julien bertaud

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10 years, 11 months ago.

is my mbed microcontroler ok ?

Hi,

I followed the exemple for the can Hello world (http://developer.mbed.org/handbook/CAN).

But after launch the Loopback example, the Mbed can't read nothing. If I have a look on the signal on the pin 10, I have nothing, Just a 3v3.

If i switch the can1 and can2 to write from p29 CAN can2(p9, p10); CAN can1(p30, p29);

i can see : /media/uploads/julienB/scope_0.png

Do you think I can have a problem with my board ?

Thanks Julien

Mbed NXP LPC1768 Transceiver mcp2551

Question relating to:

Last updated: 30 Apr 2020

2 Answers

David Smart
10 years, 11 months ago.

Hi Julien,

Here's a few thoughts -

  • Is your wiring like that shown at Handbook - CAN?
  • There might be some tips on this page CAN - Getting Started, but with the scope you have access to, you might be well beyond anything you'll find there.
  • If you disconnect the transceivers completely, and create a simple test program to wiggle the pins directly as DigitalOut lines, you might be able to see if they are working ok.
  • Is your module on a PCB where there might be another circuit (or a short circuit) on any of those pins?
  • If it is in a white protoboard , then perhaps the protoboard has a short - remove your module and test w/o the proto.
  • Can you share your code in case there is subtle error we might spot?
  • The scope trace you provided doesn't look like a CAN packet to me. Even w/o knowing what message ID and Data you tried to send, it doesn't look familiar. CAN will automatically append a CRC, so even if you were sending "FF" and the occasional pulse were a "stuff bit", I would expect to see something more complex near the end of the frame where the CRC would be. See the example diagram: Wikipedia - CAN Bus, and scope traces on Kvaser - CAN tutorial (you need to look at slide 6 for scope traces).

Accepted Answer

the code and the wiring are exactly like that shown at http://developer.mbed.org/handbook/CAN

- My module is on a white protoboard.

- if I disconnect the transceivers completely and connect directly the pin 9 to the pin 29 and the pin 10 to the pin 30, the Loopback example doesn't work. I tested it without the white protoboard.

- with a simple test program to wiggle the pins directly as DigitalOut lines, I can see the value change.

- I agree with you. The scope trace I provided doesn't look like a CAN packet. So I ask me if my mbed board don't have problem with CAN protocol.

posted by julien bertaud 03 Nov 2014
Wim Huiskamp
10 years, 11 months ago.

Did you check all the wiring? Are the two terminating resistors in place?