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Single DDR3 routing

key2 , 06-13-2016, 06:41 AM
I am doing an opensource FPGA + DDR3 design, I was watching your great tutorial about routing with altium and particularely the tutorials about DDR3 routing, but one thing that is not clear:

When you have a SoC or FPGA connected to a single DDR3, what are the groups ?

If I got it well:

1) data0-7 + dqs0_pair + LDM
2) data8-15 + dqs1_pair + UDM
4) DDR CKE_pair

Am I correct or could it be split more than that ?

What is the tolerence that is acceptable ?


mairomaster , 06-13-2016, 07:47 AM
I think you got it right, just for 4) it is cock pair, not clock enable pair but I guess that is just a typo.

The tolerance should be specified by the manufacturer of the FPGA you are using. Standard values that I am using are something like withing 0.5 mm (+/- 0.25 mm) for the signals withing a group and 0.2 mm withing a differential pair (+/- 0.1 mm between the positive and negative signal in the pair).
robertferanec , 06-14-2016, 05:34 AM
@mairomaster is right. Everything is correct except the typo in 4). Tolerance should be followed according to the chip manufacturer (sometimes the numbers are different). Normally we use:

- "within 5 mils" between P/N of differential pair,
- "within 10 mils" for Data group (we keep DQS longest from the group)
- "within 20mils" for ADDR/CMD/CTL group.

Have a look at this iMX6 Design guide and if you do not have any other documents, you could possibly use it as a reference (page 40): http://cache.freescale.com/files/32b...6DQ6SDLHDG.pdf

JohnsonMiller , 07-19-2016, 02:32 AM
From board design point of view, what is major difference between DDR3 and DDR4? Can you recommend a tutorial of help document?
robertferanec, 07-19-2016, 06:20 PM
E.g. Freescale / NXP has something: Hardware and Layout Design Considerations for DDR4 SDRAM Memory Interfaces http://www.nxp.com/files/32bit/doc/a...n&fileExt=.pdf
JohnsonMiller , 07-19-2016, 02:36 AM
In DDR2/DDR3 T-branch routing there are two sets of constraint: 1) inside group (ex. data lines), 2) between groups. For example clock signal need to have some relation with DQS. Question is that in Fly-by routing of DDR3 it is not possible to follow this rule! Am I missing or misunderstanding something?
robertferanec, 07-19-2016, 06:17 PM
In fly-by you can do this between group length matching in the first segment of signal (the place between CPU and first memory chip) e.g. make the first segment of CLK a little bit longer than ADDR/CMD/CTL group, then CLK will always arrive a little bit later then ADDR/CMD/CTL.
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