MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
37-1
Chapter 37
Buffer Descriptors and Connection Tables
The communications processor module (CPM) manages ATM traffic through the UTOPIA and serial
interfaces by means of transmit and receive buffer descriptors (BDs) and transmit and receive connection
tables (TCTs and RCTs). The BDs are grouped into circular tables of pointers into the data buffer space in
external memory. The following sections describe the structure and configuration of the BDs, TCTs, and
RCTs.
37.1
ATM Buffer Descriptors (BDs)
ATM segmentation and reassembly (SAR) operates as a multi-channel protocol, segmenting and
reassembling each channel’s transmit and receive data to and from different sets of memory buffers
simultaneously. The buffer descriptor (BD) implementation for ATM operation builds on the traditional
SCC buffering method in which each controller has one pair of BD tables for receiving and transmitting.
To accommodate multiple ATM channels, each channel number is given its own pair of BD tables located
in external memory. The base pointers to a channel’s BD tables are programmed as part of the
channel-specific information in the channel’s RCT and TCT; see
Section 37.2, “Receive and Transmit
Connection Tables (RCTs and TCTs).”
Each transmit channel has a separate TxBD table and a TCT which holds the TxBD pointers (TBASE and
TBD_PTR). Likewise, each receive channel has a separate RxBD table and an RCT holding the RxBD
pointers (RBASE and RBD_PTR). The global parameters TBDBASE and RBDBASE, located in the
ATM-specific SCC (serial and UTOPIA) parameter RAM (see
Chapter 38, “ATM Parameter RAM”
),
define the base addresses for the blocks of external memory containing all the channels’ BD tables. The
BD base offset pointers (TBASE and RBASE) point to the beginning of a given channel’s BD tables. The
BD offset pointers (TBD_PTR and RBD_PTR) specify the next (or currently active) BD.
Figure 37-1
shows TxBD tables and buffers and their associated pointers for two example transmit
channels, channel 1 and channel 4. (The RxBD tables and buffers for receive channels have the same
structure.)
Summary of Contents for PowerQUICC MPC870
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