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DDR2 Checklist recommendation

Nitesh Singh
Intellectual 320 points

I was going through the AM335X Schematic Checlist Recommendations :

http://processors.wiki.ti.com/index.php/AM335x_Schematic_Checklist#DDR2

It mentions that DDR_VREF be derived using a resistor divider with decoupling to both DDR supply and ground, and to follow DDR2 routing guidelines of the datasheet. 

But i find that this is not implemented in the same way as mentioned in data sheet in beaglebone or EVM . So I just have a slight confusion as to what design to follow here, as I was suggested that the recommendations in the datasheet might be ideal and that implemented in beaglebone or EVM are according to practical  working conditions. 

For the meanwhile I have kept the beaglebone/EVM  DDR_VREF design because its working one. 

  • 0
    TI__Guru**** 393215 points
    Hi Nitesh,
     
    What exactly are the differences that trouble you? If it's the divider resistor values (2.2kOm vs, 1kOhm), both are acceptable, since VREF draws a very small current. As for VREF decoupling capacitors, the datasheet recommendations should be followed.
  • 0 in reply to Biser Gatchev-XID
    Intellectual 320 points

    Biser Sir, 

    Yes the datasheet recommends 0.1uF capacitors decoupled at both VDDS_DDR and DGND, but this decoupling is not done in AM335X EVM and Beaglebone and I have kinda of used  their schematics as reference. And since they are both working designs, so I am circumspect about providing those decoupling capacitors. 

    So, sir am I missing something here or sud i simply provide the decaps ??

  • 0 in reply to Nitesh Singh
    TI__Guru**** 393215 points
    Hi Nitesh,
     
    I don't know the topology of your board so it's hard to say. Generally, if VREF trace is short and is well isolated from other signal/power traces you could make it without these two decaps. However, if you have enough place on the board I would suggest you provide footprints for them. Then you can see how your board behaves and if you actually need them.