LCAP

         

LCAP Release status: experimental [box doku]
Description	Das Labor CAN Automation Protocol
Author(s)	Hansinator
Last Version	0 ()
Platform	AVR, PC
License	GPLv2+
Download	SVN, browse

CAN2.0B

The CAN2.0B standard is the wire protocol used by the LCAP protocol.

CAN 2.0B Frame Features

• 11 + 18 = 29 Bits identifier
• Remote Transmission Request flag
• 4 Bit Data Length Code
• 0..8 Byte Data
• 15 Bits CRC
  • If CRC mismatches, an error frame is generated
 by a receiving node 
and the frame is repeated
• ACK Slot (any receiver may ack, though)
• Frame types:
  • Data
  • Remote Request
  • Error
  • Overload

CAN2.0B Frame Logical View

The following C struct describes the logical view of a CAN2.0B frame.

typedef struct
{
  uint32_t id;
  uint8_t dlc;
  uint8_t data[8];
} can_message;

The field "id" is assumed to have the following structure:

• Bit 28:18 -> 11 Bit CAN Standard Identifier
• Bit 17:00 -> 18 Bit CAN Extended Identifier
• (Bit intervals are inclusive)

The field "dlc" is assumed to have the following structure:

• Bit 4 -> Remote Transmission Request (RTR) flag
• Bit 3:0 -> CAN Data Length Code (DLC)

The field data contains 0 to 8 data bytes.

Note: The RTR flag is not present when using the current labor can mcp2515 driver!

LCAP

LCAP Basic Message Format (Draft)

• 8 Bit Device or 10/12 bit Topic Address
• CAN2.0B Message ID Bit-Allocation (from left to right in [std-ident:ext-ident])
  • 1 Bit DST Type (1 = Device, 0 = Topic)
  • 8 Bit DST Device or 10/12 Bit Topic
  • in case of device + 2 Bits:
    • 1 bit (R)ACK Flag
    • 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)
  • 8 Bit SRC Device
  • 8 Bit sub-protocol (if topic addressing is 12 Bit this is 6 Bit in topic mode then)
  • 2 Bit reserved for future use (encryption flags or stuff)
must be zero
  • Implicit 4 Bit DLC
  • Implicit 0..8 Bytes Data

LCAP Basic Message Semantics (Draft)

• 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

LCAP Frame Logical View (draft)

The following C struct describes the logical view of an LCAP message.

typedef enum { dst_topic = 0x0, dst_device = 0x1, req_topic = 0x2, req_device = 0x3 }  lcap_type_t;

typedef struct {
  lcap_type_t dst_type;
  union { struct { uint8_t dst_addr, flags; }; uint16_t dst_topic; };
  uint8_t src_addr;
  uint8_t sub_addr;
  uint8_t len;
  uint8_t *data[];
} lcap_msg_t;

Explanations will follow soon! ;-)

Ideas

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