High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface

Datasheet

9.9.1.1Receive 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 LAN9312 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.

9.9.1.2Force Receiver Discard (Receiver Dump)

In addition to the Receive data Fast Forward feature, LAN9312 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 LAN9312 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 180.

9.9.2RX 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

Revision 1.4 (08-19-08)

134

SMSC LAN9312

 

DATASHEET