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SN65LVDS387: TTL input open

Toshihiro Watanabe
Genius 10235 points
Part Number: SN65LVDS387

Hello,

I think SN65LVDS387 input pin is pull down by 300k ohm from figure 15 of datasheet. Is my understanding correct?

Pin 6(A1A) is not terminated and there is 2~3cm trace pattern. This trace is open stab so acts as antenna.

In channel 1 (pin6, pin63, pin 64) is not used case, do you think this unused channel affects to the used channel communication?

I don't think so but if there was an issue or something we saw before, it would be helpful to share the information.

Best regards,

Toshihiro Watanabe

  • 0
    TI__Genius 10250 points
    Hello Toshihiro-san,

    You can leave the unused input unconnected, as it is pulled down internally as you noted. If you anticipate high levels of noise and interference, the internal weak pull down may not be sufficient, and you can add a stronger pull down externally by connecting it to ground.

    Regards,
    Yaser
  • 0 in reply to Yaser Ibrahim
    TI__Genius 10235 points

    Hello Yaser-san,

    Thank you for your comment. I understand input pin open is typically no issue. Do you think this open input affects to active channels? I don't think but please let me know your idea. I will share the customer schematic in offline. please review it and give me your comment.

    By the way, the datasheet describe about EN pin are connected to VCC or GND when unused. Is it possible to know why datasheet doesn't describe about unsed input pin? I guess we are expecting all input are used or something.

    Best regards,

    Toshihiro Watanabe

  • 0 in reply to Toshihiro Watanabe
    TI__Genius 10250 points
    Hi Watanabe-san,

    I have looked at the schematics. Are they going to run a trace on the board from the pin as indicated in the schematics? I still don't think the unused open pin would affect the other channels, but to be on the safe side it may be a good idea to tie it to GND.
  • 0 in reply to Yaser Ibrahim
    TI__Genius 10235 points

    Hello Yaser-san,

    Thank you for your review. TTL side is on board and LVDS side is board-to-board interface. I think this is typical usage.

    Even though LVDS side is board-to-board communication, Unused pin won't be affected to used interface, right?

    Yes, for the safe side, unused pin should be terminated, I agree.

    Best regards,

    Toshihiro Watanabe

  • 0 in reply to Toshihiro Watanabe
    TI__Genius 10250 points
    Hi Watanabe-san,

    All I can say is that the unused open pin can be left open and should not normally affect the other used channels. However, if you anticipate high levels of noise and interference, the internal weak pull down may not be sufficient, and I recommend you tie it to ground to be on the safe side.

    Regards,
    Yaser