XRT79L74 Datasheet, PDF (21/70 Pages) Exar Corporation – 4 - CHANNEL DS3/E3 ATM UNI/PPP COMBO IC

PRELIMINARY
4 - CHANNEL DS3/E3 ATM UNI/PPP COMBO IC
XRT79L74
REV. P1.0.0
PIN #
B9
NAME
TxUClav/
TxPPA
TYPE
DESCRIPTION
O Transmit UTOPIA Interface - Cell Available Output Pin/Transmit POS-PHY
Interface - Packet Data Available Output pin:
The exact function of this output pin depends upon whether the XRT79L74
device has been configured to operate in the ATM UNI or PPP Mode.ATM UNI
Mode - TxUClav - Transmit UTOPIA Interface - Cell Space Available Indicator
Output pin:This output pin supports data flow control between the ATM Layer
Processor and the Transmit UTOPIA Interface block. This signal is asserted
(e.g., driven "high") whenever the TxFIFO is capable of receiving at least one
more full ATM cell of data from the ATM Layer processor. This signal is negated
(e.g., driven "low"), if the TxFIFO is not capable of receiving one more full ATM
cell of data from the ATM Layer processor. The exact behavior of the "TxUClav"
output pin, as a function of "UTOPIA Level" is presented below.
Multi-PHY Operation - UTOPIA Level 2:
When the XRT79L74 device is operating in a Multi-PHY Application and is con-
figured to operate in the UTOPIA Level 2 Mode, then this signal will be tri-stated
until the TxUClk cycle following the assertion of a valid address on the Transmit
UTOPIA Address bus input pins (e.g., when the contents on the Transmit UTO-
PIA Address bus pins, TxUAddr[4:0], match that which have been assigned to
this particular Transmit UTOPIA Interface block). Afterwards, this output pin will
be driven either "high" or "low" depending upon the current fill status of the
TxFIFO.
Multi-PHY Operation - UTOPIA Level 3:
When the XRT79L74 device is operating in a Multi-PHY Application, then this
signal will be tri-stated until two TxUClk cycles following the assertion of a valid
address on the Transmit UTOPIA Address bus input pins (e.g., if the contents of
the Transmit UTOPIA Address bus input pins, TxUAddr[4:0], match that which
have been assigned to this particular Transmit UTOPIA Interface block). After-
wards, this output pin will be driven either "high" or "low" depending upon the
current fill status of the RxFIFO.
PPP Mode - TxPPA
Transmit POS-PHY Interface Packet Space Available Indicator OutputThe
XRT79L74 device will drive this output pin "high" whenever a (programmable)
number of bytes of empty space is available (for writing more PPP packet data)
into the TxFIFO. The exact behavior of the TxPPA output pin, as a function of
"POS-PHY Level" is presented below.
POS-PHY Level 2:
When the XRT79L74 device is configured to operate in the POS-PHY Level 2
Mode, then this signal will be tri-stated until the TxPClk cycle following the asser-
tion of a valid address on the Transmit POS-PHY Address bus input pins (e.g., if
the contents on the Transmit POS-PHY Address bus pins, TxPAddr[4:0], match
that which have been assigned to this particular Transmit POS-PHY Interface
block). Afterwards, this output pin will be driven either "high" or "low" depending
upon the current fill status of the TxFIFO.
POS-PHY Level 3:
When the XRT79L74 device is configured to operate in the POS-PHY Level 3
Mode, then this signal will be tri-stated until two TxPClk cycles following the
assertion of a valid address on the Transmit POS-PHY Address Bus input pins
(e.g., if the contents on the Transmit POS-PHY Address bus pins, TxPAddr[4:0],
match that which have been assigned to this particular Transmit POS-PHY Inter-
face block). Afterwards, this output pin will be driven either "high" or "low"
depending upon the current fill status of the TxFIFO.
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