DP83867IR capacitor divider for Clock IN(XI)

Hello g.f.,

27pF capacitors are recommended starting points if the requirements of the crystal are not known. If the Max loading capacitor value is known to be 15pF, please follow the crystal datasheet. Whether or not 10pF will work also depends on the requirements mentioned in the crystal datasheet.

-Regards,

Aniruddha

Hi,

Thank you for the reply.

As it is written in DP83867IR datasheet page.115 "9.2.1.3 Crystal recommendations",
I understand that 27pF capacitor are recommended starting points when if using crystal.
But we are using external oscillator, not crystal.

Is 27pF capacitor also starting point even using external oscillator?

best regard,
g.f.

Hi g.f.,

External Oscillators are different from crystal in terms of component requirements. I would suggest checking with the oscillator datasheet for the recommended circuit.
If the design is still in the development stage, you can look in the schematics for our EVMs for reference. There is a oscillator included in the circuit and the schematics is included in the User's Guide. The Oscillator is not populated but the schematics can still be used for reference.

DP83867 QFN RGMII EVM: www.ti.com/.../dp83867ergz-r-evm
DP83867 QFP EVM: www.ti.com/.../dp83867irpap-evm

-Regards,
Aniruddha

HiAniruddha,

Thank you for the reply.
I'm sorry, but I don't understand yet.

From DP83867IR datasheet page.114 "9.2.1.2 Clock In(XI) Recommendation",
I understand as that DP83867IR are requiring 27pF capacitor divider
when 3.3V or 2.5V external clock source is used.
The following is information from datasheet:
////////////////////////////////////////////////////////////
If an external clock source is used, XO should be left floating.
For a 1.8-V clock source, XI should be tied to the clock source.
For a 3.3-V or 2.5-V clock source, a capacitor divder is recommended as shown in Figure 29.
The recommended values for the C1 and C2 capacitors is 27 pF.
//////////////////////////////////////////////////////////////
www.ti.com/.../dp83867ir.pdf

And in DP83867EVM User's guide page.7,
it said that required capacitors are 33pF or 27pF which is for capacitor divider.
www.ti.com/.../snlu176.pdf

I guess that this recommended capacitor divider are independent
from recommended circuit written in oscillator datasheet .

As I wrote at above post, our external oscillator's MAX load capacitor is 15pF.
If using 27pF for capacitor divider, it might be unable to drive 27pF.
So, is it allow to use 10pF for capacitor divider,
or should I select oscillator which MAX load capacitor is more than 27pF?

best regards,
g.f.

Hi Anirunddha,

Actually this question is from my customer and they are planning to use NZ2520SD(Oscillator) for DP83867IR Clock Input.
And I need to anwser to my customer as soon as possible.
So, can you please give me answer for the following questions.
1.Why 27pF is recommended value for capacitor divider?
2.Can we use other capacitor?
3.If 27pF is must, will adding gate logic between Osc and PHY Clock Input be workaround?

best regards,
g.f.

Hi Aniruddha,

Thank you for the reply and your support.
I understood.

best regards,
g.f.