á
PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
149
between the Transmit UTOPIA Interface block and
the ATM Layer processor.
Detecting and discarding “Runt” cells and insuring
that the Tx FIFO can resume normal operation
following the removal of the runt cell.
Insuring that the Tx FIFO can respond properly to
an “Overrun” condition, by generating the “Tx FIFO
Overrun Condition” interrupt, discarding the result-
ing “runt” or errored cell, and resuming proper
operation afterwards.
Transmit UTOPIA FIFO Manager Features
and Options
This section discusses the numerous features that
are provided by the Transmit UTOPIA FIFO Manager.
Additionally, this section discusses how the user can
customize these features to suit his/her application
needs.
The Transmit UTOPIA FIFO Manager provides the
user with the following options.
Handshaking Mode (Octet Level vs Cell Level)
User selected Operating Tx FIFO Depth
Resetting the Tx FIFO
Monitoring the Tx FIFO
6.1.2.3
Selecting the Handshaking Mode
(Octet Level vs Cell Level)
The Transmit UTOPIA Interface block offers two
different data flow control modes for data transmission
between the ATM Layer processor and the UNI IC.
These two modes are: “Octet-Level” Handshaking
and “Cell-Level” Handshaking; as specified by the
UTOPIA Level 2, Version 8 Specifications, and are
discussed below.
6.1.2.3.0.1
The UNI will be operating in the “Cell-Level” Hand-
shaking Mode following power up or reset. Therefore,
the user has to set bit 5 (Handshaking Mode) of the
UTOPIA Configuration Register to “0” in order to
configure the UNI into the “Octet-Level” Handshake
mode. The main signal that is responsible for data
flow control, between the ATM Layer processor and
the Transmit UTOPIA Interface block is the TxClav
output pin. The ATM Layer processor is expected to
monitor the TxClav output pin in order to determine if
it is OK to write data into the Tx FIFO. The TxClav
output pin exhibits a role that is similar to CTS (Clear
to Send) in RS-232 based data transmission systems.
As long as TxClav is at a logic “high”, the ATM Layer
processor is permitted to write more cell data bytes
(or words) into the Transmit UTOPIA Interface block
(and in turn, the TxFIFO). However, when the TxClav
pin toggles “l(fā)ow”, this indicates that the Tx FIFO can
only accept 4 (or less) more write operations from the
ATM Layer processor. Once the TxClav pin returns
high, this indicates that the TxFIFO can accept more
than 4 write operations from the ATM Layer processor,
and that the ATM Layer processor can resume writing
data to the Transmit UTOPIA Interface block.
In other words, if the UTOPIA Data bus is configured
to be 8-bits wide, then the TxClav signal will toggle
“l(fā)ow” when the TxFIFO can only accept 4 (or less)
bytes of ATM cell data, from the ATM Layer processor.
If the UTOPIA Data bus is configured to be 16-bits
wide; then the TxClav signal will toggle “l(fā)ow” when
the TxFIFO can only accept 8 (or less) bytes of ATM
cell data from the ATM Layer processor.
Figure 33 presents a timing diagram illustrating the
behavior of TxClav during writes to the Transmit
UTOPIA Interface block, while operating in the
Octet-Level Handshaking Mode.
Octet-Level Handshaking