MT90500 Mitel Semiconductor, MT90500 Datasheet - Page 64

• Manufacturer Part Number: MT90500
• Description: Multi-Channel ATM AAL1 SAR
• Manufacturer: Mitel Semiconductor

MT90500
c) The cell’s VPI field (8 bits) is examined. A bit by bit comparison of the VPI is performed using the contents of
both the VPI Match Register (4012h) and the VPI Mask Register (4014h). If a bit value in the VPI Mask
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 Mask Register is ‘1’, the comparison result will only be true if
the received VPI bit and the corresponding VPI Match Register bit are identical. The cell will only be processed
further (i.e. proceed to step (d)) if each of the 8 bit comparisons produces true results. Otherwise, the cell will
be discarded.
d) The cell’s VCI field (16 bits) is then examined. A bit by bit comparison of the VCI is performed using the
contents of both the VCI Match Register (4016h) and the VCI Mask Register (4018h). If a bit value in the VCI
Mask Register is ‘0’, no comparison is performed on the corresponding bit in the VCI Match Register. If a bit
value in the VCI Mask Register is ‘1’, the comparison result will only be true if the received VCI bit and the
corresponding VCI Match Register bit match. Step (e) will only be executed if each and every one of the 16 bit
comparisons produces true results. Otherwise, the cell will be discarded.
Note: The VPI/ VCI match and mask filter serves two important purposes. It can eliminate non-unique look-up-
table entries (important as the look-up-table space is smaller than the entire VPI/VCI space of 16M addresses).
It can also reduce the number of unnecessary look-up-table accesses (and unnecessary memory-access
bandwidth) by eliminating cells with VPI/VCI not destined for the MT90500. The user is advised to set the VPI/
VCI match and mask filter as narrowly as practical for the application.
e) Any cell which passes through both the VPI and VCI match filtering will be placed in the 32-cell FIFO of the
UTOPIA module. Cells are then read out by another internal process. As mentioned in step (a) above, OAM
cells which are located in the Primary Receive Queue are automatically placed into the Receive Data Cell
FIFO. On the other hand, non-OAM cells are passed to the lookup engine of the UTOPIA module, as explained
in step (f).
(f) Within the look-up engine, the N least significant bits of the VCI and the M least significant bits of the VPI are
concatenated together to form a 15-bit word. If M + N is smaller than 15, the missing most significant bits of the
15-bit word are zeroed. Two least significant zeroes are appended automatically (by H/W) to this word to form a
17-bit pointer aligned on a double-word boundary. Note: This is explained more fully in the register description
for the VPI/VCI Concatenation Register at address 4010h. This pointer is added to the contents of the Look-up
Table Base Address Register at address 401Eh to form a memory pointer into the VC Look-up Table, which is
composed of 32-bit entries. The look-up engine then examines the “T” bits of each look-up table entry. These
bits indicate the type of information being carried by a particular cell and therefore determine the final
destination of the cell:
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“00” indicates an undefined cell type. In this case, the cell is either discarded or treated as a non-
CBR data cell which is placed in the Receive Data Cell FIFO. This final cell routing is dependent
on the setting of the UKSEL (Unknown Routing Select) bit in the UTOPIA Control Register at
4000h.
“01” represents a non-CBR data cell. In this case, the cell is stored in a 64-byte long structure
within the Receive Data Cell FIFO (see Figure 29).
“10” indicates a CBR cell. In this case, the RX Structure Address in the look-up table, (Figure 27)
is used to access the RX_SAR Control Structure (see Figure 22 - RX_SAR Control Structure) to
determine how to process the cell payload data.