STE100 datasheet(42/66 Pages) STMICROELECTRONICS

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STE10/100

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6.2 Network Packet Buffer Management

6.2.1 Descriptor Structure Types

During normal network transmit operations, the STE10/100 transfers the data packets from transmit buffers in

the host’s memory to the STE10/100’s transmit FIFO. For receive operations, the STE10/100 transfers the data

packet from its receive FIFO to receive buffers in the host’s memory. The STE10/100 makes use of descriptors,

data structures which are built in host memory and contain pointers to the transmit and receive buffers and main-

tain packet and frame parameters, status, and other information vital to controlling network operation.

There are two types of structures employed to group descriptors, the Ring and the Chain, both supported by

the STE10/100 and shown below. The selection of structure type is controlled by RCH (RDES1 bit 24) and TCH

(TDES1 bit 24).

The transmit and receive buffers reside in the host’s memory. Any buffer can contain either a complete or partial

packet. A buffer may not contain more than one packet.

Ring structure: There are two buffers per descriptor in the ring structure. Support receive early interrupt.

Figure 5. Ring structure of frame buffer

own

Data Buffer

Length 1

Data

Length 2

Buffer1 pointer

Buffer2 pointer

Length 2 Length 1

Descriptor

Descriptor Pointer

CSR3 or CSR4

End of Ring

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