XRT7245
DS3 UNI FOR ATM
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PRELIMINARY
REV. 1.03
146
TxEnB*—Transmit UTOPIA Data Bus—Write
Enable input pin
TxClav/TFullB*—TxFIFO Cell Available
Each of these signals are briefly discussed below.
TxData[15:0] —Transmit UTOPIA Data Bus inputs
The ATM Layer Processor will write its ATM Cell Data
into the Transmit UTOPIA Interface block, by placing
it, in a byte-wide (or word-wide) manner on these in-
put pins. The Transmit UTOPIA Data Bus can be con-
figured to operate in the “8-bit wide” or “16-bit wide”
mode (See Section 6.1.2.1.2). If the “8-bit wide”
mode is selected, then only the TxData[7:0] input
pins are active and capable of receiving data. If the
“16-bit wide” mode is selected, the all 16 input pins
(e.g., TxData[15:0]) are active. The Transmit UTOPIA
Data bus is tri-stated while the active-low TxEnB*
(Transmit UTOPIA Data Bus—Write Enable) input
signal is “high”. Therefore, the ATM Layer processor
must assert this signal (e.g., toggling TxEnB* “l(fā)ow”) in
order write the cell data, on the Transmit UTOPIA Data
bus, into the Transmit UTOPIA Interface Block. The
data on the Transmit UTOPIA Data Bus is sampled
and latched into the Transmit UTOPIA Interface
block, on the rising edge of the Transmit UTOPIA In-
terface Block Clock signal, TxClk.
Additionally, the Transmit UTOPIA Interface block will
only process one cell worth of data (e.g., 52, 53 or 54
bytes, as configured via the CellOf52Bytes option—
See Section 6.1.2.1.3), following the latest assertion
of the TxSoC (Transmit-Start of Cell) pin. Afterwards,
the Transmit UTOPIA Data bus will become tri-stated
and will cease to process any more data from the
ATM Layer Processor until the next assertion of the
TxSoC pin. Once the Transmit UTOPIA Interface
block reaches this condition, it will ignore the asser-
tions of the TxEnB* pin, and will keep the Transmit
UTOPIA Data bus input pins tri-stated until the ATM
Layer Processor pulses the TxSoC input pin, once
again.
If the Transmit UTOPIA Interface block detects a
“runt” cell (e.g., if the amount of data that is read into
the TxFIFO is less than that configured via the
“CellOf52Bytes” option), then the Transmit UTOPIA
Interface block will discard this cell, and resume nor-
mal operation.
TxAddr[4:0]—Transmit UTOPIA Address Bus
inputs
These input pins are used only when the UNI is oper-
ating in the Multi-PHY mode. Therefore, for more
information on the Transmit UTOPIA Address Bus,
please see Section 6.1.2.3.2.
TxClk—Transmit UTOPIA Interface Block Clock
signal input pin
The Transmit UTOPIA Interface block uses this signal
to sample and latch the data on the Transmit UTOPIA
Data bus into the Transmit UTOPIA Address block
(for Multi-PHY operation) into the Transmit UTOPIA
Interface block. This clock signal can run at frequencies
of 25 MHz, 33 MHz, or 50 MHz.
TxEnB*—Transmit UTOPIA Data Bus—Write
Enable input
The Transmit UTOPIA Data Bus is tri-stated while
this input signal is negated. Therefore, the ATM Layer
Processor must assert this “active-low” signal (toggle
it “l(fā)ow”) in order to write the byte (or word) on the
Transmit UTOPIA Data Bus, into the Transmit
UTOPIA Interface block.
TxPrty—Transmit UTOPIA—Odd Parity Bit
Input Pin
The ATM Layer Processor is expected to compute
the odd-parity value of each byte (or word) of ATM
Cell data that it intends to place on the Transmit
UTOPIA Data bus. The ATM Layer Processor is then
expected to apply this parity value at the TxPrty pin,
while the corresponding byte (or word) is present on
the Transmit UTOPIA Data Bus.
TxSoC—Transmit UTOPIA—“Start of Cell”
Indicator
The ATM Layer processor is expected to pulse this
signal “high”, for one clock period of TxClk, when the
first byte (or word) of a new cell is present on the
Transmit UTOPIA Data Bus. This signal must be kept
“l(fā)ow” at all other times.
Note:
Once the ATM Layer Processor has pulsed the TxSoC
pin “high”, the Tansmit UTOPIA Interface Block will proceed
to read in and process only one cell of data (e.g., 52, 53, or
54 bytes, as configured via the “CellOf52Bytes” option—
See Section 6.1.2.1.3) via the Transmit UTOPIA Data Bus.
Afterwards, the Transmit UTOPIA Interface block will cease
to process any more data from the ATM Layer Processor
until the TxSoC pin has been pulsed “high” once again.
This phenomenon is more clearly defined in “Example-1”
below.
Further, if the ATM Layer Processor pulses the TxSoC
pin before the appropriate number of bytes (as con-
figured via the “CellOf52Bytes” option—See Section
6.1.2.1.3), have been read in and processed by the
Transmit UTOPIA Interface block, then a “runt” cell