TUSB7340: Drive Level Spec for external crystal

Jacob Butler

Part Number: TUSB7340

Hi Team,

Can you help me answer a question about the drive level supported by XI/XO pins on TUSB7340?

My understanding is that the drive level is defined by the crystal, not the device.

As long as the load capacitance and ESR are within the datasheet spec our device should be able to operate with the crystal, correct?

However, how do customers calculate the maximum drive level seen by the crystal based on the values we provide in the datasheet and their chosen load capacitance/recommended 2MOhm shunt resistor?

Based on the following other thread, it looks like the answer is to confirm with crystal vendor that it can be driven by 1.8V clock and the board design and layout guideline file implies the same, but I'd like to confirm.

e2e.ti.com/.../tusb4020bi-drive-level-spec-of-tusb4020bi

Thank You,

Jacob Butler

over 3 years ago

0 Brian Zhou over 3 years ago

TI__Guru*** 145480 points

you can send 1.8v clock on XI or a crystal between X and XO. for crystal, it should meet +-50ppm and 50 ESR

0 Jacob Butler over 3 years ago in reply to Brian Zhou

TI__Intellectual 1710 points

Hi Brian,

Thank you for the confirmation of stability and ESR specs, but I may be missing the connection to drive level here.

I'm also a little confused by what the Load Capacitance in the datasheet is referring to. Is this from the perspective of the device, or how is this calculated?

This design is looking at qualifying additional crystals and would like to determine if they will be acceptable:

For Shunt capacitance, the original xtals we are using specifies 5pF max. The alternate xtals specify 7pF and 3pF max.

For Drive Level, the original xtals specify 500uW max. The alternate xtals specify 300uW max.

Thank You,

Jacob Butler

0 Brian Zhou over 3 years ago in reply to Jacob Butler

TI__Guru*** 145480 points

3pf or 7pf is Cstray, CL is the spec in datasheet. TI suggest 18pf for Ca and Cb.

0 Jacob Butler over 3 years ago in reply to Brian Zhou

TI__Intellectual 1710 points

Hi Brian,

Thank you for clarifying those capacitor values!

Any comment on qualifying crystals with drive levels of 300uW max? Or is this something we don't specify?

Best Regards,

Jacob Butler

+1 Brian Zhou over 3 years ago in reply to Jacob Butler

TI__Guru*** 145480 points

We don't specify in mW, but in voltage (1.8v).

Regards

brian

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TUSB7340: Drive Level Spec for external crystal