
14
MD400177/B
84220
2.2.5 RMII - 10 Mbps
10 Mbps RMII operation is identical to 100 Mbps RMII
operation, except:
 The CLKIN frequency remains at 50 Mhz (same as 100
Mbps operation).
 Each data di-bit must be input on TXD[1:0] for ten
consecutive CLKIN cycles.
 Each data di-bit will be output on RXD[1:0] for ten
consecutive CLKIN cycles.
2.2.6 FBI - 100 Mbps
The Five Bit Interface (FBI), or Symbol Interface, is a five
bit wide interface produced when the 4B5B encoder/
decoder is bypassed. The FBI is primarily used for
repeaters or Ethernet controllers that have integrated
encoder/decoders.
The FBI is identical to the MII, except:
 The FBI data path is five bits wide, not nibble wide like 
the MII.
 TXER pin is changed to be the fifth transmit data bit, 
TXD4.
 RXER pin is changed to be the fifth receive data bit, 
RXD4.
 CRS is asserted as long as the device is in the Link 
Pass state (CRS no longer asserted/deasserted at 
beginning/end of packet).
 COL is not valid.
 RXDV is not valid.
 TXEN is ignored.
2.2.7 FBI - 10 Mbps
The FBI  is not available in 10 Mbps mode.
2.2.8 Selection of MII, RMII, or FBI
MII is the default interface to the MAC controller. RMII is
selected by asserting the RMII_EN pin, a global control.
The FBI is automatically enabled when the 4B5B encoder/
decoder is bypassed. Bypassing the encoder/decoder
passes the 5B symbols between the receiver/transmitter
directly to the FBI without any alteration or substitutions.
The 4B5B encoder/decoder can be bypassed by setting
the bypass encoder bit in the MI serial port Channel
Configuration register.
When the FBI is enabled, it may also be desirable to
bypass the scrambler/descrambler and disable the
internal CRS loopback function.  The scrambler/
descrambler can be bypassed by setting the bypass
scrambler bit in the MI serial port Channel Configuration
register.  The internal CRS loopback can be disabled by
setting the TXEN to CRS loopback disable bit in the MI
serial port Channel Configuration register.
2.2.9 MII Disable
The MII and FBI inputs and outputs can be disabled by
setting the MII disable bit in the MI serial port Control
register. When the MII is disabled, the inputs are ignored,
the outputs are placed in high impedance state, and the
TP output is high impedance.
2.2.10 TXEN to CRS Loopback Disable
The internal TXEN to CRS loopback can be disabled by
appropriately setting the TXEN to CRS loopback disable
bit in the MI serial port Channel Configuration register.
TXEN to CRS loopback is disabled  in RMII mode.
2.3 ENCODER
2.3.1 4B5B Encoder - 100 Mbps
100BaseTX and 100BaseFX require that the data be
4B5B  encoded.   4B5B  coding  converts  the   4  bit   data
nibbles  into 5 bit data words.  The mapping of the 4B
nibbles to the 5B code words is specified in IEEE 802.3
and shown in Table 1.  The 4B5B encoder on the 84220
takes 4B nibbles from the controller interface, converts
them into 5B words according to, Table 1, and sends the
5B words to the scrambler.   The 4B5B encoder also
substitutes the first eight bits of the preamble with the
SSD delimiters (/J/K/ symbols) and adds an  ESD
delimiter (/T/R/ symbols) to the end of each packet, as
defined in IEEE 802.3 and shown in Figure 2. The 4B5B
encoder also fills the period between packets, called the
idle period, with a continuous stream of idle symbols, as
shown in Figure 2.