The Gigabit Media Independent Interface (GMII) is the IEEE defined interface for use between an Ethernet PHY and an Ethernet MAC. GMII is available on the PAP devices only. The purpose of GMII is to make various physical media transparent to the MAC layer. The GMII Interface accepts either GMII or MII data, control and status signals and routes them either to the 1000BASE-T, 100BASE-TX, or 10BASE-Te modules, respectively.

The GMII interface has the following characteristics:

• Supports 10/100/1000 Mbps operation
• Data and delimiters are synchronous to clock references
• Provides independent 8-bit wide transmit and receive data paths
• Provides a simple management interface
• Provides for Full-Duplex operation

The GMII interface is defined in IEEE 802.3 Clause 35. In each direction of data transfer, there are Data (an eight-bit bundle), Delimiter, Error, and Clock signals. GMII signals are defined such that an implementation may multiplex most GMII signals with the similar PCS service interface defined in IEEE 802.3 Clause 22. Two media status signals are provided. One indicates the presence of carrier (CRS), and the other indicates the occurrence of a collision (COL). The MII signal names have been retained and the functions of most signals are the same, but additional valid combinations of signals have been defined for 1000 Mbps operation.

The connection diagram for GMII is shown in Figure 8-7.

Figure 8-7 GMII Connections