á
PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
147
will have been detected. Whenever the Transmit
UTOPIA Interface block detects a “runt” cell, it will
generate a “Change in Cell Alignment” interrupt and
will discard the “runt” cell. This phenomenon is more
clearly defined in “Example-2” below.
Example-1
For example, if the user configures the Transmit
UTOPIA Interface block to process 53 bytes per cell;
then following the assertion of the TxSoC pin (which
is coincident with the placement of the first byte of the
cell on the Transmit UTOPIA Data bus), the Transmit
UTOPIA Interface block will read in and process 52
more bytes of data via the Transmit UTOPIA data bus
resulting in a total of 53 bytes being processed. After
the Transmit UTOPIA Interface block has read in the
53rd byte, it will no longer read in any more data from
the ATM Layer Processor, until the TxSoC pin has
been asserted.
Example-2
If the ATM Layer processor were to prematurely as-
serts the TxSoC pin, (e.g., when the 52nd byte is
present on the Transmit UTOPIA data bus, then the
Transmit UTOPIA Interface block will interpret the
previous 52 bytes of cell data as a “runt” cell. The
Transmit UTOPIA Interface block will then generate a
“Change of Cell Alignment” interrupt and will proceed
to discard this runt cell.
TxClav/TFullB*—Tx FIFO Cell Available/TxFIFO
Full*
This output signal is used to provide some data flow
control between the ATM Layer processor and the
Transmit UTOPIA Interface block. Please See Section
6.1.2.2.1 for more information regarding this signal.
Selecting the UTOPIA Data Bus Width
The UTOPIA data bus width can be selected to be ei-
ther 8 or 16 bits by writing the appropriate data to the
UTOPIA Configuration Register, as shown below.
If the user chooses a UTOPIA Data Bus width of 8
bits, then only the Transmit UTOPIA Data inputs:
TxData[7:0] will be active. (The input pins: TxData[15:8]
will not be active). If the user chooses a UTOPIA
Data bus width of 16 bits, then all of the Transmit
UTOPIA Data inputs: TxData[15:0] will be active. The
following table relates the value of Bit 0 (UtWidth)
within the UTOPIA Configuration Register, to the
corresponding width of the UTOPIA Data bus.
Note:
1. The selection of this bit also affects the width of the
Receive UTOPIA Data bus.
2. Upon power up or reset, the UTOPIA Data Bus
width will be 8 bits. Therefore, the user must write a
“1” to this bit in order to set the width of the Transmit
UTOPIA (and the Receive UTOPIA) to 16 bits.
6.1.2.1.2
Selecting the Cell Size (Number
of Octets per Cell)
The UNI allows the user to select the number of octets
per cell that the Transmit UTOPIA Interface block will
process, following each assertion of the TxSoC input
pin. Specifically, the user has the following cell size
options.
If the UTOPIA Data Bus width is set to 8 bits then
the user can choose:
– 52 bytes (with no HEC byte in the cell), or
– 53 bytes (with either a dummy or actual HEC
byte in the cell)
If the UTOPIA Data Bus width is set to 16 bits then
the user can choose:
UTOPIA Configuration Register: Address = 6Ah
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
Unused
Handshake Mode
M-PHY
CellOf52 Bytes
TFIFODepth[1, 0]
UtWidth16
RO
R/W
R/W
R/W
R/W
R/W
T
ABLE
11: T
HE
R
ELATIONSHIP
BETWEEN
THE
CONTENTS
OF
B
IT
F
IELD
0 (U
T
W
IDTH
16)
WITHIN
THE
UTOPIA
C
ONFIGURATION
R
EGISTER
AND
THE
OPERATING
WIDTH
OF
THE
UTOPIA D
ATA
BUS
V
ALUE
FOR
U
T
W
IDTH
16
W
IDTH
OF
UTOPIA D
ATA
B
US
0
8 bit wide Data Bus
1
16 bit wide Data Bus