DP83822IF: What kind of signal level is required for XI input when crystal oscillator is used for reference clock?

Hello John, 

Thank you for the query.

The oscillator input is in the VDDIO domain. The amplitude of the oscillator is required to follow the the VDDIO voltage provided to the PHY..

Regards,

Sreenivasa

Hi Sreenivasa,

Thank you for your quickly reply. it is helpful.

However you know it is difficult to find 1.8V crystal oscillator, this requirement will increase the system cost and complicated design. I am considering is it possible to drive XI input with 3.3V XO? I noted the maximum voltage rating in 'Absolute Maximum Ratings' for 'Other Inputs' is 3.8V. I understand this rating is not recommended operation condition, while it implies that the voltage rating for input pins is not related to VDDIO. If this means it can also work with 3.3V XO?

Regards,

John

Hello John, 

Thank you for the reply.

One of the option that could be considered by customer is using a capacitor divider. 

Regards,

Sreenivasa

Hi Sreenivasa,

Thank you for your proposal. However I am afraid there are some risk with capacitor divider: 

1. Capacitance typically has large tolerance, typically +/-20%, that means the tolerance of divided down level will be +/-40%!

2. I am not sure if large capacitance load is allowed by oscillator. The capacitance load will result very slow rise/drop edge, typically high speed digital device does not allow slow rise/drop input signal.

3. Large capacitance load will pull the output frequency down as i known.

Regards,

John

Hello John, 

Thank you for the inputs.

This is something we have been recommending forDP83867E and i provided similar suggestion.

Please have a look the the DP83867E datasheet.

Regards,

Sreenivasa

Hi Screenivasa,

Thank you. DP83867E is indeed different. It specifically specify Absolute Maximum Rating for XI input within -0.3V to 2.1V. Maybe this is the reason why the datasheet gives such proposal. While the Maximum Rating of DP83822 is -0.3V to 3.8V. 

Regards,

John

Hello ohn, 

Understand. I was indicating the DP83867E as a possible solution. 

Let me check if the input ca be greater than the supply. 

Regards,

Sreenivasa

Sreenivasa:

Thank you so much for help!

John

Hello John, 

Noted, i will update you when i have some inputs.

Regards,

Sreenivasa

Hello John, 

I looked at the below table for the IO range and looks like the input follow the IO voltage.

7.3 Recommended Operating Conditions

Please let me know your thoughts.

Regards,

Sreenivasa

Hi Sreenivasa,

The table '7.3 Recommended Operating Conditions' specifies the supply voltage range of VDDIO, AVD, and CT, in stead of I/O pin signal voltage range.

Regards,

John

Hello John, 

Thank you for the reply. 

Please refer to the below sections.

7.3 Recommended Operating Conditions

7.5 Electrical Characteristics

Let mw check if the IOs with the team if the IOs are rated for higher voltages than the supply range and update you.

Regards,

Sreenivasa

Hi Sreenivasa,

Thank you for your explain.

Yes there is VIH/VIL specification in 7.5 Electrical Characteristics, however it only specifies MIN/MAX threshold for VIH/VIL, means VIH must >1.3V, and VIL must < 0.5V to ensure correct operation, it does not specify what is the maximum allowable VIH. I think from this specs we cannot decide if XI can accept 3.3V CMOS input signal. 

As a reference, for an typical logic device, if its input signal is not allowed to exceed supply rail, typically the Absolute Maximum Rating voltage for input pins is VDD+0.3V. For some other devices, such as LVC family logic devices, its Absolute Maximum Rating voltage for input pins is not related to VDD, instead it is a absolute voltage, such as 6.5V, in this condition the device can accept 5V logic input when it is powered from 3.3V. I am guessing same condition apply to DP83822.

Regards,

John

Hello John, 

Thank you for the input.

Please see  the below for DP83826

I would believe this would be similar for the DP83822i.

We seem to spec the electrical characteristics the same way for all devices. I looked at DP83867 and DP83826

Regards,

Sreenivasa

Hello John, 

I do not have any additional inputs from my side and will update you if i have any additional inputs.

Regards,

Sreenivasa

Hello John, 

Thank you for waiting.

The below is the confirmation of what i hade mentioned before 

We have dual diode ESD on XI pin, hence VDDIO and Input signal level should be same.  Hence with 1.8V VDDIO, we can not have 3.3V Crystal oscillator  input.

Regards,

Sreenivasa

Hello John, 

Do you have any additional thoughts or questions ?

Regards,

Sreenivasa

Hello John, 

Do you have any additional thoughts or questions ?

Regards,

Sreenivasa