VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC880

High Performance 16x16

Serial Crosspoint Switch

G52191-0, Rev 4.2

Page 17

01/05/01

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2.5 Transmitter Operation

In Packet Mode, the transmitter sends out data words that come from the switch matrix, adding the appropriate

overhead bit information for acknowledges, response bits and flow control. Acknowledges are used to signal the

transceiver that a connection request has been granted (see the VSC870 data sheet). The response bits are used with

the Multi Queue connection request word. The flow control channel is used to pass state information from the

receiving port card to the transmitting port card. The switch redirects the flow control signals to the correct output

using the current switch connection state information. In the case of multicast where there is more than one receiver,

these channels are logically ORed before being sent back to the transmitter. The flow control channel is also used to

send response bits from the switch to the transceiver for multi queue mode (see VSC870 data sheet).

Response words are required by the transceiver in Packet Mode. These response words are simply IDLE words

or CRQ words in the data stream that are going back to the transceiver containing port connection status information.

If the transceiver receives a connection request word that requires a response, it can use the flow control channel to

force an IDLE word into the data stream. When this IDLE arrives at the switch, the switch adds the response data.

2.6 Disconnect Operation

A disconnect can be made to occur automatically after a packet is transmitted through the switch if the packet is

followed by either a CRQ (Connection Request) associated with a new packet, or a null CRQ followed by a null

header word. The CRQ associated with the new packet typically has the BRK bit set. This breaks down the old

connection before it tries to establish the new connection through arbitration. This new connection is made only when

the switch receives the header word. When the old connection is broken, the associated output ports are freed up,

becoming available for new connections. The null CRQ is a CRQ with the BRK bit set, and has no output port

selected. In this case, the switch chip will break down the old connection(s) and will not establish any new

connections. The null CRQ word must be followed by a null (empty) header word. The switch will send IDLE words

to the transceivers which have not established a new connection. During the packet transmission, if the sending link

goes out of synch, the switch will terminate the connection and send an end of packet word (CRQ with the BRK bit

set LOW) to the destination port followed by IDLE words. In this case, the receiving transceiver will receive only one

CRQ instead of the two normally received.

2.7 Flow Control Channel

The VSC880 can support a back pressure mechanism by providing a flow control channel. The flow control

channel is time shared with the signaling between the switch chip and the transceiver for acknowledgment and

response bits. Therefore, it can only guarantee to pass the state information from input pin at the receiving port card

through the switch and to the output pin at the transmitting port card. The main application for this flow control

channel is to prevent the FIFO on the receiving side from overflowing. By using this channel, when the receive FIFO

is almost full, the transmit FIFO will be disabled from sending data. During the time the switch is sending an ACK or

response bits back to the port cards, these flow control bits are dropped by the switch. During a multicast

transmission, the flow control channels are ORed in the switch. If a port is in an out of synch state, no flow control

back pressure is exerted from this output.