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Original question:

SN74LVC8T245: Power up sequence

SN74LVC8T245: Power on Sequencing

Songzhen Guo
Intellectual 2770 points
Part Number: SN74LVC8T245
Other Parts Discussed in Thread: SN74LVC4245A

Hi,

If the VCCB rise after VCCA, the delay time is 200ms. Is it OK for Power on SN74LVC8T245?

Could you review the schematic? Thanks.

  • 0
    TI__Expert 6680 points

    Hi Songzhen,

    Yes, the 200ms delay between powering VCCA & VCCB will be ok for this circuit.

    The schematic attached also looks good, however since the DIR pin is always pulled high and the OE\ pin is always pulled low, I would say that the pull-down resistors on the Bx pins are unnecessary, since they will always be driven. The circuit will still operate as usual if the pull-down resistors are there, it just might draw more current.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Intellectual 2770 points

    Hi Chad,

    Thanks for your reply.

    scea021a.pdfscea058a.pdfThe power-up sequence don't need care when AXC series device was used. See attachment for detail(scea058a.pdf)

    But it also said in datasheet of SN74AXC245: to ensure the high-impedance state of the level shifter I/Os during power up and power down.

    But there is a different description of LVC series in another document. Now customer want to use 74LVC8T245 for their project. Could you check the power-up sequence requirement for this device? Thanks.

    By the way, what's the difference between those grade？

  • 0 in reply to Songzhen Guo
    TI__Expert 6680 points

    Hi Songzhen,

    I'm not entirely sure what you're asking. The power up sequence of the AXC family is different than that of the LVC family of devices. 

    For more information on the power sequencing of the SN74LVC8T245 device, you can see the following FAQ:

    As well as additional information on the Ioff protection of the device can be found here:

    Please let me know if you have any other questions,

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Intellectual 2770 points

    Hi Chad,

    Thanks so much for you feedback.

    The question is simple:

    For ACX family, it doesn't need any power up sequence. It's OK.

    For LVC family, it has power up sequence, we can see the description like that in SCEA021A:

    VCCA need power  up first when the /OE connect pull-up resistor to VCCA.

    How long do we need to delay for VCCB? Are there any other considerations for SN74LVC8T245?

  • 0 in reply to Songzhen Guo
    TI__Expert 6680 points

    Hi Songzhen,

    There is no issue with ramping both VCCA & VCCB at the same time: Another user with a very similar situation as you I believe has posted about this in the below thread:

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Intellectual 2770 points

    Hi Chad,

    Here is the document for SN74LVC4245. Is SN74LVC8T245 same as SN74LVC4245?  Could you check it for me? Thanks.

    Our customer used SN74LVC8T245 from other supplier, it have power up problem.

    Here is the link for this document:


     

    In this document, it said clear that SN74LVC4245A need strict power up sequence.


    Here is the requirement for power up sequence:


     

    BR

    Songzhen Guo

  • 0 in reply to Songzhen Guo
    TI__Expert 6680 points

    Hi Songzhen,

    The SN74LVC8T245 is not the same device as the SN74LVC4245. That being said, I believe we have a misunderstanding. It is correct that the SN74LVC8T245 does not have any power up sequencing requirements; however, it is recommended that these devices follow the power up sequencing seen in SCEA021A.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Intellectual 2770 points

    Hi Chad,

    Thank you so much for you help.