Fast Ethernet Controller (FEC)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
45-36
Freescale Semiconductor
45.3.4.2
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 FEC confirms 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 modified.
•
If L = 1, the FEC clears the R bit and modifies the DEF, HB, LC, RL, RC, UN, and CSL status bits
after the buffer is accessed and frame transmission completes.
7
M
Miss, written by FEC.Set by the FEC for frames that were accepted in promiscuous mode but
were flagged as a miss by the internal address recognition. Thus, while promiscuous mode is
being used, the user can use the M bit to quickly determine whether the frame was destined to
this station. This bit is valid only if both the L bit and PROM bit are set.
0 The frame was received because of an address recognition hit.
1 The frame was received because of promiscuous mode.
8
BC
Set if the DA is broadcast
9
MC
Set if the DA is multicast and not broadcast
10
LG
Rx frame length violation, written by FEC. The frame length exceeds the value of
MAX_FRAME_LENGTH in the bytes. The hardware truncates frames exceeding 2047 bytes so
as not to overflow receive buffers This bit is valid only if the L bit is set.
11
NO
Rx nonoctet-aligned frame, written by FEC. A frame that contained a number of bits not divisible
by 8 was received and the CRC check that occurred at the preceding byte boundary generated
an error. NO is valid only if the L bit is set. If this bit is set the CR bit is not set.
12
SH
Short frame, written by FEC. A frame length that was less than the minimum defined for this
channel was recognized.
13
CR
Rx CRC error, written by FEC. This frame contains a CRC error and is an integral number of
octets in length. This bit is valid only if the L bit is set.
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).
2
Data
length
Data length, written by FEC. Data length is the number of octets written by the FEC into this
BD’s 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.
4
Rx buffer
pointer
Rx buffer pointer A[0–31], written by user. The receive buffer pointer, which always points to the
first location of the associated buffer, must always be a multiple of 16. The buffer must reside in
memory external to the FEC.
Table 45-35. Receive Buffer Descriptor (RxBD) Field Description (continued)
Bits
Name
Description
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