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Can i use only one channel for ddr ram?

armin.salcin , 11-08-2017, 06:25 PM
Hello Robert ,

Curently i started to work on my own project , and i choose rk3288 processor ,as it has two channel 64bits so max 4 ram chip used , my question is :

Can i use only one channel , example i want to have only 1GB of ram , can i use only two 4Gb ram chips on one channel?

Or i must use 4 ic of 2Gb ddr3 ram ?

Best Regards
robertferanec , 11-09-2017, 02:38 PM
What you mean by one channel ... like CKE0 & CKE1? Or it is completely separate channel (different data & address bus)?
Usually you can use one channel with no problems, but you may need to adjust memory controller register settings. Test it on development kit.
armin.salcin , 11-09-2017, 03:15 PM
I have DDR0 and DDR1 Channel so i have DDR0_D0 - DDR0_31 DATA ,and DDR1_D0- DDR1_31 . So its completely separate channel .
armin.salcin , 11-10-2017, 05:07 AM
I also have a some questions , but i will ask here so i dont need to open new topic is it okay ?

1. Is it okay to route ram (some tracks) example addreses on the top layer (L1) , do we need to change layer stackup becouse of impedances?
2. How to coose ram frequency is that something that is written in datasheet of processor?
3. If we have space on board is it bette to use T- topology or Fly-by topology?

Thanks in advance , hope this question will help someone other thinking same .
robertferanec , 11-10-2017, 06:12 PM
I have DDR0 and DDR1 Channel so i have DDR0_D0 - DDR0_31 DATA ,and DDR1_D0- DDR1_31 . So its completely separate channel .
It may depends on chip, but I believe you should be able to run it with one channel only. Try the register settings on development kit.

1. I route signals in group same way as it helps with length matching (so I do not route some signals from address group on top layer only and other address signals inside PCB). However, you can route groups differently e.g. one group on Layer 3 and another on layer 10.

2. yes, the memory bus speed / frequency is specified by CPU. The memory chips which you use have to be for the frequency specified by maximum memory bus speed or higher.

3. You need to follow CPU manufacturer recommendations - what they recommend. If you can use both, then it is really up to you. Fly-by will run to higher frequencies, but you need termination resistors. Also, may depend if you are planning to put the memory chips on top of each other (we usually use t-branch) or you will be placing them in one row (you may want to use fly-by)

armin.salcin , 11-11-2017, 08:04 AM
Thank you so much you helped me

1. I learnt from course that we need to route sam group same topology . But my question is can we route one group example on top player becouse of impedances.
robertferanec , 11-11-2017, 09:44 AM
1. I am not sure if this answers your question, but yes, you can have controlled impedance on top an bottom layers
armin.salcin , 11-11-2017, 09:45 AM
Yes thats answer .

Thank you !
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