8 years, 4 months ago.

CAN hardware filter returns 0

Hi all,

I use this little snippet to test out the CAN hardware acceptance filter:

CAN hw acceptance filter

#include "mbed.h"
CAN can(p30, p29);

DigitalOut led1(LED1);
Serial pc(USBTX, USBRX);

int handle;
void isrr() {
	CANMessage m;
	if(can.read(m,handle)) {

int main() {
	handle = can.filter(0x100, 0xff, CANStandard);
	pc.printf("MAIN:%d\n", handle);
	char a = 0;
	return 0;

The problem is that, handle gets the value 0. This is what the api documentation says: "Returns 0 if filter change failed or unsupported"

What can I do to fix this, so I can use the hardware acceptance filter?

Im using the mbed LPC1768.

Cheers, Boris

2 Answers

8 years, 4 months ago.

Boris, I also have the same issue (just posted a question yesterday about the same thing) and I can add that can.frequency(500000) is also returning 0 each time your using it. I looked over the forum for other examples that have been used and not had any success.

If you try this: Does it work for you?

    printf("baud rate faild");

Filter example I found

can->reset(); // reset and clear all filters (accept all messages)
h1 = can->filter(0x100, 0x700, CANStandard); // Accept standard messages with id 0x100-0x1FF
h2 = can->filter(0x200, 0x7FF, CANStandard); // Accept standard message 0x200 as well
h2 = can->filter(0x300, 0x7FF, CANStandard, h2); // Accept standard message 0x300, overwrite 0x200
CANMessage msg = CANMessage();
int r = can.read(msg, h1); // Should only return messages with id 0x100-0x1FF
int r = can.read(msg, h2); // Should only return messages with id 0x300
int r = can.read(msg);     // Returns 0x100-0x1FF if pending, otherwise 0x300, otherwise any message

8 years, 4 months ago.

Yeah i found the same example, but even when trying to reset first, it still returns 0. I'm not sure wether my can.frequency(500000) returns 0 as well, but I can confirm, that it sets the frequency to 500000 in my program (as i tested it with another device at 500000 frequency).

Hopefully someone knows the answer to this?

Boris, Have you found a solution or received an answer yet to this issue?

posted by Rob Crain 31 Mar 2014

Nope, unfortunately not. I was hoping someone of the mbed team would have an explanation for this??

posted by Boris Mattijssen 31 Mar 2014

Hi Rob, I found this snippet: http://mbed.org/users/ym1784/code/CAN_MONITOR/file/fe1f886dff76/main.cpp which works for me!

It would have been a lot nicer if the standard library would just work though.

posted by Boris Mattijssen 31 Mar 2014

Hi Boris and Rob, did you ever get the library filter to work? I'm using the same workaround too!

posted by Chris Mabey 17 Oct 2017