LAN9211 datasheet(21/144 Pages) SMSC

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High-Performance Small Form Factor Single-Chip Ethernet Controller with HP Auto-MDIX

Datasheet

SMSC

LAN9211

Revision 1.8 (06-06-07)

DATASHEET

The LAN9211 can store up to 250 Ethernet packets utilizing FIFOs, totaling 16K bytes, with a packet

granularity of 4 bytes. This memory is shared by the RX and TX blocks and is configurable in terms

of allocation. This depth of buffer storage minimizes or eliminates receive overruns.

3.2

Flow Control

The LAN9211 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,

"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 LAN9211 supports VLAN-tagged packets. The LAN9211 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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