MPC8540 datasheet(1207/1302 Pages) FREESCALE

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Functional Description

MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

Freescale Semiconductor

21-67

21.6.3.2 Receive Buffer Descriptor (RxBD)

In the RxBD the user initializes the E, I, and W bits in the first word and the pointer in second

word. If the data buffer is used, the FEC modifies the E, L, F, M, BC, MC, LG, NO, SH, CR, OV,

and TR bits and writes the length of the used portion of the buffer in the first word. The M, BC,

MC, LG, NO, SH, CR, OV, and TR bits in the first word of the buffer descriptor are modified by

the FEC only if the L bit is set. The first word of the RxBD contains control and status bits.

Figure 21-47 describes the RxBD.

Table 21-49 describes the fields of the RxBD.

Offset + 0

TXTRUNC TX truncation. Set in the last TxBD (TxBD[L] is set) when IEVENT[BABT] occurs

for the frame.

Offset + 2

0–15

Data

Length

Data length is the number of octets the FEC transmits from this BD’s data buffer.

It is never modified by the FEC. This field must be greater than zero. This field

must be greater than zero.

Offset + 4

0–31

TX Data

Buffer

Pointer

The transmit buffer pointer contains the address of the associated data buffer.

There are no alignment requirements for this address.

0123456

789

Offset + 0

RO1

BCMCLG

CROV

Offset + 2

DATA LENGTH

Offset + 4

RX DATA BUFFER POINTER

Offset + 6

Figure 21-47. Receive Buffer Descriptor

Table 21-49. Receive Buffer Descriptor Field Descriptions

Offset

Bits

Name

Description

Offset + 0

Empty. Written by the FEC (when cleared) and by the user (when set).

0 The data buffer associated with this BD is filled with received data, or data reception

is aborted due to an error condition. Status and length fields have been updated as

required.

1 The data buffer associated with this BD is empty, or reception is currently in progress.

Offset + 0

RO1

Receive software ownership bit.

This field is reserved for use by software. This read/write bit is not modified by hardware,

nor does its value affect hardware.

Offset + 0

Wrap. Written by user.

0 The next buffer descriptor is found in the consecutive location.

1 The next buffer descriptor is found at the location defined in RBASE.

Table 21-48. Transmit Data Buffer Descriptor (TxBD) Field

Descriptions (continued)

Offset

Bits

Name

Description

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