Hypertransport.
  designed to inter-connect CPUs and/or bus controllers.

  Versions 1.x, 2.0, 3.0, 3.1
 
  Clock speeds from 200 MHz to 3.2 GHz depending on version.
    
  Implements DDR data transfers - transfer on both up and down tick of clock.

  Provide point to point links.
    Chain - physical link between host bridge and a device.
      Link has host bridge on one end and single linked device on other. 
      All chains must terminatie with single link device.
 
    Tunnels - allows dual link devices to be physically daisy chained while 
      maintaining logical point to point connections.
      (Simple switching router).

    Bridges - bridges between host bus and one or more logical secondary bus.
      Routing circuitry for interface multiple devices including 
        other HT bridges. Support physical tree topology.
      Isolates each bus so signal timing applies only to that bus.

  Two classes of links.
    Link signal group.
      Links used to transfer packets in either direction.

      CAD Link (control,address,data) width 2 to 32 bits wide.
        Link uses a LVDS-like line pairs for each bit, 
          a pair for each direction.

        Requires one minimal connection in either direction.

        Links in opposite directions don't have to be same width.

      Clock pair - timing.
        one for each 8 bit set of CAD links.  1, 2, or 4

      Control(CTL) - signals if CAD transporting Control or Data.
        one for each 8 bit set of CAD links.  1, 2, or 4
        # gen 1 HT used a single CTL.

      Signalling very similar to low voltage differential signalling.

    Support signal group.
      power, reset, and various other controls. 

      Uses single end point signalling.

  Packetized protocol.

  Packets are 4 to 64 bytes in size  (32-bit words).
    always multiples of 4.

  Hypertransport uses transaction based communication
    consisting of 1 or more packets.

    Starts with a command packet. (request)
      if data transfer, includes size of data to be moved.
    Usually followed by an acknowledgement packet
    Possibly follwed by a data packet of up to 64 bytes.
      
  Uses C/BE to flag command packets. 
    Command packets 
      1st 5 bits indicate command type.
      Rest of packet provide appropriate information.
      Minimum of 32 bits.