DDR SDRAM with Xilinx Virtex 2 on self designed PCB

[Elmo]

Hello,

last week I started the development and design of a PCB with an FPGA (Xilinx
Virtex 2) and two DDR-SDRAMs in parallel. No big deal, I thought, keeping in
mind the most obvious design rules, i.e. combining the adress lines and
separating the data and strobe (DQS) lines. But now I came across the many
other signals there are, e.g. the clock signals, S0 and S1, CAS, RAS, WE,
etc. My first idea was to also combine them for both modules. Lately I
wondered if I am right with that assumption?

Regards, Elmo

 

[Allan Herriman]

Hello,

last week I started the development and design of a PCB with an FPGA (Xilinx
Virtex 2) and two DDR-SDRAMs in parallel. No big deal, I thought, keeping in
mind the most obvious design rules, i.e. combining the adress lines and
separating the data and strobe (DQS) lines. But now I came across the many
other signals there are, e.g. the clock signals, S0 and S1, CAS, RAS, WE,
etc. My first idea was to also combine them for both modules. Lately I
wondered if I am right with that assumption?

From the P.O.V. of functionality, that depends on what you want to do.
Will you need to write to individual bytes? Will you want to be able
to precharge (or whatever) one of the chips while you are performing a
different operation on the other chip?


Signal integrity implications of pairs of SDRAMs (DIMMs, actually) are
discussed in this recent c.a.f thread:
http://groups.google.com/[email protected]

Regards,
Allan

 

[Elmo]

Hello and thanks Allan!

I just browsed through the recent thread. But let me pick up your question,
anyhow: Yes, I want to be able to write to individual bytes and precharge
one of the chips while I am performing a different operation on the other
chip, i.e. I want one DIMM to load some data into memory and the other one
to read data from the memory to any sort of interface. What are the aspects
I must consider in connecting the DIMMs???

Regards, Elmo

 

[Anders F]

Hello and thanks Allan!

I just browsed through the recent thread. But let me pick up your question,
anyhow: Yes, I want to be able to write to individual bytes and precharge
one of the chips while I am performing a different operation on the other
chip, i.e. I want one DIMM to load some data into memory and the other one
to read data from the memory to any sort of interface. What are the aspects
I must consider in connecting the DIMMs???

Take GREAT care if parallelling them: You might end up with SI problems.
(I'd opt for separate lines if you have the I/Os)
And remember termination resistors/supply.... As well as matching delays.

/A