LAN9220 datasheet(53/142 Pages) MICROCHIP

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DS00002417A-page 53

LAN9220

3.13.1.1

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 RX_DP_CTRL register, the host can instruct the LAN9220 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 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, RX status FIFO, or the TX status

FIFO until the RX_FFWD bit is cleared. Other resources can be accessed during this time (i.e., any registers and/or the

TX data FIFO). After the fast-forward operation has completed and the RX_FFWD bit has been cleared, a wait time

restriction must be observed before reading the TX or RX status FIFO’s, as specified in Section 6.2.2, "Special Restric-

tions on Back-to-Back Read Cycles," on page 116. Also note that the RX_FFWD will only fast-forward the RX data FIFO,

not the RX status FIFO.

The receiver does not have to be stopped to perform a fast-forward operation.

3.13.1.2

Force Receiver Discard (Receiver Dump)

In addition to the Receive data Fast Forward feature, LAN9220 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 FIFOs will be returned to their reset state. To perform a receiver dump, the

LAN9220 receiver must be halted. Once the receiver stop completion is confirmed, the RX_DUMP bit can be set in the

RX_CFG register. The RX_DUMP bit is cleared when the dump is complete. For more information on stopping the

receiver, please refer to Section 3.13.4, "Stopping and Starting the Receiver," on page 56. For more information on the

RX_DUMP bit, please refer to Section 5.3.7, "RX_CFG—Receive Configuration Register," on page 74.

3.13.2

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 3-26.

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.

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