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Can L138’s DDR2 run at 25MHz?

Karl Garcia
Prodigy 40 points


Hi,

What is the lowest allowable frequency of a DDR2 which is connected to an L138 ?

For example, on page 30 oaf Micron MT47H64M16(product page)’s datasheet, the MAX clock cycle is listed as 8.0ns, which suggest lowest frequency is 125MHz.

However, in page 81 of the same document, it seems to have suggested that by disabling DLL, the DDR2 could even work “below” 25MHz.


 

On the L138 side, at many places in TRM SPRUH77 ch15 I read that the memory controller can disable DLL of DDR2, and register SDCR’s DDRDLL_DIS is used specifically for disabling DLL of DDR2.

 

Put these two together, should we understand it as: L138 can support MT47H64M16 running below 25MHz by setting SDCR'S DDRDLL_DIS→1? What is the lowest DDR2 frequency the L138 team has ever tested with?

 

Karl

  • 0
    TI__Guru**** 179981 points

    Karl

    Sorry for the late reply, please refer to section 5.11.1 DDR2/mDDR memory controller electrical data/timing. At 1.2/1.3V, minimum clock frequency is 125MHz for DDR2.

    Thanks

    David

     

  • 0 in reply to David (ASIC) Liu
    Prodigy 40 points

    David,

    I see it in "5.11.1 DDR2/mDDR Memory Controller Electrical Data/Timing".

    But does this 125MHz lower bound also hold when DLL of DDR2 has been disabled? Is it possible to run at the 25MHz lowest DDR2 speed as specified in the DDR2’s datasheet?

     

    Karl

  • 0 in reply to Karl Garcia
    TI__Expert 4760 points

    Karl,

    Although in some cases the DRAM may work with the DLL off, this mode of operation is not documented nor supported by JEDEC. Therefore, each
    DRAM design may behave differently when configured to run with the DLL disabled.Running the DRAM with the DLL disabled may cause the device to
    malfunction and/or violate some DRAM output timing specifications.

    So the problem is the DLL inside the DRAM chips. It depends on how theyimplemented their DLL. If you have slow clock speeds, you might not care
    about the phase relationship, so you could just turn off the DLL.So basically this means that maybe it works, maybe it doesn't. You could be lucky and have DRAM chips that support it, but you can't count on it.It might work with one shipment of chips, but might not with another. It may vary from manufacturer to manufacturer, and from die revision to die revision. So even though technically slower clock speeds should be possible, this is just something that is out of spec, and even if it happens to work the functionality might go away at any time.

    Regards

    Antony

     

     

     

  • 0 in reply to Antony Samsunil
    Prodigy 40 points

    Anthony,

    Sorry for our late reply. We will stick to the specification (125MHz) instead. Thanks!

    Karl