Freescale Semiconductor POWERPC MPC860T - Manual (Page 58)

6-26 MPC860T (Rev. D) Fast Ethernet Controller Supplement MOTOROLA
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
6.4.2 Ethernet Transmit Buffer Descriptor (TxBD)
Data is presented to the FEC for transmission by arranging it in buffers referenced by the
channelÕs TxBDs. The FEC conÞrms transmission or indicates error conditions using BDs
to inform the host that the buffers have been serviced. The user initializes
TxBD[R,W,L,TC], the length (in bytes), and the buffer pointer.
¥ If L = 0, the FEC clears the R bit when the buffer is accessed. Status bits are not
modiÞed.
¥ If L = 1, the FEC clears the R bit and modiÞes the DEF, HB, LC, RL, RC, UN, and
CSL status bits after the buffer is accessed and frame transmission completes.
The TxBD is shown in Figure 6-24.
Table 6-29 describes TxBD Þelds.
14 OV Overrun, written by FEC. A receive FIFO overrun occurred during frame reception. If OV = 1,
the other status bits, M, LG, NO, SH, CR, and CL lose their normal meaning and are cleared.
This bit is valid only if the L bit is set.
15 TR Truncate. Set if the receive frame is truncated ( ³ 2 Kbytes).
Offset+2 Data
length
Data length, written by FEC. Data length is the number of octets written by the FEC into this
BDÕs buffer if L = 0 (the value = R_BUFF_SIZE), or the length of the frame including CRC if
L = 1. It is written by the FEC once as the BD is closed.
Offset+4 Rx
buffer
pointer
Rx buffer pointer A[0Ð31], written by user. The receive buffer pointer, which always points to the
Þrst location of the associated buffer, must always be a multiple of 16. The buffer must reside in
memory external to the FEC.
Figure 6-24. Transmit Buffer Descriptor (TxBD)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+0 R TO1 W TO2 L TC DEF HB LC RL RC UN CSL
+2 DATA LENGTH
+4 Tx Data Buffer Pointer A[0Ð15]
+6 Tx Data Buffer Pointer A[16Ð31]
Table 6-29. Transmit Buffer Descriptor (TxBD) Field Descriptions
Bits Name Description
0 R Ready, written by FEC and user.
0 The buffer associated with this BD is not ready for transmission. The user can manipulate this
BD or its associated buffer. The FEC clears R after the buffer is sent or an error occurs.
1 The user-prepared buffer has not been sent or is being sent. The user cannot update the BD
while R = 1.
1 TO1 Transmit software ownership bit. This Þeld is available for use by software. This read/write bit is
not modiÞed by hardware and its value does not affect hardware.
Table 6-27. Receive Buffer Descriptor (RxBD) Field Description (Continued)
Bits Name Description
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