SMSC LAN9311i - Manual (Page 135)

Two Port 10/100 Managed Ethernet Switch with 16-Bit Non-PCI CPU Interface
Datasheet
SMSC LAN9311/LAN9311i 135 Revision 1.4 (08-19-08)
DATASHEET
9.9.1.1 Receive Data FIFO Fast Forward
The RX data path implements an automatic data discard function. Using the RX Data FIFO Fast
Forward bit (RX_FFWD) in the
Receive Datapath Control Register (RX_DP_CTRL) , the host can
instruct the LAN9311/LAN9311i to skip the packet at the head of the RX Data FIFO. The RX Data FIFO
pointers are automatically incremented to the beginning of the next RX packet.
When performing a fast-forward, there must be at least 4 DWORDs of data in the RX Data FIFO for
the packet being discarded. For cases with less than 4 DWORDs, do not use RX_FFWD. In this case
data must be read from the RX Data FIFO and discarded using standard PIO read operations.
After initiating a fast-forward operation, do not perform any reads of the RX Data FIFO until the
RX_FFWD bit is cleared. Other resources can be accessed during this time (i.e., any registers and/or
the other three FIFO’s). Also note that the RX_FFWD will only fast-forward the RX Data FIFO, not the
RX Status FIFO. After an RX fast-forward operation the RX status must still be read from the RX Status
FIFO.
The receiver does not have to be stopped to perform a fast-forward operation.
9.9.1.2 Force Receiver Discard (Receiver Dump)
In addition to the Receive data Fast Forward feature, LAN9311/LAN9311i also implements a receiver
"dump" feature. This feature allows the host processor to flush the entire contents of the RX Data and
RX Status FIFOs. When activated, the read and write pointers for the RX Data and Status FIFO’s will
be returned to their reset state. To perform a receiver dump, the LAN9311/LAN9311i receiver must be
halted. Once the receiver stop completion is confirmed, the RX_DUMP bit can be set in the
Receive
Configuration Register (RX_CFG) . The RX_DUMP bit is cleared when the dump is complete. For more
information on stopping the receiver, please refer to Section 9.9.4, "Stopping and Starting the
Receiver" . For more information on the RX_DUMP bit, please refer to Section 14.2.2.1, "Receive
Configuration Register (RX_CFG)," on page 181 .
9.9.2 RX Packet Format
The RX status words can be read from the RX Status FIFO port, while the RX data packets can be
read from the RX Data FIFO. RX data packets are formatted in a specific manner before the host can
read them as shown in
Figure 9.9 . It is assumed that the host has previously read the associated
status word from the RX Status FIFO, to ascertain the data size and any error conditions.
Figure 9.9 RX Packet Format
ofs + First Data DWORD
.
.
.
.
Last Data DWORD
0 31
Host Read
Order
1st
2nd
Last
Optional Pad DWORD0
.
.
Optional Pad DWORDn
Optional offset DWORD0
.
.
Optional offset DWORDn