SDRAM refresh cycles calculation

[pratham4647]

I have a 64Mb sdram chip which is organised as 4 banks, 2048 rows and 256 columns. It needs 4096 refresh cycles every 64ms. can anyone tell me how this 4096 number is evaluated? Also how many rows will be refreshed when a refresh cycle is issued by memory controller?

 

[heliboy]

I am not an expert in SDRAM, but as far as I know, the refresh cycles is determined by the internal structure of Bank/Row, and it is not always 4096, like the one I am using now, IS42S32160F, it needs 8192 cycles every 64mS.

There is an internal refresh address generator which generates the refresh address by itself, you don't have to care about it. What you have to do is just issuing refresh command 4096 times every 64mS (can be any pattern, does not have to be evenly distributed, can be even refresh in a burst fashion), the SDRAM itself will take care of the rest.

 

[pratham4647]

Thanks for the reply, Heliboy. whatever you said makes sense. however i have one more question. the chip i was working with is 64Mb x32 with 2048 rows - 256 columns - 4 banks. Somewhere i read that every refresh command refreshes 1 row from all banks. with that said, can you tell me why it needs 4096 refresh cycles and not 2048 every 64ms. PFA for sdram spec.

 

[heliboy]

Hi ,

After reading some documents, the refresh cycles equals the row number. That means the same row in all banks are refreshed at the same time.

As for the SDRAM you ask, I am not sure why the refresh count is still 4096. My wild guess is that it is more like a standard (either 4096/8192) which all the SDRAM controllers are designed for, so you don't have to change it. That is only my guess.



BTW why this is important? You just issue 4096 refreshes ever 64mS and you are done.

 

[KlausST]

Hi,

BTW why this is important? You just issue 4096 refreshes ever 64mS and you are done.

I agree with this.

The device has 2048 rows x 4 banks.
So maybe it can refresh just two banks with one refresh cycle, then it needs 2 cycles per row making 4096 cycles total.
But indeed it does not matter....

Klaus