AMD
TAXI Technical Information Publication #89-07
S u b je c t : S yn c h ro n o u s vs . As yn c h ro n o u s S t ro b e
Qu e s t io n :
When should synchronous vs. asynchronous strobing be employed?
An s w e r:
Inputs to the TAXI Transmitter can be strobed asynchronously, but with some limita-
tions. In local mode, the STROBE edge can rise at any time, without regard for place-
ment within any particular byte. The data associated with the STROBE (STRB) will be
latched into the Transmitter, and will be transmitted at the earliest opportunity. If some
system limitation insures that a second STRB cannot ever be within the same byte time
(80.0 ns at 125 MHz), then there is no need to observe the ACK output of the TAXI
Transmitter.
If no such guarantee exists, then ACK must be used to insure that no more than two
STRBS lie within a single byte. Again there is no restriction on STROBE placement
within a byte, since ACK will always insure that the capacity of the TAXI input buffer is
not exceeded. The TAXI input buffering can handle two STRBS within a single byte, but
this additional buffer must be flushed by the absence of a STRB in a given byte before
another two-strobe byte is encountered.
In an early revision of the 70 MHz Transmitter, there was a particular placement of
STROBE with respect to the falling edge of CLK, that could cause loss of data. This lead
to several revisions of technical literature mentioning the Prohibited Zone, and Uncer-
tainty Zone. This restriction in STROBE placement has been removed in the 125 MHz
version of the Transmitter. T6 in the data sheet, now refers only to the exact placement
of the internal byte boundary. Knowledge of this time is only important for synchronous
systems to predict in which byte the data will appear.
The only restriction on Local mode asynchronous STRB input would be in systems
which require contiguous data output from the TAXI Receiver. It is possible that when
strobing asynchronously, there will be an occasional byte with no strobe, and another
nearby byte with two strobes. The Transmitter has sufficient buffering to handle this
condition, but will pad the missing byte with a Sync character. For systems that have this
restriction, STRB should be made synchronous with the CLK of Transmitter.
TAXIchip Integrated Circuits Technical Manual
101
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