Message Mailboxes

Some other CAN driver libraries in the market feature what usually is called "mailboxes", "message objects", "user buffers" etc. CANlib, on the other hand, focuses on message queues because we feel this imposes fewer restrictions on the CAN system.

For example, a higher layer protocol that uses a sequence of CAN messages with the same identifier is hard to implement using mailboxes: when the next message arrives it overwrites the previous one. You can, in a way, simulate the behavior of mailboxes by using canReadSpecific(). You will have to call canRead() periodically in order to clear out messages not read by canReadSpecific().

Example. How to simulate a mailbox-style CAN interface.

Assume you take special interest in CAN messages with identifiers 530, 540 and 550. Your message-handling loop could then look something like this:

while (..) {
  if (canReadSpecific(hnd, 530, buf, ...) == canOK) {
    // Process msg 530
  }

  if (canReadSpecific(hnd, 540, buf, ...) == canOK) {
    // Process msg 540
  }

  if (canReadSpecific(hnd, 550, buf, ...) == canOK) {
    // Process msg 550
  }

  while (canRead(hnd, &id, buf, ...) == canOK) {
    // Handle other messages
  }
}

You can call this message-processing routine from your WM__CANLIB handler or make it the main routine in your program.

Message Queue and Buffer Sizes

Incoming and outgoing CAN messages can be buffered in three different places:

• In the CAN controller
• On the CAN board
• In the CAN driver (i.e. CANlib)

The sizes of the first two buffers are clearly hardware dependent. You might find the following information useful.

Many CAN boards from KVASER uses the SJA1000 CAN controller. This chip can buffer somewhere between 5 and 19 incoming messages. It does not buffer outgoing messages.

CAN boards with an on-board microcontroller (LAPcan, LAPcan II, USBcan) typically buffer something like 500 incoming and outgoing messages.

CAN boards using the M16C microcontroller (USBcan Rugged, USBcan II, PCIcan II) typically buffer some 100 incoming messages and around 50 outgoing messages.

CANlib drivers will, by default, buffer 256 outgoing messages per channel and 1024 incoming messages per channel. You can use canIoCtl() to adjust the size of the receive buffer.