Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

4.3.3NRZI and MLT-3 Decoding

The DSP generates the MLT-3 recovered levels that are fed to the MLT-3 converter. The MLT-3 is then converted to an NRZI data stream.

4.3.4Descrambling

The descrambler performs an inverse function to the scrambler in the transmitter and also performs the Serial In Parallel Out (SIPO) conversion of the data.

During reception of IDLE (/I/) symbols. the descrambler synchronizes its descrambler key to the incoming stream. Once synchronization is achieved, the descrambler locks on this key and is able to descramble incoming data.

Special logic in the descrambler ensures synchronization with the remote transceiver by searching for IDLE symbols within a window of 4000 bytes (40us). This window ensures that a maximum packet size of 1514 bytes, allowed by the IEEE 802.3 standard, can be received with no interference. If no IDLE- symbols are detected within this time-period, receive operation is aborted and the descrambler re-starts the synchronization process.

The descrambler can be bypassed by setting bit 0 of register 31.

4.3.5Alignment

The de-scrambled signal is then aligned into 5-bit code-groups by recognizing the /J/K/ Start-of-Stream Delimiter (SSD) pair at the start of a packet. Once the code-word alignment is determined, it is stored and utilized until the next start of frame.

4.3.65B/4B Decoding

The 5-bit code-groups are translated into 4-bit data nibbles according to the 4B/5B table. The translated data is presented on the RXD[3:0] signal lines. The SSD, /J/K/, is translated to “0101 0101” as the first 2 nibbles of the MAC preamble. Reception of the SSD causes the transceiver to assert the RXDV signal, indicating that valid data is available on the RXD bus. Successive valid code-groups are translated to data nibbles. Reception of either the End of Stream Delimiter (ESD) consisting of the /T/R/ symbols, or at least two /I/ symbols causes the transceiver to de-assert carrier sense and RXDV.

These symbols are not translated into data.

The decoding process may be bypassed by clearing bit 6 of register 31. When the decoding is bypassed the 5th receive data bit is driven out on RXER/RXD4/PHYAD0. Decoding may be bypassed only when the MAC interface is in MII mode.

4.3.7Receive Data Valid Signal

The Receive Data Valid signal (RXDV) indicates that recovered and decoded nibbles are being presented on the RXD[3:0] outputs synchronous to RXCLK. RXDV becomes active after the /J/K/ delimiter has been recognized and RXD is aligned to nibble boundaries. It remains active until either the /T/R/ delimiter is recognized or link test indicates failure or SIGDET becomes false.

RXDV is asserted when the first nibble of translated /J/K/ is ready for transfer over the Media Independent Interface (MII mode).

Revision 1.0 (05-28-09)

22

SMSC LAN8720/LAN8720i

 

DATASHEET