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[FAQ] TUSB8041: What are the power-up and reset timing requirements for the USB hub?

Part Number: TUSB8041
Other Parts Discussed in Thread: TUSB8044A, , TUSB8044, TUSB8042A, TUSB8043, TUSB8020B, TUSB8043A, TUSB8042

When using the TUSB8041, does it matter which power supply ramps first and how does it impact power on reset timing?

  • There is a requirement (note [1] below) in the TUSB8041 datasheet specifying VDD33 must ramp after VDD11 if a passive reset circuit is used.  This requirement is meant to prevent the case where a customer ramps 3.3V before 1.1V and the internal pullup on GRSTz to 3.3V rises allowing the passive reset to complete before the 1.1V rail even powers on, leading to an invalid reset. 

    In practical application, most implementations have 3.3V and 1.1V ramp simultaneously (or within us) and include a large enough cap on the passive reset circuit pin to allow for reset to be held low for the required time after the power rails are stable. This is the case on the evaluation modules.   The capacitor on the GRSTz pin will vary depending on the ramp times in the platform.  1 uF is acceptable for many applications, 2.2 uF would give more margin if they have slower ramp times.

     If there is an active reset circuit in the design, which would not rely on the internal pullup resistor on GRSTz, then power rail sequencing does not matter.

    This information also applies to TUSB8044A, TUSB8043A, TUSB8042A, TUSB8044, TUSB8043, TUSB8042, TUSB8041A,TUSB4041, TUSB4020B, TUSB8020B.