RTL8208B-LF/RTL8208BF-LF
Datasheet
7.3.4. Jabber
Jabber occurs when TX_EN is asserted for longer than 21ms. Both transmit and loopback functions are
disabled once jabber has occurred. The MII Register 1.1 (Jabber detect) bit is set high until jabber
disappears and the bit is read again. The Jabber function is supported in 10-Base-T only, and is not
implemented in 100Base-TX. The collision LED of the corresponding port will blink while Jabber occurs.
Jabber is dismissed after TX_EN remains low for at least 500ms.
7.3.5. Loopback
Loopback mode can be achieved by writing to Register 0.14=1. Loopback mode routes transmitted data at
the output of NRZ to the NRZI conversion module, back to the receiving path. This mode is used to check
the device’s connections at the 5-bit symbol bus, and verify the operation of the Phase-Locked Loop
(PLL).
7.4. 100Base-TX
An internal 125MHz clock is generated by an on-chip PLL circuit to synchronize the transmit data or
generate the clock signal for the incoming data stream.
7.4.1. Transmit Function
Upon detection of TX_EN high, the RTL8208B(F)-LF converts RMII/SMII/SS-SMII TXD to a 5-bit
code-group and substitutes J/K code-groups for the first two code-groups (Start of Stream Delimiters
(SSD)). As long as TX_EN is asserted high, 4B5B coding continues for all data. At the end of TX_EN,
T/R code-groups are appended to the last data field. These are stripped off at the remote receiving end.
During the inter-packet gap, where TX_EN deasserted, IDLE code-groups are transmitted for clocking
purposes of the remote receiver. The 5-bit serial data stream is 4B5B coded and then scrambled, as
defined by the TP-PMD Stream Cipher function, to flatten the power spectrum energy such that EMI
effects are significantly reduced.
The scrambled seed is unique for each port based on PHY addresses. After scrambling, the bit stream is
driven onto the network media in the form of MLT-3 signaling. This multi-level signaling technology
moves the power spectrum energy from high frequency to low frequency, which also benefits EMI
emission. Scrambling is not implemented in 100Base-FX.
Single-Chip Octal 10/100-TX/FX PHY Transceiver
29
Track ID: JATR-1076-21 Rev. 1.3
REALTEK [ Realtek Semiconductor Corp. ]
