Skip to content
FuturePlus SystemsMar 3, 2021 3:59:18 PM2 min read

What’s the latest for DDR4 3D Stacked DRAM?

Samsung 256GB DDR4 3DS 2933 RDIMM

Well first of all, the JESD 79-4, 3D Stacked DRAM spec has been updated.  The new revision is -1B released last month (February 2021) and you can find a copy on the JEDEC web site.

The language has been modified to remove offensive terms,  Master has been replaced with Primary.

They finally deleted the reference to TRR.  This stands for Targeted Row Refresh.  Removed from DDR4 (but present in LPDDR4), Targeted Row Refresh was once the answer to Row Hammer failures.  Alas no remedy for Row Hammer has yet to be widely implemented.

The new updated spec adds more detail as to how many Refreshes can be postponed.  For 8Gb and below density die, a maximum of 8 Refresh commands per logical rank can be postponed.  For 16Gb and above density die the number of burst refresh commands per 3D stacked PACKAGE is limited to a maximum of 16.  Multiple successive Refreshes would cause all of the die in the package to pull on additional current thus causing a potential power drop across the module.  The new spec also adds clarity on how many Refreshes can be ‘pulled in’ (done early) and specifies the maximum interval between refreshes for the various heights 2H, 3H or 4H (dies) in a 3DS package.

Some optional timing parameters were added concerning Refresh.  tRFC is the symbol that defines the parameter REF command to ACT or REF command time to the same logical rank.  For 16Gb densities, an optional timing column was added to the Refresh Parameters table that allows for a shorter time between the ACT to REF or REF to REF for the same logical rank.  These times are given in nanoseconds not clock cycles so BIOS writers take note!  You have to convert these numbers to clock cycles using the clock period that the bus is operating at and then apply the rounding algorithm per the JESD 21-C Annex L: SPD spec.  Don’t forget to add the -.025  guard band!  In case you have forgotten what the rounding controversy was all about, refer to one of my other posts here.

Finally, DDR4 3DS DIMM pricing has reached new heights!  A quick search on the web reveals prices range from $1,000.00 to $7,000.00 for a single DIMM!  WOW!  Would not want to have that fail on me.  After paying that much you would think it would last forever and never have any failures….

Need a ‘refresh’ on what DDR4 3DS is all about?  See my post here.