LAUNCHXL-F280049C: About short circuit in DCAN module

lei li

Intellectual 375 points

Part Number: LAUNCHXL-F280049C
Other Parts Discussed in Thread: SN65HVD232

Hi,

I shorted circuit CANH and CANL.And see the figure below for the waveform：

My question is:

1、According to this waveform, what frame format was this？

2、The first Low level time was 17 bit times; and the later low level time was 1 bit time. Does such a waveform have any special meaning?

Thanks!

over 3 years ago

0 Hareesh Janakiraman over 3 years ago

TI__Guru* 95315 points

You have not mentioned if there are other nodes on the bus or if 280049 Launchpad is the only node. What is also important to know is exactly what the CAN module is doing when you short the bus.

lei li said:
1、According to this waveform, what frame format was this？

It is likely an error frame.

0 lei li over 3 years ago in reply to Hareesh Janakiraman

Intellectual 375 points

Hareesh Janakiraman said:
You have not mentioned if there are other nodes on the bus or if 280049 Launchpad is the only node.

There have none other nodes on the bus. Only shorted circuit CANH and CANL in 280049 Launchpad.

Hareesh Janakiraman said:
What is also important to know is exactly what the CAN module is doing when you short the bus.

My customer is trying the experiment, and I don't know the details.

Hareesh Janakiraman said:
It is likely an error frame.

But, I noticed that it is neither "active error frame" nor "passive error frame".

0 Hareesh Janakiraman over 3 years ago in reply to lei li

TI__Guru* 95315 points

The 17-bit low-time could be an active error frame.

I did try the experiment on my side with a 280049 Launchpad and did not see the waveform shown. Without more details on exactly how this experiment was done and what the objective is, we can't help further. Refer to www.ti.com/lit/spracq3 for ideas on generating different types of error.

0 lei li over 3 years ago in reply to Hareesh Janakiraman

Intellectual 375 points

I'm so sorry! The customer clarified a fact to me. They used F28335 to try the experiment. It can generate the above waveform， if shorted circuit CANH and CANL in only one CAN node. So can you explain the waveform further?

Thanks!

0 Hareesh Janakiraman over 3 years ago in reply to lei li

TI__Guru* 95315 points

What is the CAN module doing while the CANH and CANL pins are shorted? Is it idle or is it trying to transmit? Can you attach the oscilloscope waveform of CANTX & CANRX pin when this experiment is being done? Exactly what is the objective of this experiment?

0 lei li over 3 years ago in reply to Hareesh Janakiraman

Intellectual 375 points

The CAN module was sending data frame while the CANH and CANL pins are shorted. And I attach the oscilloscope waveform of CANTX & CANRX pin in the experiment. The yellow waveform is CANTX.

In addition, the purpose of my customer is: whether the bus can be restored in the event of a CAN failure.

Thanks!

0 Hareesh Janakiraman over 3 years ago in reply to lei li

TI__Guru* 95315 points

If the objective is to determine "whether the bus can be restored in the event of a CAN failure", I don't believe this is a valid experiment. Most of the CAN transceivers offer some level of short-circuit protection. This includes shorting of CANH or CANL with supply or GND. This also includes shorting CAN_H & CAN_L together. Once this error condition is removed, the module should recover on its own after the elapsing of 128x11 recessive bit-times.

In any case, I tried this experiment on my setup and did not see the waveform you had attached in the post. I only see CANTX pin staying at 3.3v when I shorted the CANH & CANL pins.