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TMS320F28066: Tx pin voltage when not powered

Jordan Zhou
Genius 9350 points
Part Number: TMS320F28066

Hi,

Customer faced one issue about F28066 Tx (pin43). When ARM is powered with 3.3V, F28066 is not powered. The voltage of point A will be lower to 2.0V.

Customer wants to know the reason.

In customer's board, F28066 has the SCI connection with ARM. They are powered separately. Supply currents are high enough, more than 500mA.

When F28066 loses its power, the voltage of point A will be driven low to 2.0V. They tried to adjust the resistance from 3.3K to 10K. There's no improvement.

Thanks.

Br, Jordan

  • 0
    TI__Expert 4810 points
    Jordan,

    See the datasheet section 'Recommended Operating Conditions'. For Vih the maximum is VDDIO+0.3v. Also 'Power Sequencing' the statement "No voltage larger than a diode drop (0.7 V) above VDDIO should be applied to any digital pin before powering up the device."

    The situation you describe above there is power applied to the F28066 Pin 43 but there is no power to F28066 VDDIO. This is not a good condition for the device. Current is being forced into Pin 43 and 'soft powering' the F28066 through the IO. If you measure VDDIO you will see a voltage rise there.

    I recommend a redesign which either ensures 3.3v source is common for all IC's on this pin so they are powered together, or use an isolating buffer powered from the same F28066 VDDIO supply which will block this path when VDDIO is not powered.

    Best regards,
    Jason