AM7968-125JC datasheet(99/127 Pages) AMD

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TAXIchip Integrated Circuits Technical Manual

TAXI TIPs

TAXI Technical Information Publication #89-01

Subject: Receiver Response to Loss of Input Signal

Question:

It is desired that the TAXI Receiver outputs be predictable and stable during conditions

when the TAXI Transmitter may cease transmitting (power-off) or is disconnected. How

can a system designer predict the TAXI Receiver outputs or use the TAXI Receiver in a

system where the TAXI Receiver must appear

Inactive under these conditions? This

applies to both Coaxial and Fiber-Optic Systems.

Answer:

The key to this problem is interpreting the loss of incoming Tx data as a Quiet-Line-

State and either flagging the system accordingly or gating the TAXI Receiver outputs

with an

inactive flag.

In a Coaxial coupled system the loss of incoming signal drive will cause the TAXI

Receiver inputs to rest at the input termination bias voltages. The differential serial

inputs will normally be at equal potential. To properly interpret a loss of signal as a

constant

quiet state the input termination bias voltages must be altered slightly from

each other. To allow reliable interpretation of the offset as a constant logic state, the

offset voltage should typically be set to about 50 mV.

The TAXI Receiver will interpret the

Quiet-Line-State differently depending upon the

operation mode (8-bit, 9-bit, or 10-bit). In 8-bit mode the Receiver will generate continu-

ous CSTRB’s with the Command outputs at F Hex, or all HIGH. In the 9-bit and 10-bit

modes there is no defined interpretation of an incoming quiet data stream. This will

cause the TAXI Receiver to generate continuous CSTRB’s and the Violation output will

be continuously HIGH. A one-shot may be used to determine Violation=HIGH duration

and then generate an

inactive flag, or the system may interpret the Violation output

directly as an

inactive or invalid condition flag and halt data-dependent system opera-

tions during any byte with Violation=HIGH.

In a Fiber-Optic coupled system two methods may be used, depending on the Optical

Receiver construction. If the Optical Receiver has a

Carrier-Detect output this signal

may be used to flag an inactive state. If there is no

Carrier-Detect, one may be gener-

ated using an ECL one shot arrangement which will detect loss of edges after a

predetermined period of say, for example, 80 bit times. The actual time may vary

depending on the desired response to loss of data.

In either condition, if the Receiver recovered Byte Clock (CLK) is used as a system

clock, then the system must be able to tolerate a shift in the CLK frequency of typically

about +/-3% to allow for Rx-PLL offset and drift during this period. (If the Optical

Receiver begins oscillation when the fiber is dark, the TAXI Receiver PLL may attempt

to track the oscillation resulting in an indeterminate Rx recovered clock frequency.

Optical Receiver dark response thus becomes a possible system concern.)

AM7968-125JC Datenblatt(PDF) 99 Page - Advanced Micro Devices

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