DDR Memory Selection

[Lakshmi]

I am new to hardware Design in Memory section. I was going through the Memory DDR SDRAM Datasheet.
It was mentioned as
MT41J512M4 – 64 Meg x 4 x 8 Banks
MT41J256M8 – 32 Meg x 8 x 8 Banks
MT41J128M16 – 16 Meg x 16 x 8 Banks
What's the difference between these ? Which one to chose ? My Processor can be 16 bit / 32 bit

64 Meg x 4 x 8 Banks 256MB and 4 bit DDR?What exactly 8 Banks means?

[mairomaster]

You just need to do some reading on DDR internal structure. I am not an expert myself but I can try to give you a few hints.
8 Banks is the number of internal memory banks in the chip. You shouldn't really care about this.
x16 is the width of the data bus. If you use only one chip, that will be limiting your bandwidth apart from the frequency.
16 Meg is the size of a Bank.

When you multiply those 3 you get the total memory in Mbits. So basically you mostly care about the data bus width and the total memory you get. Keep in mind that some chips have not 1 but 2 or 4 dies inside, so you need chip selects to access each of them. That additionally complicates things.

Please somebody correct me if I'm speaking nonesense.

[Lakshmi]

Thank you,
I feel comparing all those 3rd one i.e. MT41J128M16 is better since 16 bit Data bus.

[Paul van Avesaath]

preach brother preach!

in all honesty i don't have a full grasp on this either.. but it sounds correct !
just google a bit more and you will find enough info!

[Lakshmi]

Thanks Paul Van

[robertferanec]

Yes, you select memory chip based on data bus width.
- The first chip has 4bit data bus (you do not want to use this, it is special),
- Then second chip has 8 bit data bus
- and the last chip had 16 bit data bus

So:
- you can go for one 16 bit memory chip and run your CPU in 16 bit mode
- or you can go for two 16 bit memory chips and run your CPU in 32bit mode (better throughput comparing to 16 bit mode as you can transfer 32 bits in one cycle)

[Tohidtnt]

Hey guys,
I get confused. How many SDRAM chips can we connect to a single MPU? Is it dependent on the number of data banks that existed on the MPU?
next question is, can we connect, for example, just 1 chip or 2 chips, even 3 SDRAM chips instead of 4 chips in the imx6 rex board?
What about the other processors like imx.2 or even an allwiner
in general, what should we pay attention to choosing the rams?

[Paul van Avesaath]

its dependend on what MCU you use.. it must support a ertain amount of memory in a certain configuration. read the datasheet and it should tell you which configurations are allowed.

[robertferanec]

Depends on memory controller capabilities. As Paul van Avesaath mentioned, you need to read datasheet.

For example:
- there will be registers which will define memory data bus width e.g. if you would like to use 8, 16, 32 or 64 bit data bus. Often memory chips are 8 or 16 bits, so you may need 1 chip or more of them to connect whole data bus.
- Also, another option are number of banks you can connect - means how many chips you can connect to one data line - this is controlled by chip selects.
- Another option is number of memory channels - these are completely independent memory busses.
etc ....

Important: smaller data bus, more cycles CPU will need to transfer data, so slower the board is going to be.