Buffer Descriptors

Table 17-37. Transmit Buffer Descriptor Field Definitions (continued)

Word

Location

Field Name

Description

 

 

 

 

Offset + 2

Bits [15:0]

Data Length

Data Length, written by user.

 

 

 

Data length is the number of octets the FEC should transmit

 

 

 

from this BD’s data buffer. It is never modified by the FEC. Bits

 

 

 

[15:5] are used by the DMA engine, bits[4:0] are ignored.

 

 

 

 

Offset + 4

Bits [15:0]

A[31:16]

Tx data buffer pointer, bits [31:16] 1

Offset + 6

Bits [15:0]

A[15:0]

Tx data buffer pointer, bits [15:0].

 

 

 

 

1The transmit buffer pointer, which contains the address of the associated data buffer, must always be evenly divisible by 4. The buffer must reside in memory external to the FEC. This value is never modified by the Ethernet controller.

NOTE

Once the software driver has set up the buffers for a frame, it should set up the corresponding BDs. The last step in setting up the BDs for a transmit frame should be to set the R bit in the first BD for the frame. The driver should follow that with a write to TDAR which will trigger the FEC to poll the next BD in the ring.

MOTOROLA

Chapter 17. Fast Ethernet Controller (FEC)

17-51

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Motorola MCF5281, MCF5282 user manual 17-51