Crystal selection for LM3S6965

Hello,

   I'm working on a product with the LM3S6965 and I noticed that the crystal specifications are quite tight. I'm trying to follow closely the EK-LM3S6965 design. According to the Datasheet:

Main 8MHz crystal: +/-50ppm tolerance, +/-25ppm temperature stability

On the Ethernet Reference Crystal: +/-50ppm tolerance, +/-5ppm temperature stability

I saw another post from someone in TI that the crystal needs to meet the tolerance, but not the stability, since it's so tight.

So, could something like this work for the 25MHz or 8MHz crystal?

http://search.digikey.com/us/en/products/ABLS2-25.000MHZ-D4YF-T/535-9883-2-ND/2001250

It has 30ppm tolerance, 30ppm temperature stability.

Also, which crystals were used in the EK-LM3S6965? I checked the hardware guide but it had the schematics but not the BOM.

Thanks,
Derek

  • Hi Derek,

       The specs for frequency tolerance (deviation from expected frequency at 25degC) and stability (change in frequency across temperature range) can be added together when selecting a crystal.  So for the ethernet crystal a +/- 50ppm tolerance and a +/-5 ppm stabilitiy == 55ppm total.  If you have a crystal with +/-30ppm tolerance and +/-20ppm stability which totals to +/-50ppm you will meet the needed specs.

    The  NX5032GA-25.000000MHZ crystal is used on the EK-LM3S6965 is such a crystal. 

    The crystal you selected above would most likely be ok but it slightly violates the total frequency accuracy of (30+30)=60ppm and slighly violates the load cap typical target of 16pf.  For load capacitance it is ok to go smaller but typically want to avoid going larger or you risk issues with startup.  

    The 8Mhz crystal used on the EKLM3S6965 is less critical when it comes to tolerance and stability as the communication interfaces this crystal drives have more overall tolerance than eithernet.   

    The NX8045GB-8.000000MHZ crystal is used on the EK-LM3S6965.

     

    - Ken

  • In reply to Stellaris Ken:

    Thanks, this is exactly what I was looking for. I replaced the HC49/US crystal I was going to use with the ones you mentioned.

    --Derek