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AM2634: Buffer on SOP boot mode pins

Francois Charlot
Mastermind 28257 points
Part Number: AM2634

Hello,

Our boards include a '245 buffer that drives the user's boot mode selection onto the SOP pins around PORz. This buffer is tristated after the SOP pins sensing. In my understanding, this is to avoid loading the corresponding SPI and QSPI pins, which would affect the timings (rise and fall times) during normal operation after PORz. Is this correct?

The concern is that this increases the bill of materials. Customers may want to avoid this buffer and just have the pull resistors on the SOP lines. Do we have simulation data or guidelines to help customers achieving the maximum QSPI operating frequency while avoiding such a buffer?


Best regards,
François.

  • 0
    TI__Expert 7586 points

    Hi François,

    The ticket is assigned to expert.

    Please expect some delay in response.

    Regards

    Sri Vidya

  • +1 in reply to Sri Vidya Gunturi
    TI__Mastermind 22685 points

    Hi François,

    Yes, you are correct.  The purpose of the buffers is to remove the resistive loads from the pins once boot is complete and normal operation of the device has begun.

    No, we do not have any simulation data or formal guidelines for designing without the buffer.  You would need to do timing analysis on the specific board with the resistors in place to determine if you can still meet the timing requirements of the QSPI/OSPI device.  You can increase the resistor size to decrease the current that is routed through the pull resistors when in QSPI/OSPI mode, but you need to be careful to not increase them so large that they don't effectively pull for the bootmode selection during boot.

    Thanks,
    Mike

  • 0 in reply to Mike Pridgen
    TI__Mastermind 28257 points

    Hi Mike,

    Thank you for your answer.


    Best regards,
    François.