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TUSB8042: Question about XI when using external oscillator

Simeon Aleksandrov
Intellectual 920 points
Part Number: TUSB8042

Hello,

Please allow me to introduce myself, my name is Simeon Aleksandrov. I am an FAE intern at TI BeNeLux.

I have a question regarding a TUSB8042 (USB3 Hub) schematic design:

I was looking at the requirements when using an external oscillator instead of a crystal for clock generation.

 

Section 8.3.6 of the datasheet says: “XI should be tied to the 1.8-V clock source…”

Is 1.8V really correct?

The reason why I’m asking is that I can’t see any reason why this should work with a 1.8V clock as the supplies are defined with VDD33=3.3V and VDD=1.1V.

Also the input logic levels are defined as:

VIH_min = 2V

VIH_max = VDD33

Additionally I can’t find any specific logic level definitions for XI input, so I would assume that I need a 3.3V clock source?

Could you provide some insight here? Looking forward to your inputs!

Thank you,

Simeon Aleksandrov

  • 0
    TI__Guru 60870 points

    Hi Simeon,

    Yes, the clock cell is a 1.8V cell.  The Vih is defined for the external inputs and bidrectional buffers only, not the clock input.  The max range of the XI pin is listed here:

    Regards,

    JMMN