

GND

6.2 Layout using Fiber Optic Data Links

Because of their small signals levels, fiber optic data links require some care in layout.

Fiber optic data Link receivers consist of a photo sensitive diode and an amplifier. The

photo-diode converts light pulses into currents of around a few hundred nano-amps.

This signal current is then amplified and translated into an ECL signal.

TAXI Receivers and most digital chips switch hundreds of milliamps. If switching noise

from the digital section of the board gets coupled into the optical data link, the signal

from the light pulse data can be corrupted. To prevent the coupling of the optical data

link output with other digital signals, the user must ensure that small signal and digital

switching currents do not flow in the same path. This is done by separating both the

optical VCC plane and the optical ground plane from the VCC and Ground planes used by

other digital circuitry. See Figure 6-5.

AM7968-125JC Datasheet(PDF) 77 Page - Advanced Micro Devices