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Original question:

TCAN1042V-Q1: Bus recessive level outside expected electrical characteristics

TCAN1042V-Q1: CANH, CANL signal quality deteriorated

Shyama Agrawal
Intellectual 860 points
Part Number: TCAN1042V-Q1

Hi,

Please find below CANH-GND and CANL-GND and CANH-CANL in a system with only 1 node and no CAN termination resistor (Schematic below). Vcc here is 5V and Vio is 3.3V. Please suggest what causes the drive strength to be so low. This is noticed with a few products and not all. As soon as we connect another node (CAN Analyzer) to the system, the signal improves, and communication is established. With few other types of nodes however, the communication doesn't get established (haven't checked this waveform yet). 

Above is CANH-GND

Above is CANL-GND

Above is CANH-CANL

Above is schematic, R18, R19 not populated.

  • 0
    TI__Mastermind 26716 points

    Shyama,

    First, please use TCAN1044AV-Q1 for future projects in place of TCAN1042V-Q1. This is in a newer technology with increasing supply over time. TCAN1042V-Q1 supply over time will not increase to support new designs.

    I must admit I can't really decipher what's being shown in the images you included. What are you applying to TXD? And what are you seeing on RXD?

    CAN transceivers expect to see a total of 60 Ω of bus termination between CANH and CANL. I can't quite tell what is being shown in the oscilloscope plots, but from the schematic, you mention that the bus termination R18 and R19 are not populated. CANalyzer systems might apply termination to the bus, which could explain the phenomenon.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    Intellectual 860 points

    I'll get the waveforms for TXD too. We are sending data on CAN every 500ms but we are not seeing CANH, CANL change at the same frequency. Again this behavior occurs only on field in a few cases

  • 0 in reply to Shyama Agrawal
    TI__Mastermind 26716 points

    Shyama,

    Ok, that would be helpful to see.

    From your description, I think I'm getting a clearer picture. Have you confirmed the termination on the bus?

    Also, are you using a CAN controller, or are you sending a square wave on TXD?

    It could be worth checking on the status of the STB pin, which will need to be held low to keep the device in Normal mode.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    Intellectual 860 points

    Hi, 

    Below is TX-GND

    The software is trying to send at 500ms period, can the TCAN1042 IC hold it forcefully to high and only occasionally let it go to low. The microcontroller we use has an internal CAN controller. We are using that send data to TCAN. STB is stable and low through a 10kohm pull down.

    Yes, there is NO CAN termination resistor in the network. Only 1 node.

  • 0 in reply to Shyama Agrawal
    TI__Mastermind 26716 points

    Shyama,

    If you are using a CAN controller, your oscilloscope period is way too short to see what's going on. It looks to be around 1 second. Note that CAN would most likely operate in the 500 kbit/s range for arbitration. If an error frame is occurring here, we wouldn't be able to tell.

    Do you have access to any other oscilloscopes? To accurately diagnose part behavior, we usually need to see more than one plot on the screen at the same time.

    It is possible that the improper CAN bus loading is causing the dominant-to-recessive edge to be too slow, which would cause the controller to see a dominant bit during transmission of a recessive bit. This would be interpreted as arbitration loss and would cause it to stop communicating.

    Best,

    Danny