• State TI Thinks Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 5 answers
  • Subscribers 34 subscribers
  • Views 286 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TCAN1044V-Q1: STB, TXD pull up

Zena Lee
Intellectual 2740 points

Part Number: TCAN1044V-Q1

Hi team,

I have a question regarding internal/ external pull up.

1. TXD and STB is drawn as below in Datasheet functional block diagram. Is this integrated pull up or just the current source? (STB is specified to have integrated pull up)

2. Do we recommend any external pull up resistor for STB and TXD? 

Thank you.

  • 0
    Guru 285730 points

    1. The datasheet says "pull-up", not "pull-up resistor". As shown in the block diagram, the pull-up is a pull-up current source.

    2. The STB pin can source up to 20 µA when pulled low (see its IIL specification). You can add an external resistor if you want more pull-up current for reduced noise sensitivity.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2740 points

    Hi,

    Thanks for the reply.

    Could you give comments about TXD also?

    And for pull-up resistor value that is connected to MCU, there are no particular value for recommendation?

  • 0 in reply to Zena Lee
    Guru 285730 points

    The TXD pin also has an IIL specification.

    The size of the external pull-up resistor would depend on the requirements of your circuit. A common value is 10 kΩ.

  • 0 in reply to Zena Lee
    TI__Mastermind 26736 points

    Zena,

    Clemens's comments are correct. This is usually a consideration when there is a weak driving source for TXD/STB or when the driving source is open-drain only. Based on your customer's implementation it may or may not be required. It will not hurt to add an additional pull-up resistor as mentioned.

    Best,

    Danny

  • 0 in reply to Danny Bacic
    TI__Intellectual 2740 points
    Danny Bacic said:
    when the driving source is open-drain only

    Hi Danny,

    Could you explain more about open-drain output? What is the open-drain output of CAN transceiver?

  • 0 in reply to Zena Lee
    TI__Intellectual 2740 points

    So basically there's no problem if customer does not use pull up resistor?

    Or could you check if I share their schematic? 

  • 0 in reply to Zena Lee
    Guru 285730 points

    Most CAN controllers drive the TXD signal with a push/pull output and therefore do not need a pull-up resistor. (You would configure the CAN TX output as open-drain only when you need to do level shifting, or to connect multiple transceivers.)

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2740 points
    Clemens Ladisch said:
    Most CAN controllers drive the TXD signal with a push/pull output and therefore do not need a pull-up resistor.

    Is this same for STB also?

  • 0 in reply to Zena Lee
    TI__Guru 50835 points

    Hi Zena,

    An active MCU will most likely drive the STB pin with a push/pull output. The reason why this pin has an internal bias (and often an external one) is so that the input pin remains in a known state when the MCU pin is high, often the case before the software has configured its IO. Because of this case, all of the input pins internal biases will put the transceiver into a state that will be least impactful to the system - recessive input to TXD and low power modes for mode control pins. 

    The reason for including external biasing resistor is to override the integrated bias if a different behavior is desired or to increase the signal's noise immunity by providing a stronger bias that will not be as easily modified by nearby coupled signals. 

    Let me know if you have any more questions. 

    Regards, 
    Eric Schott