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SN74LVC8T245-Q1: Have a High Level Pulse During Power Up Even When OE Pull Up to VCC

Yazhu Liu
Intellectual 1410 points
Part Number: SN74LVC8T245-Q1
Other Parts Discussed in Thread: SN74LXC8T245, , SN74LXC8T245-Q1

Hi Team,

My customer observe that SN74LVC8T245-Q1 will have a high level pulse during power up even when OE pin pulled up to VCC. But in the datasheet, it says that "To ensure the high-impedance state during power up or power down, OE should be tied to VCC through a pullup resistor". May I know how to avoid this high level pulse? Or do we have p2p device, like SN74LXC8T245 which will keep high impedance state during power up?

Thanks.

Best Regards,

Livia

  • 0
    TI__Mastermind 42600 points

    Hey Livia,

    Yes the SN74LXC8T245-Q1 is a p2p device with glitch free power up. I recommend that device for this.

  • 0 in reply to Dylan Hubbard
    TI__Intellectual 1410 points

    Hi Dylan,

    May I ask is there difference inside SN74LXC8T245 compared with SN74LVC8T245-Q1 which will guarantee glitch free power up? As in SN74LVC8T245-Q1 datasheet, it also says if we pull up OE to VDD, I/O pins will be high-impedance during power-up. But customer do observe a high level pulse as below.

    CH1(yellow) is 12V main power, and is transferred to 5V through a BUCK, then to 3.3V through a LDO. VCCA=3.3V, VCCB=5V, DIR is A->B.

    In customer's design 5V will be ready first, and a pulse at SN74LVC8T245-Q1 output(CH3, purple) will be observed right after 5V is ready. And the output will become low after 3.3V is ready.

    I asked customer to adjust the power up sequence, if VCCA is ready before VCCB, there won't be a high level pulse at the output. But in customer's design, 3.3V is generated from 5V, so if they need to set VCCA before VCCB, a layout change will be required which customer do not want... Would you please help to confirm if change to SN74LXC8T245 will solve this issue?

    Thanks.

    Best Regards,

    Livia

  • 0 in reply to Yazhu Liu
    TI__Mastermind 42600 points

    Hey Livia,

    Ya this is due to the control circuitry being supplied with VCCA. As they power up there can be some internal race conditions as that control circuitry is being supplied. Since the output supply is already there, this opens up the potential for there to be a brief glitch when the direction is realized. 

    The new SN74LXC8T245 will definitely solve this issue with its glitch suppression circuitry.