mt90528ag2 Zarlink Semiconductor, mt90528ag2 Datasheet - Page 54

• Manufacturer Part Number: mt90528ag2
• Description: 28-port Primary Rate Circuit Emulation Aal1 Sar
• Manufacturer: Zarlink Semiconductor

If the VPI comparison is true, the same process is applied to the cell’s VCI field, using the VCI Match Register
(4006h) and the VCI Match Enable Register (4008h). The cell is only processed further if each of the 16 bit-
comparisons produces a true result. Otherwise the cell is discarded.
If the VCI comparison is true, the most significant bit of the PTI field in the cell header is examined to determine if
the cell is identified as carrying OAM information. If the received cell is not an OAM cell, it is automatically sent to
the internal RX UTOPIA FIFO. If the received cell is an OAM cell and the GLOBAL_OAM_SEL bit in the UTOPIA
Control Register is set, the cell is sent to the internal RX UTOPIA FIFO; OAM cells that are sent to this internal
FIFO are treated as non-CBR data cells and are eventually sent to the Receive Data Cell Buffer in external
memory. If the received cell is an OAM cell, but the GLOBAL_OAM_SEL bit is cleared, the cell is discarded.
Note: The VPI/VCI Match and Match Enable filter serves two important purposes. It can eliminate non-unique look-
up table entries (this is important, because the look-up-table space is smaller than the entire VPI/VCI space of 16 M
addresses), and it eliminates null cells. The Match and Match Enable filter can reduce the number of unnecessary
look-up table accesses (and therefore unnecessary memory-access bandwidth) by eliminating cells with VPI/VCI
combinations known not to be destined for the MT90528 device. The user is advised to set the VPI/VCI Match and
Match Enable filter as narrowly as practical for the application.
RX UTOPIA FIFO
The primary role of the RX UTOPIA FIFO is to act as a buffer between the OAM & VPI/VCI Filter and the UDT VPI/
VCI comparison and look-up table search sub-modules. This buffer is necessary because the RX UTOPIA Interface
may be receiving cells while the next filtering modules are not yet ready to process them. The 32-cell deep FIFO
therefore allows these cells to be stored until they can be processed.
UDT VCI/VPI Comparison and Look-up Table
This sub-module’s primary function is to determine the destination of the cells; the module is responsible for
determining to which of the RX_SARs (UDT, SDT or Data) cells should be directed. The RX_SARs are responsible
for processing the received cells and transferring their contents to either a buffer in external memory or the TDM
data bus.
UDT Operation
In the UDT case, the routing task is performed using a number of pre-defined match registers (one per port) which
identify which cells are associated with a particular TDM port.
Up to twenty-eight (28) bits of the cell header may be used to perform this search: the GFC<3:0> field, the
VPI<7:0> field, and the VCI<15:0> field. The 4-bit GFC field is only used when NNI-type cells are received. A global
register bit (UNI_N_NNI in the UTOPIA Configuration Register) determines whether the GFC field is used in the
search. Because the comparison sub-module operates as a pseudo-CAM, the incoming cell header is compared
with all of the UDT search registers (4200h to 426Eh) simultaneously.
If the incoming cell is matched with a particular port, an additional comparison is performed to see whether the cell
contains OAM data:
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If external memory is present in the system configuration containing the MT90528, any received cells whose
headers do not match those in the UDT search registers are passed to the look-up table sub-module and are
processed as detailed in the SDT Operation section that follows. This additional processing is provided primarily to
If the cell contains OAM data (determined by the most significant bit of the PTI header field being set to ‘1’)
and the ‘OAM_SEL’ bit is set in the corresponding UDT VPI Register (located at 4202h + p*4h), the UTOPIA
module sends the cell to the Data RX_SAR. The Data RX_SAR (if enabled) is responsible for copying the
cell into the Receive Data Cell Buffer in external memory.
If the cell is carrying OAM data but the user has selected not to process OAM cells for that port (i.e.,
‘OAM_SEL’ bit for port = 0), the UTOPIA module discards the cell.
If the cell is not carrying OAM data, the UTOPIA module sends the cell to the UDT RX_SAR for processing.
Zarlink Semiconductor Inc.
MT90528
54
Data Sheet