SDRAM controller

Hi all,

can anybody tell how autorefresh in SDRAM exactly works? suppose in SDRAM specifications it is mentioned that 64 ms, 4096 cycle refresh(15.6 us/row) what exactly it means and how we need to generate the autorefresh cycles.

actually i am trying to upgrade the 512k*4*32 SDRAM(Fujitsu Make) to Micron Make 1M*4*32 SDRAM.

FUJITSU specifies that 4K refresh cycles every 16ms, auto refresh (3.9 us) and micron specifies that 64 ms, 4096 cycle refresh(15.6 us/row).

Did any body tried to do this kind of upgradation. we have the CPLD which controls the Fujitsu SDRAM and other we want to upgrade the SDRAM as i mentioned above.

Anybody can help in this regard please reply to .i can send u the data sheets of both the SDRAMs,if you want.

thanks and regards,


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in general the 64mS says that you need to complete all the 4096 cycles in 64 mS period. how you do it it is your choice (based on the application) = you can do 4096 refresh cycles in a clock by clock (NOT real-life option) and after these 4096 clocks go to sleep or do your algorithm until the next 64mS starts or (another scenario) you can do some N refresh cycles together and repeat this so 4096/N such events will take less than 64mS, etc. as the SDRAM takes care inside for the refresh it actauly means that you will "visit" every SDRAM line in 64 mS and this will prevent the data from disappearing.

as for the upgrade -->

the X mS period is the MIN. as you see Fujitsu asks for 16 mS while Micron wants 64 mS = actualy you need to do NOTHINg as what will happen with the Micron is that it will "over-" refreshed (x4) but this odes NOT make any damage at all ! doing the oposite (Micron --> Fujitsu) will NOT work as than if the design is for 64 mS and the device expects 16 mS the data might disappear (might as if you still keep accessing the SDRAM it will take longer time for this effect).

hope this helps.

                           yours -
                                     Arie Z.
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Arie Zychlinski

And one of the many advantages of an FPGA is that if you know enough about the application to know that every row will be visited anyway you don't need to refresh at all.

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