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TCAN1044V-Q1: Internal pullup on TXD, RXD, STB pin

Akihiro Hirata
Expert 6190 points

Part Number: TCAN1044V-Q1

Hello CAN team,

TI has CAN IC with Vio (V version) and without Vio (Non-V version) such as TCAN1042(V)-Q1 and TCAN1044(V)-Q1.

In this simplified internal block diagram, there are pullup to "VCC or VIO" on the TXD, RXD, STB pins.

For the CAN without Vio (Non-V version) this pull-up is connected to VCC, and for the CAN with Vio (V version) this pullup is connected to Vio, correct?

For the CAN with Vio (V version), No voltage will show up on the TXD, RXD STB pin when it is after supplying VCC 5V but before supplying Vio (3.3V for example), correct?

Regards,

Hirata  

  • +1
    TI__Guru 53515 points

    Hi Hirata-san,

    Your understanding here is correct. These digital pins will be pulled-up to the Vio voltage source if available. For non-'V' devices, this source is tied to Vcc. 

    For 'V' devices, it Vio is not supplied, the pull-up source will not be available, thus the pull-up path will lead to GND (or floating value of Vio supply line). Note that other leakage paths may exist in this state so the pin states may not necessarily be at GND potential, though this would be expected in most cases. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 6190 points

    Hi Eric-san,

    Thanks for your detailed answer!!

    Note that other leakage paths may exist in this state so the pin states may not necessarily be at GND potential,

    When device is after supplying VCC but before supplying Vio, are there any chance that high voltage can show up at the TXD, RXD, STB because of the leakage which you mentioned? If high voltage shows up before supplying Vio, it is problematic and need to avoid from the system point of view.

    Do we need pull down resistor at the TXD, RXD, STB line for measures or no issue without pull down resistor?

    I believe there is no issue without pull down resistor but let me double check.

    Thanks for your support.

    Regards,

    Hirata

  • 0 in reply to Akihiro Hirata
    TI__Guru 53515 points

    Hi Hirata-san,

    No, this would not be possible as no significant path would exist between these pins and the Vcc supply. Such pull-down resistors would not be necessary for this purpose. What I meanly meant by this comment is that if Vio is floating to some value (say 0.5V) while not active, these pins may drift between this voltage and GND. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 6190 points

    Hi Eric-san,

    Thanks for your detailed explanation. I understood.

    I appreciate your support, as always.

    Regards,

    Hirata