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PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
279
This section presents a detailed description of “Single-
PHY” operation. Whenever the ATM Layer processor
is responsible for receiving cell data from the Receive
UTOPIA Interface block, it must do the following.
Check the level of the RxClav pin
If the RxClav pin is “high” then the RxFIFO contains
some ATM cell data that needs to be read by the ATM
Layer processor. In this case, the ATM Layer proces-
sor should begin to read the cell data from the Receive
UTOPIA Interface block. However, if the RxClav pin is
“l(fā)ow”, then the RxFIFO does not contain any cell data
that can be read. In this case, the ATM Layer proces-
sor should wait until RxClav toggles “high” before
attempting to read any more cell data from the
“Receive UTOPIA Interface block”.
1.
Note:
The actual meaning associated with RxClav toggling
“high” or “l(fā)ow” depends upon whether the UNI is operating
in the “Cell Level” or “Octet Level” handshake modes.
Assert the RxEnB* pin and read the first byte (or
word) of the new cell from the Receive UTOPIA
Data Bus.
Once the ATM Layer processor has detected that
RxClav has toggled “high”, then it should assert the
RxEnB* input pin (e.g., toggling it “l(fā)ow”). Once the
Receive UTOPIA Interface block has determined that
the RxEnB* input pin is “l(fā)ow”, then it will begin to
place some cell data onto the Receive UTOPIA Data
Bus. If this first byte (or word) is the beginning of a
2.
new ATM cell, then the ATM Layer processor should
verify that this byte (or word) is indeed the beginning
of a new cell, by observing the RxSoC output pin (of
the UNI IC) pulsing “high” for one clock period of RxClk.
Compute the odd-parity of the byte (or word) that
is being read from the Receive UTOPIA Data
bus, and compare the value of this parity bit with
that of the RxPrty output pin.
This operation is optional, but should be done concur-
rently while checking for the assertion of the RxSoc
output pin.
When reading in the subsequent bytes (or words) of
the cell, the ATM Layer must do the following.
Repeat Steps 1 and 2.
If the UNI is operating in the Octet-Level Hand-
shake mode, then the ATM Layer processor should
check the RxClav level prior to asserting the
RxEnB* (Receive UTOPIA Interface—Output Enable)
pin. The ATM Layer processor should only attempt
to read the contents of the Receive UTOPIA Data
Bus if the RxClav signal is “high”.
If the UNI is operating in the Cell-Level Handshake
mode, then the ATM Layer processor should check
the RxClav signal level just as it (the ATM Layer
processor) is reading in the very last byte (or word)
of a given cell. If the RxClav level is “high”, then the
ATM Layer processor should proceed to read in the
next cell from the Receive UTOPIA Interface block.
3.
F
IGURE
88. S
IMPLE
I
LLUSTRATION
OF
S
INGLE
-PHY O
PERATION
DS3 UNI
ATM Switch
(ATM Layer Device)
TxData[15:0]
TxClav
TxSoC
TxEnB*
TxPrty
RxData[15:0]
RxClav
RxSoC
RxEnB*
RxPrty
TxPOS
TxNEG
TxLineClk
RxPOS
RxNEG
RxLineClk
Tx Flow Control Input
Tx Start of Cell Output
Tx Write Enable Output
Tx Utopia Data Bus Parity
Tx FIFO Clock Signal
To/From
DS3 LIU
TxClk
RxClk
Rx Flow Control Input
Rx Start of Cell Input
Rx Read Output Enable Signal
Rx Utopia Data Bus Parity
Rx FIFO Clock Signal
Rx ATM Cell Data
Tx ATM Cell Data