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CHAPTER 5 SAR FUNCTION
183
5.5.4 Setting Receive Lookup Table
The lookup table exists in control memory and is used to map the VPI/VCI to be received according to the
receive VC table. The lookup table always starts from 0000H in control memory, and its size is determined by
the setting of the VRR register to be received, as well as the pattern of VPI/VCI to be received.
The
μ
PD98405 internally converts the 24 bits of the VPI/VCI included in the receive cell into a 16-bit logic
code. This conversion is performed according to the setting of the "SHIFT" and "MASK" fields of the VRR
register. Based on the converted logic code, whether the cell of the VPI/VCI is to be received is set in the
receive lookup table entry, by means of the following procedure.
<1>
The VPI is shifted to the VCI side by an amount equal to the number set in the "SHIFT" field of the VRR
register. At this time, the VPI can be shifted by up to 15 bits. When it has been shifted by 8 bits or more,
0 is added to the high-order bit positions.
<2>
The 16 bits created in
<1>
and the contents of the "MASK" field of the VRR register are ANDed.
<3>
The high-order 15 bits of the ANDed 16 bits are part of the address of the receive lookup table.
<4>
The least significant bit ("L" in Figure 5-37) indicates the high-order 16 bits or low-order 16 bits of the
word corresponding to the address. If the least significant bit is "0", the high-order 16 bits are indicated.
If it is "1," the low-order 16 bits are indicated.
<5>
The host stores a 16-bit code that combines the enable bit ("ENBL" bit) and the "VC NUMBER" of the VC
to the lookup table address created in this way.
When the enable bit is set to "1," the VC enters the active status.
When the
μ
PD98405 receives a cell from a PHY layer device, it creates the address of the receive lookup
table in the same manner as described above, in accordance with the VPI/VCI in the header of the receive cell,
and the setting of the VRR register. The
μ
PD98405 accesses the lookup table entry from that address,
receiving cells if the enable bit is "1," or discarding cells if the enable bit is "0." To receive cells, the address of
the corresponding VC table is created from the "VC NUMBER" stored together with the 16-bit code, and the
setting to be used for processing is either confirmed or updated.
The size occupied by the lookup table in control memory is determined from the number of VPI/VCI patterns
to be received. The capacity of this table must be sufficient to store the specified number of lookup table
entries in the VPI/VCI to be received.
Caution
"VC NUMBER" is not a physical address that indicates the beginning of the VC table in the
free block pool. Instead, it is a code obtained by shifting the lower 4 bits of the physical
address.
When reducing the VPI/VCI from 24 to 16 bits, the
μ
PD98405 can select whether to discard or receive the
reception packet in which the area ignored by "SHIFT" has a value other than 0. This function can be specified
by the VFM it of the VRR register. For example, when VFM = 0 and SHIFT = 4, the
μ
PD98405 discards the
reception packet in which the high-order 4 bits of the VPI or VCI have a value other than 0. The configuration of
the VRR register and a VPI/VCI reduction example are shown below.