LCAP

CAN 2.0B Protocol Features

• 11 + 18 = 29 Bits Identifier
• 8 Byte Data
• 4 Bit Data Length
• 15 Bits CRC
  • If CRC mismatches, an error frame is generated
(by which receiving node? all? id-matched ones?)
and the frame is repeated
• ACK Slot (any receiver may ack, though)
• Remote Transmission Request Flag
• Frame types:
  • Data
  • Remote Request
  • Error
  • Overload

LCAP Basic Message Format (Draft)

• 8 Bit Device or Topic Address
• Message ID Allocation
  • 1 Bit DST Type (1 = Device, 0 = Topic)
  • 8 / 10(12?) Bit DST Device / Topic
  • in case of device + 2 Bits:
    • 1 bit long packet start or sequence bit
      • DLC == 8 && set -> start long packet
      • DLC < 8 && long packet -> stop
      • DLC == 8 && long packet -> sequence bit (toggle)
    • 1 bit (R)ACK Flag
  • 8 Bit SRC Device
  • 8 Bit sub-protocol
  • 2 Bit reserved for future use (encryption flags or stuff)
must be zero
  • 8 Bytes Data
• if reserved bits are not zero, the message must be ignored
• 0x(3)FF is the broadcast address
  • for devices this addresses all devices
  • for topics, this is the system topic
• request messages
  • requesting from normal devices
    • to devices -> ping request or ack
      • ping is answered with an ack data frame if
the requested sub-protocol is understood
      • if the rack bit is set, this is an ack packet
    • to topics -> request device to publish topic data
  • requesting from broadcast device address
    • to devices -> ignore
    • to topics -> any device may answer
      • may be useful as a sync for freshness counter stuff
  • request to system topic
    • request service, either from any master or a specific one, depending on the src address
    • if broadcast address is src address, all masters may try to answer, but must abort if they lost bus arbritation (higher priority system master overrides)
  • request to broadcast address
    • from normal device -> request ping
      • rack bit set -> request from all devices (all devices must answer)
      • rack bit not set -> any device may answer
    • from broadcast address -> ignore

Configure can chip to use rec buffer 0 for device messages, having one filter set to device address and the other to broadcast use rec buffer 1 for topic messages, supporting pre-filtering of 4 topics or groups