Freescale Semiconductor MPC860T Ethernet Transmit Buffer Descriptor TxBD, describes TxBD Þelds

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

Table 6-27. Receive Buffer Descriptor (RxBD) Field Description (Continued)

Bits	Name	Description
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 (≥ 2 Kbytes).
Offset+2	Data	Data length, written by FEC. Data length is the number of octets written by the FEC into this
	length	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	Rx buffer pointer A[0Ð31], written by user. The receive buffer pointer, which always points to the
	buffer	Þrst location of the associated buffer, must always be a multiple of 16. The buffer must reside in
	pointer	memory external to the FEC.

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.

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 describes TxBD Þelds.

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.

6-26MPC860T (Rev. D) Fast Ethernet Controller Supplement MOTOROLA

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