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TAXIchip Integrated Circuits Technical Manual

3.2 Violation Logic

The TAXI Receiver logic has been designed to detect the most common types of

transmission errors. It detects these errors by completely decoding the incoming data

patterns, and recognizes the following types of VIOLATIONS:

1. Illegal, reserved or unused data patterns (pure Violations).

2. Unused COMMAND combinations.

3. COMMAND in Upper half and DATA in Lower half pattern.

4. DATA in Upper half and COMMAND in Lower half pattern.

Type 1 and 2 VIOLATIONS are decoded and interpreted as either DATA or COMMAND

outputs with the appropriate STRB output. Type 3 & 4 VIOLATIONS are decoded as

COMMAND outputs with a CSTROBE output (even though one half would have been

transformed DATA/COMMAND or COMMAND/DATA by an error), since there is no

information available to the TAXI Receiver to indicate where the error lies. The user

needs to be aware of this possible transposition (possible with all four types of VIOLA-

TION), since the system must account for it. VIOLATION will always be the flag for

these detectable errors.

This method of detection is not 100% effective. As Appendix B shows, it will detect

approximately 50% of the possible double bit errors in Data. Double bit errors in

Command will be detected 99.8% of the time or more, depending upon the pattern

width. Appendix B contains a more detailed treatment of the efficiency of the violation

logic for the various data bit modes.

The method of detecting violations, is effective enough to be used to give an early

warning of transmission problems before the host’s error detection system would detect

the errors. It should not be used alone in fault sensitive systems, since it misses a

significant number of transmission errors which cause one valid DATA pattern to alias to

another VALID DATA PATTERN.

3.3 TAXI PLL Characteristics

The Phase Locked Loop in the TAXI Receiver is used to recover the data encoded in

the serial bit stream sent by the TAXI Transmitter. In order to ensure accurate data

recovery, the Receiver PLL must lock on to the underlying code rate of the Transmitter,

and must track minor changes in frequency and phase while rejecting noise superim-

posed on the bit stream. This noise includes both amplitude and phase/frequency

disturbances. Amplitude variations are dealt with in the Receiver’s input amplifier

(SERIN+/-), and are not passed through to the PLL, except for phase effects.

Phase/frequency noise, or jitter, can come from many sources, and can have many

different characteristics. Jitter can be introduced by the Transmitter, the Receiver, the

media interface or by the media itself. Examples of media induced jitter include reflec-

tions and edge perturbations caused by improper line termination, pulse width spreading

due to frequency dependent cable attenuation, and pulse dispersion caused by fiber

optic cable effects. Examples of media interface jitter include low light effects in optical

receivers and pulse width distortion caused by baseline shift (changing DC offset) in AC

coupled amplifiers.

The TAXI PLL has been optimized to allow correct data recovery in the presence of the

largest jitter possible. To this end, the PLL parameters, most notably loop bandwidth,

have been chosen to enhance the jitter tolerance of the TAXI Receiver.

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