Datasheet

DP83848VYB

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SNLS266D –MAY 2007–REVISED APRIL 2013

6 Functional Description

The DP83848VYB supports several modes of operation using the MII interface pins. The options are

defined in the following sections and include:

— MII Mode

— RMII Mode

— 10 Mb Serial Network Interface (SNI)

The modes of operation can be selected by strap options or register control. For RMII mode, it is required

to use the strap option, since it requires a 50 MHz clock instead of the normal 25 MHz.

In each of these modes, the IEEE 802.3 serial management interface is operational for device

configuration and status. The serial management interface of the MII allows for the configuration and

control of multiple PHY devices, gathering of status, error information, and the determination of the type

and capabilities of the attached PHY(s).

6.1 MII INTERFACE

The DP83848VYB incorporates the Media Independent Interface (MII) as specified in Clause 22 of the

IEEE 802.3u standard. This interface may be used to connect PHY devices to a MAC in 10/100 Mb/s

systems. This section describes the nibble wide MII data interface.

The nibble wide MII data interface consists of a receive bus and a transmit bus each with control signals

to facilitate data transfer between the PHY and the upper layer (MAC).

6.1.1 Nibble-wide MII Data Interface

Clause 22 of the IEEE 802.3u specification defines the Media Independent Interface. This interface

includes a dedicated receive bus and a dedicated transmit bus. These two data buses, along with various

control and status signals, allow for the simultaneous exchange of data between the DP83848VYB and

the upper layer agent (MAC).

The receive interface consists of a nibble wide data bus RXD[3:0], a receive error signal RX_ER, a receive

data valid flag RX_DV, and a receive clock RX_CLK for synchronous transfer of the data. The receive

clock operates at either 2.5 MHz to support 10 Mb/s operation modes or at 25 MHz to support 100 Mb/s

operational modes.

The transmit interface consists of a nibble wide data bus TXD[3:0], a transmit enable control signal

TX_EN, and a transmit clock TX_CLK which runs at either 2.5 MHz or 25 MHz.

Additionally, the MII includes the carrier sense signal CRS, as well as a collision detect signal COL. The

CRS signal asserts to indicate the reception of data from the network or as a function of transmit data in

Half Duplex mode. The COL signal asserts as an indication of a collision which can occur during half-

duplex operation when both a transmit and receive operation occur simultaneously.

6.1.2 Collision Detect

For Half Duplex, a 10BASE-T or 100BASE-TX collision is detected when the receive and transmit

channels are active simultaneously. Collisions are reported by the COL signal on the MII.

If the DP83848VYB is transmitting in 10 Mb/s mode when a collision is detected, the collision is not

reported until seven bits have been received while in the collision state. This prevents a collision being

reported incorrectly due to noise on the network. The COL signal remains set for the duration of the

collision.

If a collision occurs during a receive operation, it is immediately reported by the COL signal.

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