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SN74CB3T3245: Unused pin / Control Input

Kurumi Hasegawa
Expert 6841 points
Part Number: SN74CB3T3245

Hi team,

On the datasheet of SN74CB3T3245, it is mentioned that “All unused control inputs of the device must be held at VCC or GND to ensure proper device operation.”.

Which pins are the control inputs?

At first I thought those are A1~A8, but I found this thread. Are A&B pins I/O pins and OK to left floating? And by control pin does it mean OE-pin?

If the A1~A8 are the control pins, then, when A1 and B1 are not used, only A1 should be connected to Vcc/GND? Or, Both A1 and B1 should be connected to Vcc/GND?

 

Best regards,

Kurumi

  • +1
    TI__Genius 10974 points

    Hi Kurumi,

    The control inputs for all of our devices refer to the pins that control which channels are enabled or are being addressed. In this case for this device, there is only one control input, the OE-pin (on enable pin). These logic pins must be held in a high or low state to prevent the pin floating into an unwanted state or oscillating. The input/output pins (Ax/Bx for this device) can be left floating as long as it is not a high frequency application where floating pins can possibly pick up stray noise.

    Thanks!

    Bryan  

  • 0 in reply to Bryan Marshall
    TI__Expert 6841 points

    Hi Bryan,

     Thank you for the input, I understand control input is OE-pin in SN74CB3T3245.

    Bryan Marshall said:
    The input/output pins (Ax/Bx for this device) can be left floating as long as it is not a high frequency application where floating pins can possibly pick up stray noise.

    When used for high frequency application, connecting IO pins to GND would be recommended, is it correct?

    Also, could you share rough range of the frequency that stray noise would be a problem? The customer would like to know just as a reference, so it does not have to be exact number.

    Best regards,

    Kurumi

  • +1 in reply to Kurumi Hasegawa
    TI__Genius 10974 points

    Hi Kurumi,

    That is correct, connecting the IO pin to GND through a 50 ohm resistor is normally recommended in these high frequency situations. For a rough range, I would say once you start getting into the GHz range you will may start seeing more appreciable noise if the inputs are left floating, but again, depends on layout and actual frequency of the system.

    Thanks!

    Bryan

  • 0 in reply to Bryan Marshall
    TI__Expert 6841 points

    Hi Bryan,

    Thank you so much for the additional input. 

    Best regards,

    Kurumi