LAN9220 datasheet(16/142 Pages) MICROCHIP

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LAN9220

DS00002417A-page 16

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3.2

Flow Control

The LAN9220 Ethernet MAC supports full-duplex flow control using the pause operation and control frame. It also sup-

ports 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 param-

eter 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 cur-

rent 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 trans-

mission. 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, "VLAN Frame", 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 LAN9220 supports VLAN-tagged packets. The LAN9220 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 (nor-

mally 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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