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TMS320F280039C: SCI TX/RX connecting to ISO7741

Wayne Huang
Genius 17165 points

Part Number: TMS320F280039C
Other Parts Discussed in Thread: ISO7741, SYSCONFIG

Dear Champs,

I am asking this for our customers.

The user uses F28003x SCI to connect to another MCU via TI ISO7741.

Questions:

1. For TX/RX, does the user needs to use any pull-up resistors like 10KOhm between F28003x TX/RX and ISO7741 if they use default SCI Sysconfig pinmux setting below?

2. If the user does not use any pull-up resistors between F28003x TX/RX and ISO7741, do they need to modify the Sysconfig SCI pinmux setting to enable the internal pull-up like below?

or they can just keep the default setting below without needing to add external pull-up between F28003x and ISO7741?

  • +1
    TI__Mastermind 26170 points

    Hi Wayne,

    I don't know the specific details of communication with the ISO7741 device, but in terms of F28003x there should always be an external or internal pull-up resistor on SCI-RX. There are some details on SCI-TX as well that can be found on this SCI debug page for reference: 

    "Similar to the SCI-RX pin, the SCI-TX pin expects that the other device’s RX pin will have a pull-up on the other end for both noise immunity and enforcing a return-to-high logic level. If this pull-up resistor is missing, the other device may not return to the proper logic level, and will receive incorrect signals. Note that the C2000 SCI-TX pin will return-to-high logic level when the module is enabled, but any transceivers in between the C2000 device and the other device may not enforce return-to-high without a pull-up."

    Here are my answers to your questions:

    1. The default SCI pin type settings in SysConfig you have in the screenshot include an internal pull-up resistor on the input, so no external pull-up resistor is needed here.

    2. Yes, they can keep the default SCI pin type settings for F28003x as you showed in the very last screenshot.

    Best Regards,

    Delaney

  • +1 in reply to Delaney Woodward
    TI__Mastermind 26170 points

    However, another note is that an external pullup would be needed if they need to guarantee the pin stays high during the bootup of the F28003x. One example of this would be if there's something external to the device that cannot be "floating" while the F28003x is still booting up (or is off).

    Best Regards,

    Delaney