NXP Semiconductors freescale semiconductor ColdFire MCF51CN128 Series, Reference Manual

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Fast Ethernet Controller (FEC)
Freescale Semiconductor 16-25
MCF51CN128 Reference Manual, Rev. 6
NOTE
When the software driver sets an E bit in one or more receive descriptors,
the driver should follow with a write to RDAR.
16.5.1.3 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).
0 1
OV
Overrun. Written by the FEC. A receive FIFO overrun occurred during frame reception. If this bit is
set, the other status bits, M, LG, NO, CR, and CL lose their normal meaning and are zero. This bit
is valid only if the L-bit is set.
0 0
TR
Set if the receive frame is truncated (frame length > 2047 bytes). If the TR bit is set, the frame must
be discarded and the other error bits must be ignored as they may be incorrect.
2 15–0
Data
Length
Data length. Written by the FEC. Data length is the number of octets written by the FEC into this
BD’s data buffer if L equals 0 (the value is equal to EMRBR), or the length of the frame including
CRC if L is set. It is written by the FEC once as the BD is closed.
4 15–0
A[31:16]
RX data buffer pointer, bits [31:16]
1
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 — — — — — — — — —
2 Data Length
4 Tx Data Buffer Pointer - A[31:16]
6 Tx Data Buffer Pointer - A[15:0]
Figure 16-25. Transmit Buffer Descriptor (TxBD)
Table 16-26. Receive Buffer Descriptor Field Definitions (continued)
Word Field Description