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TCAN1145-Q1: CAN physical layer test found that the signal symmetry test failed

Alan Xia
Genius 9680 points
Part Number: TCAN1145-Q1

Tool/software:

Hi team，

At present, the customer used TCAN1145DMTTQ1 to do CAN physical layer test and found that the signal symmetry test failed. The result is as follows. Please help to analyze it. The attachment is the TCAN1145 design drawing. Thank you!

TCAN1145.pdf

  • 0
    TI__Guru 53575 points

    Hi Alan,

    The ISO CAN datasheet defines the load conditions for the Vsym measurement as listed below. Do you know what the "Maximum capacitive load" is that is referenced in this document? The values in the schematic suggest that the capacitive load on the bus is 47pF which is outside the conditions specified in the standard. We would not be able to guarantee the Vsym parameter at this unspecified load. 
    RL = 60 Ω (tolerance ≤ ±1 %) C1 = 4,7 nF (tolerance ≤ ±5 %)

    The CANL signal on the recessive-to-dominant transition looks a bit odd in the scope shots here. I would not expect that the signal have the sudden rise during the transition before continuing to fall. It seem that this deviation is what is causing the larger than normal common mode voltage. Do you know if there is another device active on the bus during this test or some glitch appearing on the TXD pin? I would be interested to see this transition in more detail if a zoomed in scope shot of this can be captured. 

    Regards, 
    Eric Schott 

  • 0 in reply to Eric Schott1
    TI__Genius 9680 points

    Hi Eric，

    The customer tested the 47PF capacitor on the CAN bus and the result was still NG. Please support us. Thank you!

  • 0 in reply to Alan Xia
    TI__Guru 53575 points

    Hi Alan,

    Thanks for checking on the capacitive load on this setup. Were you able to confirm that there is nothing else on the CAN signal lines during this test? The CAN waveform here still looks odd to me and does not match how I expect our transceiver would behave in a typical system.
    Please check that there are no cables or other nodes connected to the CAN lines during this measurement. 
    Is it possible to share the layout of the CAN pins on the board?

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Genius 9680 points

    Hi Eric，

    Below are screenshots of the test platform and PCB layout built by the customer.

  • 0 in reply to Alan Xia
    TI__Guru 53575 points

    Hi Alan,

    The Vsym specification is measured on the transceiver only with minimal impact from the measurement setup. In the setup you show here there is a cable harness and multi-node harness. This network is likely the cause of the deviant waveform we are observing. Particularly in the waveform we can see a reflection that happens right after the drive state changes which is causing the spike in the common mode. 

    This measurement should be conducted when the device is not connected to a cable harness. I recommend disconnecting the cables connected after the oscilloscope probes and repeating this test to see if this results in a passing measurement. 

    Regards, 
    Eric Schott 

  • 0 in reply to Eric Schott1
    TI__Genius 9680 points

    Hi Eric，

    PATEO feedback: The OEM test clearly requires the addition of a terminal network for testing. Since PATEO's products are not terminals, some terminal networks are NC. PATEO can shorten the terminal network connection line for verification. If it still fails, how to deal with it? Is it because TCAN1145's products cannot meet the OEM's indicator requirements?

  • 0 in reply to Alan Xia
    TI__Genius 9680 points

    Hi Eric，

    How about your suggestion？

  • 0 in reply to Alan Xia
    TI__Guru 53575 points

    Hi Alan,

    Is the Vsym measurement with the termination network an explicit requirement for the OEM? This would seem odd to me. The Vsym behavior from the ISO 11898-2 CAN standard is a transceiver-level characteristic and is not necessarily expected to be maintained in a system environment. Applying the transceiver-level requirement to the system without allowing more allowance for the impact of the system seems very strict to me and not entirely realistic as we cannot guarantee these tight parameters in every possible CAN network. 

    I think we need to push back more on this requirement as I believe there's some confusion between the OEM requirement and what the customer is asking for. I suspect that any other CAN transceiver would similarly struggle to maintain Vsym in such a setup. 

    Regards,
    Eric Schott