DS580 Datasheet, PDF (6/42 Pages) Xilinx, Inc – Optional MDIO interface for PHY access

LogiCORE IP XPS Ethernet Lite Media Access Controller
Half-Duplex
1. Even when it has nothing to transmit, the Ethernet Lite MAC monitors the bus for traffic by watching the carrier
sense signal (PHY_crs) from the external PHY. Whenever the bus is busy (PHY_crs = ’1’), the Ethernet Lite
MAC defers to the passing frame by delaying any pending transmission of its own.
2. After the last bit of the passing frame (when carrier sense signal changes from true to false), the Ethernet Lite
MAC starts the timing of the interframe gap.
3. The Ethernet Lite MAC will reset the interframe gap timer if carrier sense becomes true.
Full-Duplex
1. The Ethernet Lite MAC does not use the carrier sense signal from the external PHY when in full duplex mode
because the bus is not shared and only needs to monitor its own transmissions. After the last bit of an Ethernet
Lite MAC transmission, the Ethernet Lite MAC starts the timing of the interframe gap.
Carrier sense multiple access with collision detection (CSMA/CD) access method
A full duplex ethernet bus is by definition, a point to point dedicated connection between two ethernet devices
capable of simultaneous transmit and receive with no possibility of collisions.
For a half duplex ethernet bus, the CSMA/CD media access method defines how two or more stations share a com-
mon bus.
To transmit, a station waits (defers) for a quiet period on the bus (no other station is transmitting (PHY_crs = ’0’))
and then starts transmission of its message after the interframe gap period.
If, after initiating a transmission, the message collides with the message of another station (PHY_col - ’1’), then each
transmitting station intentionally continues to transmit (jam) for an additional predefined period (32 bits for 10/100
Mbps) to ensure propagation of the collision throughout the system.
The station remains silent for a random amount of time (backoff) before attempting to transmit again.
A station can experience a collision during the beginning of its transmission (the collision window) before its trans-
mission has had time to propagate to all stations on the bus. Once the collision window has passed, a transmitting
station has acquired the bus.
Subsequent collisions (late collisions) are avoided because all other (properly functioning) stations are assumed to
have detected the transmission and are deferring to it.
The time to acquire the bus is based on the round-trip propagation time of the bus (64 byte times for 10/100 Mbps).
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DS580 September 21, 2010
Product Specification