CDCS502: Input signal requirements

Yusuke Mukuno

Expert 7395 points

Part Number: CDCS502

Hi expert,

My customer is using CDCS502 for adding SSCG function.

However they have an issue about amplitude for input signal.

Their waveform meet VI threshold, but it didn't meet VIL/VIH.

input signal

Vdd = 3.3V

Max = 1.9V < Vdd x 0.7 = 2.31V

Min = 1.14V > Vdd x 0.3 = 0.99V

Could you please give me your advice? Will they need meet VIL/VIH spec?(or is it no problem because the waveform is oscillate around Vi threshold?)

Thanks

Muk

over 5 years ago

+1 Hao Z over 5 years ago

TI__Mastermind 18070 points

Hi Muk,

The reference clock for CDCS502 is expected to be LVCMOS. So the input requirements need to be met.

Regards,
Hao

0 Yusuke Mukuno over 5 years ago in reply to Hao Z

TI__Expert 7395 points

Hi Hao,

Do you have any idea for solving?

They are using DSX321G (10.0352MHz, Load Capacitance:16pF(custom), 80ohm max).

Should they change the crystal?

Do you have any recommendation for selecting the crystal? Which parameter of the crystal affected this problem?

Also, do you have any idea for external circuit, PCB layout, etc?

Thanks

Muk

+1 Hao Z over 5 years ago in reply to Yusuke Mukuno

TI__Mastermind 18070 points

Hi Muk,

As long as the crystal meets the recommendations in the datasheet, there's no problem.

There's no particular requirement for layout. Just general rules such as keeping the distance between signal traces at least 3x the trace width. Solid ground layer to avoid crosstalk and etc.

Regards,
Hao

0 Yusuke Mukuno over 5 years ago in reply to Hao Z

TI__Expert 7395 points

Hi Hao,

How do you think the reason of this issue that is not enough the amplitude for input signal?

Customer think, the DSX321G is no problem against datasheet recommendation.

Thanks

Muk

+1 Hao Z over 5 years ago in reply to Yusuke Mukuno

TI__Mastermind 18070 points

Hi Muk,

There's some misunderstanding. The chip accepts two kinds of inputs - XTAL (crystal resonator, this is a passive device) and external reference clock (clock signal from an active device, such as XO - crystal oscillator).

The VIH/VIL requirements are for the active clock source, not for XTALs. There's no point of measuring the voltage of a XTAL. More background knowledge on XTAL can be found here:

https://www.ti.com/lit/an/snaa331/snaa331.pdf?ts=1620709272315&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FCDCE6214-Q1

Regards,
Hao

0 Yusuke Mukuno over 5 years ago in reply to Hao Z

TI__Expert 7395 points

Hi Hao-san

Thank you very much for your answer!

Muk

Clock & timing

Clock & timing forum

CDCS502: Input signal requirements