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TUSB8041: GRSTz with just putting capacitance case

Oba
Genius 17485 points
Part Number: TUSB8041


Hello

 

TUSB8041 datasheet says that

“The 1 μF capacitor on the GRSTz pin can only be used if the VDD11 supply is stable before the VDD33 supply.” on P34.

 

I think GRSTz is just active low rest input pin. Why does just putting capacitance on this pin work as reset signal?

And why does it work only VDD33 supply first case?

 

Regards,

oba

  • 0
    TI__Guru**** 179576 points

    Oba-san

    The TUSB8041 has an internal pullup on GRSTz, so this internal pullup and the external capacitor creates a RC time constant delay. For most applications, this passive reset circuit created by the external cap and internal pullup provides an adequate reset pulse on GRSTz, but you may need to change the capacitor value to meet the timing requirement as listed on Page 11 of the datasheet. Also, if a customer system ramps 3.3V before 1.1V, an active reset may be required because the internal pullup on GRSTz is connected to the 3.3V rail.

    Thanks
    David

  • 0
    TI__Guru 64326 points

    TUSB8041 has an internal pull-up which will charge the capacitance on GRST. VDD should rise before VDD33 to ensure that the output drivers have a defined input and the state machine is initialized preventing TUSB8041 from entering any unknown states.