XSwitch

The XSwitch is an interconnect used by the XCore processor. The interconnect protocol is defined by XMOS, and is based around routing messages comprising 9-bit tokens between cores on a network. The protocol is specifically designed for on-chip and board-level communication, but using LVDS drivers it can also run over longer cables.

The interconnect routes sequences of messages. A message consists of a header that specifies the core address, a sequence of tokens, and an END-token. There are 512 tokens, 256 of which are data tokens, and 256 of which are control tokens. Data tokens are used to transport data (e.g. an audio data stream), the control tokens can be used to send in-band control data, in order to implement protocols over the interconnect. A message is routed through a sequence of switches interconnected by XLinks.

There are two control tokens that can be used to end a message: END which ends the message, and PAUSE which suspends the message. Both tokens will free the route through the switches. The difference between END and PAUSE is that the END token will be delivered to the receiver, whereas the PAUSE token will be silently thrown away by the last switch.

Since messages are terminated by a token (rather than have a packet length), the sender may chose not to terminate a message. This creates an open circuit with a guaranteed bandwidth and latency. Applications can use, for example, one series of links to form a circuit to stream audio, and use another series of links for small packets containing control data. Circuits guarantee in order delivery of all tokens transmitted. Messages may be delivered out of order if there are multiple paths. Circuits permanently occupy a series of links, whereas messages only occupy links for short periods of time.

The XSwitch interconnect architecture implements the media layer of the OSI model.

At the physical layer the signal is either transmitted using a serial protocol over two wires, or using a fast protocol over five wires. In both cases the signal is transmitted as a series of transitions using a 1 out of M code.

On a two-wire system a transition on wire 0 signals a '0' bit, and a transition on wire 1 signals a '1' bit. A token is transmitted as a sequence of exactly 10 transitions, the first eight signal the eight data bits (MSB first) then one bit to signal whether this is a control token, and then finally a return-to-zero bit that returns the wires to zero (after nine transitions one wire will be high).

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