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SN74AXC4T774: SN74AXC4T774

Zeev Leshem
Prodigy 70 points
Part Number: SN74AXC4T774

Hello,

I'm using SN74AXC4T774BQBR at a design that translate 3.3V signals from Raspberry Pie (RSBP) side to 1.8V at FPGA side.

At 2 different boards this device stop functioning for a particular signal.

VCCB is 3.3V ;  VCCA is 1.8V ; Dir1..4 = GND ; OE = GND via 10K resistor

One signal is connected to pin14 (B1) with a 3.3V 10K pull up - it is floating from RSBP side. 

Instead of getting 1.8V at pin 3 (A1) I'm getting '0'

I've measured the resistance between that pin to GND and got 150 ohm. I've measured the resistance between this signal and 3.3V and got 900 ohm instead of the 10K (of pull up).

After removing the pull up I've measured 1K.

Clearly something damaged the buffer of B1 and although not driving by any signal and actually was floating, the potential was '0' thus A1 had '0' as well.

Either removing the device or either disabling it by OE=3.3V solve the problem and now pin A1 got it's '1' via 1.8V pull up.

I don't expect that pin B1 got higher than Absolute maximum rating (4.2V) as it was driving by RSBP (3.3V), moreover - other signals such as B3, B4 are connected the same but didn't malfunctioned

  • 0
    TI__Expert 7610 points

    Hello Zeev,

    Thanks for reaching out. We are currently looking into this and will respond as soon as possible.

    Regards,

    Josh

  • 0
    TI__Expert 7610 points

    Hello Zeev,

    Looking at the schematic and over your comments, I don't see issues with the setup that would force B1 → A1 to stop working. What most likely happened was a voltage transient occurred at input B1. We note the inputs can be above the supply voltage as long as it's below the absolute maximum input voltage which is 3.6V.

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Prodigy 70 points

    HI Josh,

    I'll try to see if I can catch such spike.

    In case that RSBP is connected externally and driving signals but my board and VCCB is powered off - is there any issue?

    According DS:

    8.3.4 VCC Isolation
    The inputs and outputs for this device enter a high-impedance state when either supply is <100mV.

    Thanks,

    Zeev.

  • 0 in reply to Zeev Leshem
    TI__Expert 7610 points

    Hello Zeev,

    There shouldn't be any problem with providing a signal to the inputs when VCCB is off (unless it's above the absolute maximum ratings). In scenarios in which input signals are provided while one power supply is off, the AXC4T774 has both Ioff and Vcc Isolation features.

    Regards,

    Josh