TMS570LC4357: Can't Get Can Bus Out of Bus off with ABO turned on

Hello Diana,

The CAN controller has 2 error counters: transmit error counter and receive error counter (DCAN ERRC). When the transmit error counter exceeds the value of 255 (greater than or equal to 256), it transitions to bus-off state. Please read RP field of DCAN ERRC register to check if it is in bus off state.
Since the error counters will be reset at the end of the Bus-Off recovery sequence, please disable ABO before checking RP field.

Hi QJ,

As outlined above, I am getting to Bus Off state by physically disconnecting the bus. I am checking that I am in this state by using the BOff bit of the Error and Status Register (DCAN ES). I enabled ABO at initialization and set the time to 0 (or 10). I never see the bus leave the Bus Off state. My over all test runs for 10 minutes.

Does ABO work?

Should it work in my test?

Hello Diana,

Hercules DCAN module supports CAN protocol version 2.0 part A, B. It has ABO (Auto-Bus-On) feature. The DCAN certificate (Authentication on CAN Performance) is attached in the datasheet (Chapter of module certifications).

Hi QJ,

Since it is supposed to work, is my test the correct way to exercise this functionality?

Is there anyway that ABO could possibly start the recovery sequence but never finish it?

Is there anything else I should be setting other that the ABO bit to get ABO to recover the bus?

Hello Diana,

I am not sure if disconnecting the DCAN bus will force the CAN node into bus off state. If you have tools like Vector CANstress, you can exactly manipulate the data to ensure the proper bus off. Can you try to short CAN high and CAN low to generate bus off? I will do this test tomorrow if I have time.

Hello Diana,

On Part A - page 26, and Part B - page 63 of DCAN specification:

Start-up / Wake-up:

If during start-up only 1 node is online, and if this node transmits some message, it will get no acknowledgment, detect an error and repeat the message. It can become ’error passive’ but not ’bus off’ due to this reason.

When you disconnect the CAN bus, the CAN node is the only node on the bus and hence it will remain in Error Passive, but it will never go to the BUS_OFF state.

Hi OJ,

I understand that that happens, however you have to have the bus correctly terminated for that to occur. If I disconnect the bus, from the termination (therefore not having the correct termination on the bus), I get different errors then the ACK error, and the node doesn't know it's the only one on the bus. This can be a real problem in the field (someone not terminating the bus correctly), so I'm simulating it. In this case, should the bus go back to error active using ABO?

Thanks

Diana

Hi Diana,

I understand. If the bus is not in 'bus off', it will not go back using ABO.

Hi Qj,

Yes, I understand that. My board is getting to bus off when the bus is disconnected without the correct termination, but when I re-connect it with the correct termination, I can't get it back to error active. Should this test work?

Thanks
Diana

Hello Diana,

Does your "correct termination" mean the 120 ohms termination resistors?

Hi QJ,

Yes, that is what I mean.

Thanks
Diana

Hi Diana,

I did a test using CAN1 and CAN2 on HDK board. The CAN bus-off is by shorting CAN_H and CAN_L very short time using a blue wire. The CAN bus is recovered automatically aithout any problem.

Hi QJ,

How long did you short it for? What if you short it for a minute or two?

Thanks

Diana

Hi Diana,

Yes, there are two 120 ohms termination resistors on the CAN bus. For CAN, both ends of the pair of signal wires (CAN_H and CAN_L) must be terminated. This is because communication flows both ways on the CAN bus. If there are multiple devices along the cable, only the devices on the ends of the cable need termination resistors.

ISO11898 (CAN standard) requires a cable with a nominal impedance of 120 Ω; therefore, we should use 120 Ω resistors for termination.

Hi QJ,

I was asking about how long you shorted CAN_H and CAN_L for? What should happen if you short it for a minute or 2? I am assuming that you are keeping the bus with the correct termination on it when doing this.

Thanks
Diana

I am consistently getting to bus-off as I keep the correct termination on the bus when I disconnect it. I can't get it to recover with ABO after that when it is reconnected.

My bus was in bus off with ABO and did not recover.

You need to correct the problem which causes the bus off first. For example, remove the wire which is used to short CAN_H and CAN_L.

Hi QJ,

I understand that. I only disconnect it for 1 or 2 minutes and then reconnect it, but it does not recover. Should this work? Your testing said it would for very short periods of shorting, should it work for longer periods?