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DDR Memory Selection

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  • DDR Memory Selection

    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?

  • #2
    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
      Lakshmi commented
      Editing a comment
      Thank you,
      I feel comparing all those 3rd one i.e. MT41J128M16 is better since 16 bit Data bus.

  • #3
    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
      Lakshmi commented
      Editing a comment
      Thanks Paul Van

  • #4
    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

    - 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)