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Original question:

SN74LVCC3245A: possibility of damage VCCA:3.3V and VCCB:0V

SN74LVCC3245A: When VccA=3.3 and VccB=Open, DIR/OE is Low, what is Output(A bus) status?

善太 加藤
Prodigy 10 points
Part Number: SN74LVCC3245A
Other Parts Discussed in Thread: SN74LXC8T245

Dear Specialists

 

I use SN74LVCC3245A and have a question.

I would be grateful if you could advise.

When VccA = 3.3V, VccB = open, DIR is Low(B toA ), OEn is Low(active)

Is the state of Output (A data) is affected by the Input (B data)?

or 

Is the state of Output(A data) is unknown?

I met that the output signal is VccA otherwise output line is connected to GND by 4.7k resister,

when VccB is open and Input voltage(B) is Low because of connecting to GND by resister.

I appreciate your great help and cooperation.

Best regards.

Z. Kato

  • 0
    TI__Mastermind 24970 points

    Hi Z,

    Please help note that VCCB open means VCCB unknown.

    To further answer your question, if device is set for B to A and the B supply is open; the output A will switch when the input B switches. However, further note that the B port tracks VCCB and the A ports track VCCA i.e dependent on supplies.

    If the goal is to isolate the VCC supplies please see SN74LXC8T245 with VCC isolation and Ioff features, i.e have the outputs low while one supply is at GND, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 10 points

    Hello Michale.

    Thank you for your answer.

    I understood that the A/B port tracks VCCA/VCCB.

     

    In normal case, the B port and VCCB is used in condition of connecting to other module of Vcc by cable.

    When I set up the CPU board (which uses the SN74LVCC3245A), the other module is not connected, so VCCB is open.

    I think that the B port is unknown when VCCB supply is open, so the output A port is unknown.

    Is my understanding correct?

    Zenta Kato

    Best regards.

  • +1 in reply to 善太 加藤
    TI__Mastermind 24970 points

    Hi Zenta,

    Yes, your understanding is correct. You may also see the LXC recommendation with internal pulldowns. Hence with inputs floating, outputs will be low and not unknown such as your concern, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 10 points

    Hello Michale.

     

    Thank you for your answer.

     

    I'm sorry, but I have an additional question.

     

    I found that OE was Low, otherwise I control OE was High.

     

    The datasheet of SN74LVCC3245A says that “the control circuitry (DIR, OE) is powered by VCCA”.

    In other words, OE is not affected by VCCB supply,

    Is my understanding correct?

    Zenta Kato

    Best Regards.

  • 0 in reply to 善太 加藤
    TI__Mastermind 24970 points

    Hi Zenta,

    It can be tied either supply for a HIGH. However, powered or referenced to VCCA implies the electrical characteristics would be associated to VCCA's characteristics i.e it will switch for highs and lows based on VCCA = 3.3 V even if tied to VCCB, thanks.

    Best Regards,

    Michael.