3 ethernet transmit buffer descriptor (txbd), 3 ethernet transmit buffer descriptor (txbd) -29 – Motorola ColdFire MCF5281 User Manual
Page 339
Fast Ethernet Controller (FEC)
Freescale Semiconductor
17-29
NOTE
When the software driver sets an E bit in one or more receive descriptors,
the driver should follow with a write to RDAR.
17.5.1.3
Ethernet Transmit Buffer Descriptor (TxBD)
Data is presented to the FEC for transmission by arranging it in buffers referenced by the channel’s TxBDs.
The Ethernet controller confirms transmission by clearing the ready bit (TxBD[R]) when DMA of the
buffer is complete. In the TxBD, the user initializes the R, W, L, and TC bits and the length (in bytes) in
the first longword and the buffer pointer in the second longword.
The FEC clears the R bit when the buffer is transferred. Status bits for the buffer/frame are not included in
the transmit buffer descriptors. Transmit frame status is indicated via individual interrupt bits (error
conditions) and in statistic counters in the MIB block. See
Section 17.4.1, “MIB Block Counters Memory
for more details.
0ffset + 4
15–0
A[31:16]
RX data buffer pointer, bits [31:16]
1
Offset + 6
15–0
A[15:0]
RX data buffer pointer, bits [15:0]
1
The receive buffer pointer, containing the address of the associated data buffer, must always be evenly divisible by 16. The
buffer must reside in memory external to the FEC. The Ethernet controller never modifies this value.
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Offset + 0
R
TO1
W
TO2
L
TC
ABC
—
—
—
—
—
—
—
—
—
Offset + 2
Data Length
Offset + 4
Tx Data Buffer Pointer - A[31:16]
Offset + 6
Tx Data Buffer Pointer - A[15:0]
Figure 17-26. Transmit Buffer Descriptor (TxBD)
Table 17-30. Transmit Buffer Descriptor Field Definitions
Word
Field
Description
Offset + 0
15
R
Ready. Written by the FEC and you.
0 The data buffer associated with this BD is not ready for transmission. You are free to manipulate
this BD or its associated data buffer. The FEC clears this bit after the buffer has been transmitted
or after an error condition is encountered.
1 The data buffer, prepared for transmission by you, has not been transmitted or currently transmits.
You may write no fields of this BD after this bit is set.
Offset + 0
14
TO1
Transmit software ownership. This field is reserved for software use. This read/write bit is not modified
by hardware nor does its value affect hardware.
Table 17-29. Receive Buffer Descriptor Field Definitions (continued)
Word
Field
Description
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3