Ethernet ModuleMCF5272 ColdFire ® Integrated Microprocessor User’s Manual, Rev. 3Freescale Semiconductor 11-3711.6.1.2 Ethernet Transmit Buffer DescriptorData is presented to the FEC for transmission by arranging it in buffers referenced by the channel’s TxBDs.The FEC confirms transmission or indicates error conditions using the BDs to inform the host that thebuffers have been serviced. In the TxBD, the user initializes the R, W, L, and TC bits and the length (inbytes) in the first word and the buffer pointer in the second words.If L = 0, then the FEC sets the R bit to 0 in the first word of the BD when the buffer is sent as a DMA.Status bits are not modified.If L = 1, then the FEC sets the R bit to 0 and will modify the DEF, HB, LC, RL, RC, UN, and CSL statusbits in the first word of the BD after the buffer is sent as a DMA, and frame transmission is complete.The TxBD fields are detailed in Table 11-35.15 14 13 12 11 10 9 8 7 6 5 2 1 0+0 R TO1 W TO2 L TC DEF HB LC RL RC UN CSL+2 DATA LENGTH+4 Tx Data Buffer Pointer A[31–16]+6 Tx Data Buffer Pointer A[15–0]Figure 11-28. Transmit Buffer Descriptor (TxBD)Table 11-35. TxBD Field DescriptionsBits Name Description15 R Ready. Written by the FEC (= 0) and user (= 1).0 The data buffer associated with this BD is not ready for transmission. The user is free tomanipulate this BD or its associated data buffer. The FEC clears this bit after the buffer hasbeen transmitted or after an error condition is encountered.1 The data buffer, which has been prepared for transmission by the user, has not beentransmitted or is currently being transmitted. No fields of this BD may be written by the useronce this bit is set.14 TO1 Transmit software ownership bit.Reserved for use by software. This read/write bit is not modifiedby hardware, nor does its value affect hardware.13 W Wrap. Written by the user.0 The next buffer descriptor is found in the consecutive location.1 The next buffer descriptor is found at the location defined in ETDSR.12 TO2 Transmit software ownership bit. Reserved for use by software. This read/write bit is not modifiedby hardware, nor does its value affect hardware.11 L Last in frame. Written by the user.0 The buffer is not the last in the transmit frame.1 The buffer is the last in the transmit frame.10 TC Tx CRC. Written by the user and is only valid if L = 1.0 End transmission immediately after the last data byte.1 Transmit the CRC sequence after the last data byte.9 DEF Defer indication. Written by the FEC and is only valid if L = 1. The FEC had to defer while tryingto transmit a frame. This bit is not set if a collision occurs during transmission.
