MT90221 Zarlink Semiconductor, MT90221 Datasheet - Page 40

• Manufacturer Part Number: MT90221
• Description: Quad Inverse Multiplexing For Atm (IMA) Device With Flexible Ima And Uni Mode
• Manufacturer: Zarlink Semiconductor

15 and the minimum value is 0 (in the case the PHY
port is not to be used). The size of the TX Link FIFO
is defined on a per group using the TX IMA Control
registers.
The device will not accept a cell from the UTOPIA
Interface if the internal Cell Ram is full. Status bit 0 in
the General Status register is set to 1 to indicate the
’no free cell in TX Cell RAM’ condition. The status bit
can be cleared by overwriting it with 0.
The UTOPIA Input block has the option to verify the
HEC of the cell coming from the ATM layer. Four
different options are available and are selected by bit
1 and 0 of the UTOPIA Input Control register.
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5.2
The MT90221 supports a 53 byte cell stream via the
ATM output port. Cells received at the ATM output
port are stored in the RX UTOPIA FIFO before being
processed by the UTOPIA Interface. The output of
the UTOPIA Interface can be stopped by the ATM
Layer device by de-asserting the RxENB* signal.
The start of a cell is marked with the SOC signal,
which is active during the transmission of the first
byte of a cell. The following 52 bytes are expected to
belong to the same cell.
The ’00’ option is used to always accept a cell
from the ATM layer. The HEC is verified and if
wrong, the UTOPIA Input counter associated
with the UTOPIA port for cells with bad HEC is
incremented. The MT90221 will re-generate a
valid HEC based on the content of the 4-byte
header that was received.
The ’01’ option is used to verify the HEC of an
incoming cell. If the HEC value is wrong and if it
can be corrected (1 bit error), then the cell is
corrected and accepted as a good cell. The bad
HEC counter is not incremented if the HEC is
corrected. The bad HEC counter is incremented
if the HEC value cannot be corrected. In this
mode, the cell is always accepted. The
MT90221 will re-generate a valid HEC based on
the content of the 4-byte header that was
received.
The ’10’ option is used to verify the HEC on the
incoming cell and discard the cell if the HEC
value is wrong. The bad HEC counter is
incremented if a cell is discarded.
The ’11’ option is similar to mode ’01’ except
that if the HEC value cannot be corrected, then
the cell is discarded. If the HEC value is
corrected, the bad HEC counter is not
incremented.
ATM Output Port
The RX byte clock (RxClk) can be up to 25 MHz and
is checked against the system clock. If the incoming
byte clock frequency is lower than 1/128 of the
system clock, bit 3 of the General Status register
will be set. This bit is cleared by overwriting it with
0.The RxClk signal has to be synchronized with the
System Clock for the proper operation of the
MT90220. Typically, both frequencies are equal but
the RxClk frequency can be lower.
Overflow conditions in the RX UTOPIA FIFO
associated with any of the 8 PHY RX Addresses
cause a status bit to be set in either the IRQ UTOPIA
UNI Overflow Status or IRQ IMA Group Overflow
Status register. These status bits are cleared by
overwriting them with 0. Additionally, for each status
bit there is an Interrupt Enable bit in the associated
RX UTOPIA Link FIFO Overflow Enable or RX
UTOPIA
register. When enabled, the status bit is reported in
an Interrupt register. See 6.2 Interrupt Block for more
details.
The size of the RX UTOPIA FIFO is fixed at two cells
for the UNI PHY Addresses and four cells for the IMA
Group PHY Addresses.
5.3
A single ATM layer device with a UTOPIA L2 MPHY
port can be connected to the ATM input port of one
MT90221. Another ATM-Layer device using the
UTOPIA L2 MPHY input interface is used to receive
ATM cells from the MT90221.
The address pins should be set to the value
programmed by the management interface.
In this mode, the bit 6 and 5 of the Test 1 register are
not to be set to 1 for the proper operation of the RX
Utopia port.
5.4
When more than one MT90221 are connected to a
single ATM Layer device the single TxClav and
RxClav scheme is used. Direct Status Indication and
Multiplexed
supported. The necessary polling is performed by
the ATM-Layer device.
The
addresses, provided by the ATM-Layer device, are
used to de-multiplex the ATM-cell stream to as many
as
bandwidth for four E1 lines served by each MT90221
device is 2 MBytes/s.
eight
UTOPIA Operation With a Single PHY
UTOPIA Operation with Multiple PHY
UTOPIA
IMA
MT90221s.
Status
Group
Interface
Polling
The
FIFO
transmit
maximum
schemes
Overflow
MT90221
and
are
available
Enable
receive
not
31