What is the difference using 1GB 16-bit DDR3 RAM or four 512MB 16-bit DDR3 RAM?

[MertkanTuna]

Hi all;

I'm developing a pcb design with reference schematics. In reference design, four 512MB 16-bit DDR3 RAM used by creator. But i don't want to use 4 DDR3 RAM because i need more spaces on board and pcb design will be harder. When i choose same property RAM like clock frequency etc., can i get the same level of processing capacity? What differences happens between these situations?

Thank you.

[Paul van Avesaath]

as i am not fully understanding the question here.. i am assuming that you want to replace 4 chips with one big chip.
there is the obvious differenc of room and size, but with the 1GB you will need more address lines.

bigger chips are not as fast as two smaller devices, (bandwith is bigger with multiple devices)


so it depends on your needs.

[MertkanTuna]

Sorry for my bad describing. Actually i want to replace 4 DDR3 devices with 2 DDR3 devies. Finally both design will have same RAM memory size. I can not predict how much performance difference there will be. Did you have any experince about this situation? Can I get approximate results?

Thank you.

[Paul van Avesaath]

It depends on the way you connect it. but if you use 4x 512bm @ 16 bits, your bandwith is bigger than 2 x 1 gb @ 16 bits.

So lets say you have 4x512Mb,16 bits, @ 800Mhz
Then it's 800x2(because its ddr) x (4*16bits=) 64 = 102.4 Gbits /s = 12.8 Gbyte/s bandwidth
So now lets take the new scenario wich is
800x2x32 = 51.2 Gbits /S = 6,4 Gbyte/s bandwidth


So preformance will be less to my best knowledge,
The amount of data you can acces/store is the same though..

[robertferanec]

As Paul van Avesaath explained, you are going to loose on throughput.

If you use 4x 16b you are transferring 4x16 = 64bits in one clock cycle. If you are going to connect only two memory chips, 2x 16b, then you will be only transferring 32bits in one cycle - and you will need two cycles to transfer the same amount of data as in the first case. So, you will be basically slowing down your memory speed to half.

PS: But it is not clear from your description, if all the memories are on the same chipselect and they are all connected to different data bits (CPU is using 64 bit data bus) or if they are connected on two chipselects and they share some databits (if CPU has 32 bit memory interface).

[MertkanTuna]

Actually, the CPU has 2x 32-bit memory interface and i will use only one. In this situation, memory speed down to half.

Thank you for all your answers.