Freescale Semiconductor MPC8260 - Manual (Page 893)

Multi-Channel Controllers (MCCs)
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 28-45
The data length and buffer pointer are described as follows:
• Data length. Data length is the number of octets written by the CP into this BD’s data buffer. It is
written by the CP when the BD is closed. When this is the last BD in the frame (L = 1), the data
length contains the total number of frame octets (including two or four bytes for CRC). Note that
memory allocated for buffers should be not smaller than the contents of the maximum receive
buffer length register (MRBLR). The data length does not include the time stamp.
• Rx buffer pointer. The receive buffer pointer points to the first location of the associated data
buffer. This value must be equal to 8*n if CHAMR[TS] = 0 and equal to 8*n - 4 if CHAMR[TS]
= 1 (where n is any integer larger than 0).
28.9.2 Transmit Buffer Descriptor (TxBD)
Figure 28-22 shows the TxBD.
11 NO Rx nonoctet-aligned frame. A frame of bits not divisible exactly by eight was received. NO = 1 for
any type of nonalignment regardless of frame length. The shortest frame that can be detected is of
type FLAG-BIT-FLAG, which causes the buffer to be closed with NO error indicated.
The following shows how the nonoctet alignment is reported and where data can be found.
To accommodate the extra word of data that may be written at the end of the frame, it is
recommended to reserve MFLR + 8 bytes for each buffer data.
12 AB Rx abort sequence. A minimum of seven consecutive 1s was received during frame reception. Abort
is not detected between frames. The sequence
Closing-Flag, data, CRC, AB, data, opening-flag...
does not cause an abort error. If the abort is long enough to be an idle, an idle line interrupt may be
generated. An abort within the frame is not reported by a unique interrupt but rather with a RXF
interrupt and the user has to examine the BD.
13 CR Rx CRC error. This frame contains a CRC error. The received CRC bytes are always written to the
receive buffer.
14 — Reserved, should be cleared.
SF SS7 mode only: Short frame indication. Set if the received frame is less than 5 octets.
15 — Reserved, should be cleared.
Table 28-22. RxBD Field Descriptions (continued)
Bits Name Description
msb lsb
1 000...... 0
Valid data Invalid data
xxx .................................... xx