LAN89218 datasheet(18/148 Pages) MICROCHIP

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LAN89218

DS60001255B-page 18

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3.2

Flow Control

The LAN89218 Ethernet MAC supports full-duplex flow control using the pause operation and control frame. It also

supports half-duplex flow control using back pressure.

3.2.1

FULL-DUPLEX FLOW CONTROL

The pause operation inhibits data transmission of data frames for a specified period of time. A Pause operation

consists of a frame containing the globally assigned multicast address (01-80-C2-00-00-01), the PAUSE opcode, and

a parameter indicating the quantum of slot time (512 bit times) to inhibit data transmissions. The PAUSE parameter

may range from 0 to 65,535 slot times. The Ethernet MAC logic, on receiving a frame with the reserved multicast

address and PAUSE opcode, inhibits data frame transmissions for the length of time indicated. If a Pause request is

received while a transmission is in progress, then the pause will take effect after the transmission is complete. Control

frames are received and processed by the MAC and are passed on.

The MAC also transmits control frames (pause command) via both hardware and software control. The software

driver requests the MAC to transmit a control frame and gives the value of the PAUSE time to be used in the control

frame. The MAC Function constructs a control frame with the appropriate values set in all the different fields (as

defined in the 802.3x specification) and transmits the frame to the MII interface. The transmission of the control frame

is not affected by the current state of the Pause timer value that is set because of a recently received control frame.

3.2.2

HALF-DUPLEX FLOW CONTROL (BACKPRESSURE)

In half-duplex mode, back pressure is used for flow control. Whenever the receive buffer/FIFO becomes full or

crosses a certain threshold level, the MAC starts sending a Jam signal. The MAC transmit logic enters a state at the

end of current transmission (if any), where it waits for the beginning of a received frame. Once a new frame starts,

the MAC starts sending the jam signal, which will result in a collision. After sensing the collision, the remote station

will back off its transmission. The MAC continues sending the jam to make other stations defer transmission. The

MAC only generates this collision-based back pressure when it receives a new frame, in order to avoid any late

collisions.

3.2.3

VIRTUAL LOCAL AREA NETWORK (VLAN) SUPPORT

Virtual Local Area Networks or VLANs, as defined within the IEEE 802.3 standard, provide network administrators

one means of grouping nodes within a larger network into broadcast domains. To implement a VLAN, four extra bytes

are added to the basic Ethernet packet. As shown in Figure 3-1, the four bytes are inserted after the Source Address

Field and before the Type/Length field. The first two bytes of the VLAN tag identify the tag, and by convention are

set to the value 0x8100. The last two bytes identify the specific VLAN associated with the packet; they also provide

a priority field.

The LAN89218 supports VLAN-tagged packets. The LAN89218 provides two registers which are used to identify

VLAN-tagged packets. One register should normally be set to the conventional VLAN ID of 0x8100. The other register

provides a way of identifying VLAN frames tagged with a proprietary (not 0x8100) identifier. If a packet arrives bearing

either of these tags in the two bytes succeeding the Source Address field, the controller will recognize the packet as

a VLAN-tagged packet. In this case, the controller increases the maximum allowed packet size from 1518 to 1522

bytes (normally the controller filters packets larger than 1518 bytes). This allows the packet to be received, and then

processed by host software, or to be transmitted on the network.

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