Rainbow Electronics W90N745CDG - Manual (Page 98)

W90N745CD/W90N745CDG
Publication Release Date: September 22, 2006
- 93 - Revision A2
6.5.1 EMC Functional Description
MII Management State Machine
The MII management function of EMC is compliant to IEEE 802.3 Std. Through the MII management
interface, software can access the control and status registers of the external PHY chip. Tow
programmable register MIID (MAC MII Management Data Register) and MIIDA (MAC MII
Management Data Control and Address Register) are for MII management function. Set the bit BUSY
of MIIDA register will trigger the MII management state machine. After the MII management cycle is
finished, the BUSY bit will be cleared automatically.
Media Access Control (MAC)
The function of W90N745 MAC fully meets the requirements defined by the IEEE802.3u specification.
The following paragraphs will describe the frame structure and the operation of the transmission and
receive.
The transmission data frame sent from the transmit DMA will be encapsulated by the MAC before
transmitting onto the MII bus. The sent data will be assembled with the preamble, the start frame
delimiter (SFD), the frame check sequence and the padding for enforcing those less than 64 bytes to
meet the minimum size frame and CRC sequence. The out going frame format will be as following
110101010 --- 10101010 10101011 d0 d1 d2 - dn Padding CRC31 CRC30 --- CRC0
As mentioned by the above format, the preamble is a consecutive 7-byte long with the pattern
“10101010” and the SFD is a one byte 10101011 data. The padding data will be all 0 value if the sent
data frame is less than 64 bytes. The padding disable function specified in the bit P of the transmit
descriptor is used to control if the MAC needs to pad data at the end of frame data or not when the
transmitted data frame is less than 64 bytes. The padding data will not be appended if the padding
disable bit is set to be high. The bits CRC0 ... CRC31 are the 32 bits cyclic redundancy check (CRC)
sequence. The CRC encoding is defined by the following polynomial specified by the IEEE802.3. This
32 bits CRC appending function will be disabled if the Inhibit CRC of the transmission descriptor is set
to high.
The MAC also performs many other transmission functions specified by the IEEE802.3, including the
inter-frame spacing function, collision detection, collision enforcement, collision back off and
retransmission. The collision back-off timer is a function of the integer slot time, 512 bit times. The
number of slot times to delay between the current transmissions attempts to the next attempt is
determined by a uniformly distributed random integer algorithm specified by the IEEE802.3. The MAC
performs the receive functions specified by the IEEE 802.3 including the address recognition function,
the frame check sequence validation, the frame disassembly, framing and collision filtering.