Receiving more than 8 bytes from CAN

Projects Networking, Protocols, and Devices
1

Hello.
I've been stuck for some time with my project and would greatly appreciate some help. I'm rather far from programming and my brains are boiling already)

I need to read some exact bytes from CAN of my car and use it to calculate and display values like RPM etc.

Engine ECU sends 8 byte long messages, so I have no problem with reading it and dealing with any byte I want. For example, for RPM, I need bytes 5 and 6 from this:
5 62 20 2 B B8

 CAN1.readMsgBuf(&len, rxBuf); // : len = data length, buf = data byte(s)
      rxId = CAN1.getCanId(); 
    }
  }
  if (rxId == moduleIdAnswer && rxBuf[1] == PID[1] && rxBuf[2] == PID[2] && rxBuf[3] == PID[3]) {
    //  tthe above it to get correct bytes for a value.  A = rxBuf[4], B = rxBuf[5], C = rxBuf[6], D = rxBuf[7]
    switch (parameter) {
      case 0:
        // RPM
        retValue = (256 * rxBuf[4] + rxBuf[5]) / 4.0;
        break; 
    }
  }
  return retValue;
}

But another ECU sends messages 18 bytes long. And for my calculation I need byte 18.

This is an example line, that I'm dealing with, excluding the first byte, which I don't know yet, but it doesn't matter:
x 61 22 3 0 0 2 20 0 0 0 34 80 fa 9 0 0 38

I need to be able to use "38" . How do I get it?

Is below correct or I don't understant how buffers work?(

      CAN1.readMsgBuf(&len, rxBuf); // 
      CAN1.readMsgBuf(&len1, rxBuf1); // do I really need &len1 and &len2?... it's still 8 bytes per frame.
      CAN1.readMsgBuf(&len2, rxBuf2);
      
      rxId = CAN1.getCanId(); //
      
      Serial.println("CAN_BUS GET DATA");
      Serial.print("CAN ID: ");
      Serial.println(rxId);
      
      Serial.print("data len = ");
      Serial.println(len);
      //This loops through each byte of data and prints it
      for(int i = 0; i<len; i++)    // print the data
      {
        Serial.print(rxBuf[i]);
        Serial.print("\t");
      }
      
      Serial.print("data len1 = ");
      Serial.println(len1);
      //This loops through each byte of data and prints it
      for(int i = 0; i<len1; i++)    // print the data
      {
        Serial.print(rxBuf1[i]);
        Serial.print("\t");
      }

      Serial.print("data len2 = ");
      Serial.println(len2);
      //This loops through each byte of data and prints it
      for(int i = 0; i<len2; i++)    // print the data
      {
        Serial.print(rxBuf2[i]);
        Serial.print("\t");
      }
     
      Serial.println();
      delay(1000);

  if (rxId == moduleIdAnswer && rxBuf[1] == PID[1] && rxBuf[2] == PID[2]) {
    // A = rxBuf[4], B = rxBuf[5], C = rxBuf[6], D = rxBuf[7], ... O = rxBuf2[0]
    switch (parameter) {
      case 0:
        // ExtTemp
        retValue = (rxBuf2[0] - 40);
        break; 
    }
  }
  return retValue;
}

Sorry for the long post...

2

A single message longer than 8 bytes implies the use of CAN FD instead of regular CAN.
If you are using a MCP2515 then you won't be able to receive CAN FD messages. Is that what you are using?

3

Here is an ODB data processing example using the ELMduino library

4

Right, MCP2515 is the one I have.

5

Search the ECU documentation for the phrase "CAN FD" or "Flexible Data-Rate". If it really is using CAN FD then you'll need a interface board with a chip such as the MCP2517FD.

If you cannot find any references to CAN FD then the documentation may be referring to multiple 8 byte messages over regular CAN which you have to combine in your code to give a longer message.

6

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