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SN74AVC4T245: Power VCCB when VCCA is off

Michael Grenader
Prodigy 130 points
Part Number: SN74AVC4T245

Tool/software:

Hey, im using two sn74avc4t245 for uart debug interface of a processor.

I connect them the following way:

So X_VCCA and Y_VCCA comes from the card and VCCB comes from the connector (for example plugging ftdi cable). 

I dont have for now extra pin on the connect to divide vcc_b between both level shifters so i shorted them

My question is what happens for the following situations:

1. Device X and Y is off so X_VCCA and Y_VCCA is off and i plug ftdi cable and give VCC_B? Or only one device is on while VCC_B is on for both? 

2. Device X is On and i plug ftdi cable without applying VCC_B while keeping OE high? What if OE will be low?

3. I can maybe somehow make room for extra pin to divide VCC_B, but i prefer not to to save one pin on the connector (if not possible i will make the change)

Thanks,

Michael

  • 0
    TI__Mastermind 22420 points

    Hello Michael,

    See my comments below

    1. This part does not support VCC disconnect, meaning when you supply one voltage pin while you leave the other floating, the device will be off. It won't be able to voltage translate any signals.

    2. This is the same scenario as question one. X_VCCA is on while VCC_B is off. Device will be off even if you enable OE pin.

    3. When you say divide VCC_B, what are you referring to? Dividing VCC_B so each device can have it's own rail?  

    Regards,

    Josh