Fast Ethernet Controller (FEC)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
45-34
Freescale Semiconductor
45.3.3.2.1
Descriptor Controller Initialization
In the FEC, the descriptor control machine initializes a few registers whenever ECNTRL[ETHER_EN] is
set. The transmit and receive FIFO pointers are reset, the transmit backoff random number is initialized
and the transmit and receive blocks are activated. After the descriptor controller initialization sequence,
hardware is ready for operation, waiting for R_DES_ACTIVE and X_DES_ACTIVE to be asserted by the
user.
45.3.3.2.2
User Initialization (after Setting ECNTRL[ETHER_EN])
The user must initialize portions of the FEC after setting ECNTRL[ETHER_EN]. The exact values depend
on the application. The sequence resembles that shown in
Table 45-34
. (though these steps could also be
done before setting ETHER_EN).
45.3.4
Buffer Descriptors (BDs)
Data for Fast Ethernet frames must reside in external memory. 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
flexibility, 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 traffic,
respectively. The hardware reads the BDs and processes the buffers. After the DMA transfer of the data
and the updating of the BD status bits, 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 significant word of the BD) before ETHER_EN is set.
15
Initialize (empty) TxBD ring
16
Initialize (empty) RxBD ring
17
Set Port D PDPAR register
18
Set Port D PDDIR register
Table 45-34. User Initialization (After Setting ECNTRL[ETHER_EN])
Step Description
1
Fill RxBD ring with empty buffers
2
Set R_DES_ACTIVE
Table 45-33. User Initialization (Before Setting ECNTRL[ETHER_EN]) (continued)
Step Description
Содержание PowerQUICC MPC870
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