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PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
275
Note:
regarding Figure 86
1. The Receive UTOPIA Data bus is configured to be
16 bits wide. Hence, the data which the Receive
UTOPIA Interface block places on the Receive
UTOPIA Data bus is expressed in terms of 16 bit
words (e.g., W0–W26).
2. The Receive UTOPIA Interface block is configured
to handle 54 bytes/cell. Hence, Figure 86 illustrates
the ATM Layer processor reading 27 words (W0
through W26) for each ATM cell.
In Figure 86, RxClav is initially “l(fā)ow” during clock edge
#1. However, shortly after clock edge 1, the RxFIFO
receives ATM cell data from the Receive Cell Processor
block. At this point, the RxClav signal toggles “high”
indicating that the RxFIFO contains at least one
“read-cycle” worth of cell data. The ATM Layer pro-
cessor will detect this “assertion of RxClav” during
clock edge #2. Consequently, in order to begin read-
ing this cell data, the ATM Layer processor will then
assert the RxEnB* input pin. At clock edge #3, the
Receive UTOPIA Interface block detects RxEnB*
being “l(fā)ow”. Hence, the Receive UTOPIA Interface
block then places word W0 on the Receive UTOPIA
Data bus. The ATM Layer processor latches and
reads in W0, upon clock edge #4. In this figure,
shortly after the ATM Layer processor has read in
word W1 (at clock edge #5), the RxFIFO is depleted
which causes RxClav to toggle “l(fā)ow”. In this figure,
the ATM Layer processor will keep the RxEnB* signal
asserted, and will read in an “invalid” word which is
denoted by the “X” in Figure 86. Shortly thereafter,
the RxFIFO receives some additional cell data from
the Receive Cell Processor, which in turn causes
RxClav to toggle “high”. The ATM Layer processor
then continues to read in words W2 and W3. After-
wards, the ATM Layer processor is unable to continue
reading the ATM cell data from the Receive UTOPIA
Interface block; and subsequently negates the RxEnB*
signal; at clock edge #8. The Receive UTOPIA Inter-
face block detects that RxEnB* is “high” at clock edge
#8, and in turn, tri-states the Receive UTOPIA Data
Bus at around clock edge # 9. Finally, prior to clock
edge #11, the ATM Layer processor is able to resume
reading in ATM cell data from the Receive UTOPIA
Interface block, and indicates this fact by asserting
the RxEnB* (e.g., toggling it “l(fā)ow”). The Receive
UTOPIA Interface block detects this state change at
clock edge #11 and subsequently places word W4 on
the Receive UTOPIA Data bus.
7.4.2.2.1.2
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 RxClav
output is at a logic “1”, it means that the Rx FIFO
contains at least one complete ATM cell of data that
is available for reading by the ATM Layer Processor.
When RxClav toggles from “high” to “l(fā)ow”, it indicates
that Rx FIFO contains less than one complete ATM
cell. As in the “Octet-Level” Handshake mode, the ATM
Layer processor is expected to monitor the RxClav
output, and quickly respond and read the Rx FIFO,
whenever the RxClav output signal is asserted.
The UNI can operate in either the “Octet-Level” or
“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 7.4.2.2.2.
Cell Level Handshaking
F
IGURE
86. T
IMING
D
IAGRAM
OF
R
X
C
LAV
/R
X
E
MPTY
B
AND
VARIOUS
OTHER
SIGNALS
DURING
READS
FROM
THE
R
ECEIVE
UTOPIA,
WHILE
OPERATING
IN
THE
O
CTET
-L
EVEL
H
ANDSHAKING
M
ODE
.
RxClk
RxClav
RxEnB*
RxData[15:0]
RxSoC
W2
W3
W4
X
W1
W0
1
2
3
4
5
6
7
8
9
10
11
12