AMD
TAXI Technical Information Publication #89-11
S u b je c t : Us in g Re c e ive r CLK Ou t p u t t o Ru n a TAXI Tra n s m it t e r
Qu e s t io n :
Is it possible to use the Receiver CLK output to drive the X1 input of a TAXI Transmitter?
An s w e r:
To assure accurate transmission of data, the Transmitter must have a stable, jitter free,
byte rate reference to its multiplying PLL. This is typically derived from a crystal and can
be connected to any crystal controlled and noise free TTL source.
The Receiver synchronizes its internal clock with the incoming signal and recovers data
and clock for use by the receiving host system.
In the process of sending high speed data over typical serial links, the data may be
affected by noise from various sources. The PLL in the Receiver removes this noise and
delivers a synchronized clock to the Receiver logic and to the host system. However,
some of the noise may feed through the PLL and appear on the CLK output. The CLK
output can jitter as much as 2 ns when recovering data from a noisy link. This will not
typically affect normal logic functions, and can be ignored. If the Receiver must realign
its byte boundary, it will stretch CLK to the new alignment position and thus protect the
host logic from shortened CLK cycles.
These noise and phase jumps make the Receiver CLK output undesirable for use as a
Transmitter frequency source.
For systems that MUST use synchronized clocks (for example to avoid FIFO re-timing
logic) it is possible to filter the Receiver CLK output and make an adequate reference for
the Transmitter.
There are two basic approaches to provide this filter. The first is to use a crystal filter
(Figure 12). When placed between the Receiver CLK and Transmitter Xl, the crystal
filter can be effective in attenuating system jitter to levels nearly comparable to crystal
controlled reference clock levels. By the nature of a crystal filter, as the frequency of the
crystal used in the filter and the data rate frequency vary, the phase of the output varies.
This will make the filter seem to have a variable delay (+ or -) which must be accommo-
dated by the users logic.
The second method is to use a PLL tracking filter (Figure 13). The jitter attenuation
through the PLL is less than that through the crystal filter because the PLL has a
bandwidth several orders of magnitude larger. The PLL provides a solution whose
merits lie between the simplicity of the crystal filter and the need for tight crystal tracking
and matching. The PLL filter is relatively straight forward. Attention to proper grounding
and board layout should be followed. The PLL filter is more tolerant of component and
environmental variations than the crystal filter.
TAXIchip Integrated Circuits Technical Manual
105
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