XRT74L73 Datasheet, PDF (88/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
Note: regarding Figure 5
1. The Transmit UTOPIA Data Bus is configured to be
16 bits wide. Hence, the data which the ATM Layer
processor places on the Transmit UTOPIA Data Bus is
expressed in terms of 16-bit words: (e.g., W0–W26).
2. The Transmit UTOPIA Interface block is configured
to handle 54 bytes/cell. Hence, Figure 5 illustrates the
ATM Layer processor writing 27 words (W0 through
W26) for each ATM cell.
In Figure 5 , TxUClav is initially “high” during clock edge
# 1. However, shortly after the ATM Layer processor
writes in word W20, TxUClav toggles “low”, indicating
that the TxFIFO is starting to fill up. The ATM Layer pro-
cessor will detect this “negation of TxUClav” during clock
edge #2; while it is writing word W21 into the Transmit
UTOPIA Interface block. At this point, the ATM Layer
processor is only permitted to execute four more “write”
operations with the Transmit UTOPIA Interface block.
Therefore, the ATM Layer processor will proceed to write
in words: W22, W23, W24 and W25 before negating
TxUEn. The ATM Layer processor must keep TxUEn
negated until it detects that TxUClav has once again
returned “high”. In Figure 5 , TxUClav is asserted after
clock edge #8. The ATM Layer processor detects this tran-
sition in TxUClav at clock edge #9; and subsequently,
asserts TxUEn. The ATM Layer resumes writing in more
ATM cell data into the Transmit UTOPIA Interface block.
2.1.2.3.0.2Cell-Level Handshaking
The UNI will be operating in the “Cell-Level” Hand-
shaking mode following power up or reset. In the
“Cell-Level” Handshaking mode, when the TxUClav is
at a logic “1”, it means that the TxFIFO has enough
remaining empty space for it to receive at least one
more full cell of data from the ATM Layer processor.
However, when TxUClav toggles from “high” to “low”,
it indicates that the very next cell (following the one
that is currently being written) cannot be accepted by
the TxFIFO. Conversely, once TxUClav has returned
to the logic “1” level, it indicates that at least one
more full cell may be written into the TxFIFO by the
ATM Layer processor. As in the “Octet-Level” Hand-
shake mode, the ATM Layer processor is expected to
poll the TxUClav output towards the end of transmis-
sion of the cell currently being written and to proceed
with transmission of the next cell only if TxUClav is at
a logic “high”.
The UNI can operate in either the “Octet-Level” or the
“Cell-Level” Handshake mode, when operating in the
Single-PHY mode. However, only the “Cell-Level”
Handshake Mode is available when the UNI is oper-
ating in the Multi-PHY mode. For more information on
Single PHY and Multi PHY operation, please see
Section 6.1.2.3.
The UNI can be configured to operate in one of these
two handshake modes by writing the appropriate data
to Bit 5 (Handshake Mode) within the UTOPIA Con-
figuration Register, as depicted below.
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