MT90520AG ZARLINK [Zarlink Semiconductor Inc], MT90520AG Datasheet - Page 53

• Manufacturer Part Number: MT90520AG
• Description: 8-Port Primary Rate Circuit Emulation AAL1 SAR
• Manufacturer: ZARLINK [Zarlink Semiconductor Inc]

In the Match and Match Enable filter, the cell’s VPI field is examined first (all 12 bits, including the GFC). A bit-by-bit
comparison of the VPI is performed using the contents of both the VPI Match Register (400Ah) and the VPI Match
Enable Register (400Ch). If a bit value in the VPI Match Enable Register is ‘0’, no comparison is performed on the
corresponding bit in the VPI Match Register (and the bit is automatically accepted). If a bit value in the VPI Match
Enable Register is ‘1’, the comparison result is only true if the received VPI bit and the corresponding VPI Match
Register bit are identical. The cell is only processed further if each of the 12 bit-comparisons produces a true result.
Otherwise the cell is discarded.
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 16M
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 MT90520 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:
•
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
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MT90520
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Data Sheet