Filter
Top Products
6-24 MPC860T (Rev. D) Fast Ethernet Controller Supplement MOTOROLA
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
(though these steps could also be done before setting ETHER_EN).
6.4 Buffer Descriptors (BDs)
Data for Fast Ethernet frames must reside in memory external to the MPC860T device.
Frame data is placed in one or more buffers, each of which is pointed to by a BD, which
also contains the current state of the buffer. For maximum user ßexibility, BDs are also
located in external memory.
A buffer is produced by setting TxBD[R] or RxBD[E]. Writing to either X_DES_ACTIVE
or R_DES_ACTIVE indicates that a buffer is in external memory for the transmit or receive
data trafÞc, respectively. The hardware reads the BDs and processes the buffers. After the
data DMA completes and the BD status bits are written by the DMA engine, hardware
clears TxBD[R] or RxBD[E] to signal that the buffer was processed. Software can poll the
BDs or may rely on the buffer/frame interrupts to detect when buffers have been processed.
ECNTRL[ETHER_EN] operates as a reset to the BD/DMA logic. When ETHER_EN is
cleared, the DMA engine BD pointers are reset to point to the starting TxBDs and RxBDs.
The BDs are not initialized by hardware during reset. At least one TxBD and one RxBD
must be initialized by software (write 0x0000_0000 to the most signiÞcant word of the BD)
before ETHER_EN is set.
The BDs operate as a ring. R_DES_START deÞnes the starting address for the RxBD ring
and X_DES_START deÞnes the starting address for TxBD ring. The last BD in each ring
is indicated by the wrap (W) bit. When set, W indicates that the next BD in the ring is at the
location pointed to by R_DES_START and X_DES_START for the receive and transmit
rings, respectively. BD rings must start on a double-word boundary.
6.4.1 Ethernet Receive Buffer Descriptor (RxBD)
The RxBD is shown in Figure 6-23. The Þrst word of the RxBD contains control and status
bits. The user initializes RxBD[E,W] and the Rx buffer pointer. When the buffer has been
accessed by a DMA, the FEC modiÞes RxBD[E,L,M,BC,MC,LG,NO,SH,CR,OV,TR] and
writes the length of the used portion of the buffer in the Þrst word. The FEC modiÞes
RxBD[M,BC,MC,LG,NO,SH,CR,TR,OV] only if L = 1.
Table 6-27. User Initialization (after Setting ECNTRL[ETHER_EN])
Step Description
1 Fill RxBD ring with empty buffers
2 Set R_DES_ACTIVE
Fr
eescale S
emiconduct
or
, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
nc...