LAN91C111 datasheet(21/125 Pages) MICROCHIP

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DS00002276A-page 21

LAN91C111

RXD0-RXD3 should always be aligned to packet nibbles, therefore, opening flag detection does not consider misaligned

cases. Opening flag detection expects the 5Dh pattern and will not reject the packet on non-preamble patterns.

CRS100 is used as a frame envelope signal for the CSMA/CD MAC state machines (deferral and backoff functions),

but it is not used for receive framing functions. CRS100 is an asynchronous signal and it will be active whenever there

is activity on the cable, including LAN91C111 transmissions and collisions.

7.6

Serial EEPROM Interface

This block is responsible for reading the serial EEPROM upon hardware reset (or equivalent command) and defining

defaults for some key registers. A write operation is also implemented by this block, that under CPU command will pro-

gram specific locations in the EEPROM. This block is an autonomous state machine and controls the internal Data Bus

of the LAN91C111 during active operation.

7.7

Internal Physical Layer

The LAN91C111 integrates the IEEE 802.3 physical layer (PHY) internally. The EXT PHY bit in the Configuration Reg-

ister is 0 as the default configuration to set the internal PHY enabled. The internal PHY address is 00000, the driver

must use this address to talk to the internal PHY. The internal PHY is placed in isolation mode at power up and reset. It

can be removed from isolation mode by clearing the MII_DIS bit in the PHY Control Register. If necessary, the internal

PHY can be enabled by clearing the EXT_PHY bit in the Configuration Register.

The internal PHY of LAN91C111 has nine main sections: controller interface, encoder, decoder, scrambler, descrambler,

clock and data recovery, twisted pair transmitter, twisted pair receiver, and MI serial port.

The LAN91C111 can operate as a 100BASE-TX device (hereafter referred to as 100Mbps mode) or as a 10BASE-T

device (hereafter referred to as 10Mbps mode). The difference between the 100Mbps mode and the 10Mbps mode is

data rate, signaling protocol, and allowed wiring. The 100Mbps TX mode uses two pairs of category 5 or better UTP or

STP twisted pair cable with 4B5B encoded, scrambled, and MLT-3 coded 62.5 MHz ternary data to achieve a throughput

of 100Mbps. The 10Mbps mode uses two pairs of category 3 or better UTP or STP twisted pair cable with Manchester

encoded, 10MHz binary data to achieve a 10Mbps throughput. The data symbol format on the twisted pair cable for the

100 and 10Mbps modes are defined in IEEE 802.3 specifications and shown in Figure 7-3.

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