DP83867CR: Clock input: Crystal vs Oscillator - why did TI use both?

Robbie Valentine86

Part Number: DP83867CR

Hello,

Looking at the DP83867-evm I noticed there is a crystal as well as an oscillator used for the input clock for this part. Why did TI load both types (25Mhz oscillator and 25Mhz crystal)? Is one better than the other? Screenshot attached.

over 7 years ago

0 Rob Rodrigues over 7 years ago

TI__Mastermind 19780 points

Hi Robbie,

We placed both on the EVM to allow evaluation with either reference clock source.

There are many reasons to choose either a crystal or oscillator reference and there is a lot to consider at the system level to make the choice.

A crystal solution is relatively simple to implement, relatively low cost, and will have very very good jitter performance.

An oscillator solution may be extremely inexpensive if the MAC layer can provide a clean 25MHz to the PHY eliminating the cost of a crystal. Also an external reference may need to be provided if synchronous ethernet is desired to be used.

Best Regards,

0 Robbie Valentine86 over 7 years ago in reply to Rob Rodrigues

Expert 1550 points

Ok so it sounds like going with the crystal for standard ethernet is the most appealing path for me. I can just use the crystal you guys used and shouldn't have to worry about too much. I'm just shooting for standard 1000Mbit ethernet.

0 Rob Rodrigues over 7 years ago in reply to Robbie Valentine86

TI__Mastermind 19780 points

Hi Robbie,

If you use the crystal we specified, I would recommend replacing R54 with 330 ohms to keep from overdriving the crystal.

Best Regards,

0 Robbie Valentine86 over 7 years ago in reply to Rob Rodrigues

Expert 1550 points

Will do!

Is that because of the input capacitance of XIN?

0 Rob Rodrigues over 7 years ago in reply to Robbie Valentine86

TI__Mastermind 19780 points

Hi Robbie,

The current limiting resistor is needed because the crystal will be over driven. The modern AT cut crystals are using smaller blanks than historically, and as such, we need the current limiting resistor to ensure you don't cause rapid degradation of the crystal by driving too much power into it.

Best Regards,

Interface

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DP83867CR: Clock input: Crystal vs Oscillator - why did TI use both?