# Rx Buffer overflow in MCP2515 CAN controller

Here is my context: I connect to automobile drivetrain CAN bus (Volkswagen Passat) and sniffer all the traffic. The problem is that when MCP2515 got a message from CAN and I read it via SPI, the message remains in the buffer. Here is corresponding code snippet (using autowp/arduino-mcp2515 lib with Teensy 3.2):

//loop:
if (status == MCP2515::ERROR_OK) {
...
}


The code above gets into if always in the loop since we get the first message from CAN. I see in the mcp2515 lib that it runs modifyRegister(MCP_CANINTF, rxb->CANINTF_RXnIF, 0); after the message has been read, so it should clear the buffer, or not?

I also tried couple of other libs - nothing changes. Also tried force cleanup after the message has been read, still with no result:

mcp2515.clearRXnOVR();
mcp2515.clearMERR();
mcp2515.clearInterrupts();


Also tried interrupt mode. In this case all goes well when messages from CAN appear not often, so the microprocessor reads them prior to buffer overflow (ex. with filtering just single ID). But since overflow happened the interrupt stops triggering. Here is corresponding code:

volatile int canMessageQueue = 0;
void canBusInterrupt() {
canMessageQueue++;
}

// setup:
attachInterrupt(digitalPinToInterrupt(2), canBusInterrupt, FALLING);

// loop:
if (canMessageQueue > 0) {
uint8_t irq = mcp2515.getInterrupts();
Serial.println(irq);
if (irq & MCP2515::CANINTF_ERRIF)
mcp2515.clearRXnOVR();
if (irq & MCP2515::CANINTF_RX0IF) {
canMessageQueue--;
}
if (irq & MCP2515::CANINTF_RX1IF) {
canMessageQueue--;
}
if (irq & MCP2515::CANINTF_WAKIF)
mcp2515.clearInterrupts();
if (irq & MCP2515::CANINTF_ERRIF)
mcp2515.clearMERR();
if (irq & MCP2515::CANINTF_MERRF)
mcp2515.clearInterrupts();
if (canMessageQueue > 0) {
Serial.print("! overflow");
canMessageQueue = 0;
}


*The code above taken from CAN Hacker (processInterrupt() in https://github.com/autowp/arduino-canhacker/blob/master/CanHacker.cpp )

Does anybody has any idea why the rx buffer still not getting clear?

• Do you have a schematic of the setup? Or are you using a breakout - if so the schematics of that one or an image will help. I noticed that the library has two setBitrate() functions where the one that takes the canSpeed sets the default clock to MCP_16MHZ. Is that matching your oscillator? Oct 5 '19 at 18:08
• The setup is pretty simple: standard SPI pins, pin 2 for interrupts and 5V power source from USB. I've tried Teensy 3.2 and Arduino Nano - both produce the same issue. As for bitrate - I set 8MHz clock: mcp2515.setBitrate(CAN_500KBPS, MCP_8MHZ). Oct 7 '19 at 6:16