Freescale Semiconductor MPC8260 - Manual (Page 961)

ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 30-41
30.10.2 Connection Tables (RCT, TCT, and TCTE)
The receive and transmit connection tables, RCT and TCT, store host-initialized connection parameters
after connection set-up. These include AAL type, connection traffic parameters, BD parameters and
temporary parameters used during segmentation and reassembly (SAR). The transmit connection table
extension (TCTE) supports special connections that use ABR, VBR or UBR+ services. Each connection
table entry resides in a 32-byte space.
Table 30-15 lists sizes for RCT, TCT, and TCTE.
NOTE
An ATM channel is considered internal if its tables are in an internal
dual-port RAM; it is considered external if its tables are in external
memory.To improve performance, store parameters for fast channels in
internal dual-port RAM and parameters for slower channels in external
memory. Connection tables for external channels are read and written from
external memory each time the CP processes a cell. The CP does, however,
minimize memory access time by burst fetching the 32-byte entry and
writing back only the first 24 bytes.
In all connection tables, fields which are not used must be cleared.
30.10.2.1 ATM Channel Code
Each ATM channel has a channel code used as an index to the channel’s connection table entry. The first
channel in the table has channel code one, the second has channel code two, and so on. Codes of 255 or
less indicate internal channels; codes greater than 255 indicate external channels. Channel code one is
reserved as the raw cell queue and cannot be used for another purpose. The channel code is used to specify
14 — Reserved, should be cleared.
15 ALM Address look-up mechanism. See Section 30.4, “VCI/VPI Address Lookup Mechanism .”
0 External CAM lookup.
1 Address compression.
1 MPC8264 and MPC8266 only: GMODE[REM] must be set to disable receive emergency mode. If receive emergency
mode were enabled, it would result in IMA transmit protocol violations in cases of system overload, potentially avoiding
protocol errors in the IMA receiver at the expense of generating IMA transmit protocol violations to the far end. Rather
than causing erratic transmit operation, it is better to set GMODE[REM] and deal with the IMA receive protocol
violations locally. Note further that the system should be designed such that overload problems never occur in the
field; any errors due to overload should be eliminated during system design and debug.
Table 30-15. Receive and Transmit Connection Table Sizes
ATM Service Class RCT TCT TCTE
CBR, UBR service 32 bytes 32 bytes —
ABR, VBR, UBR+ service 32 bytes 32 bytes 32 bytes
Table 30-14. GMODE Field Descriptions (continued)
Bits Name Description