MPC8540 datasheet(751/1302 Pages) FREESCALE

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Functional Description

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

Freescale Semiconductor

14-125

14.6.3 Buffer Descriptors

The TSEC buffer descriptor (BD) is modeled after the MPC8260 Fast Ethernet controller BD for

ease of reuse. Drawing from the MPC8260 FEC BD programming model, the TSEC descriptor

base registers point to the beginning of BD rings. The 8-byte data BD format is similar to the

MPC8260 BD model.

Data buffers are used in the transmission and reception of Ethernet frames (see Figure 14-116).

Data BDs encapsulate all information necessary for the TSEC to transmit or receive an Ethernet

frame. Within each data BD there is a status field, a data length field, and a data pointer. The BD

completely describes an Ethernet packet by centralizing status information for the data packet in

the status field of the BD and by containing a data BD pointer to the location of the data buffer.

Software is responsible for setting up the BDs in memory. Because of pre-fetching, a minimum of

two buffer descriptors per ring are required. This applies to both the transmit and the receive

descriptor rings. Software also must have the data pointer pointing to memory. Within the status

field, there exists an ownership bit which defines the current state of the buffer (pointed to by the

data pointer). Other bits in the status field in the buffer descriptor are used to communicate

status/control information between the TSEC and the software driver.

The status field of the BD is 16-bit field, as is the length field. The data buffer pointer is a 32-bit

field. Therefore, the BDs should be accessed with the following C structure:

typedef unsigned short uint_16; /* choose 16-bit native type */

typedef unsigned int uint_32; /* choose 32-bit native type */

typedef struct bd_struct {

uint_16 flags;

uint_16 length;

uint_32 bufptr;

};

Because there is no next BD pointer in the transmit/receive BD (see Figure 14-117), all BDs must

reside sequentially in memory. The TSEC increments the current BD location appropriately to the

next BD location to be processed. There is a wrap bit in the last BD that informs the TSEC to loop

back to the beginning of the BD chain. Software must initialize TBASE and RBASE that point to

the beginning transmit and receive BDs for the TSEC.

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