Electronics > FPGA

Mixing DDR3 data lines between bytes

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Boscoe:
Hi all, I just realised (mid through my PCBs being fabricated) that I mixed the data lines on my DDR3 memory and Zynq between the byte lanes. I understand that data lines can be mixed to some extent to optimise PCB layout but will this cause me problems when accessing single bytes from memory? Attached is a picture of the schematic.

If this is a problem are there anyways to mitigate it?

Thanks,
George

Berni:
Yep this will likely cause problems, you can only safely swap within the same byte. Mostly because timings are also based in groups of 8.

asmi:

--- Quote from: Boscoe on September 08, 2023, 09:51:50 am ---If this is a problem

--- End quote ---
Yes it is. And the biggest problem is figuring out how such glaring error managed to pass unnoticed through schematic checkups/reviews/etc.


--- Quote from: Boscoe on September 08, 2023, 09:51:50 am ---are there anyways to mitigate it?

--- End quote ---
If the length of bytes lanes is sufficiently similar, you might be able to get away with it, possibly along with lowering the memory frequency. Otherwise you will have to do a respin.

Boscoe:
Thank you for the reply. Is the timing the only concern here? THe the byte masking not be an issue as well? If there's a single byte access?

asmi:

--- Quote from: Boscoe on September 08, 2023, 07:05:03 pm ---Thank you for the reply. Is the timing the only concern here? THe the byte masking not be an issue as well? If there's a single byte access?

--- End quote ---
I don't think Zynq does per-byte accesses because it's got cache on chip, and cache controller typically reads/writes entire cache lines at a time. Not sure how it would behave if your have a PL peripheral which would request a narrow transfer directly to/from DDR controller though. But sinse that's probably within your control, you can either design those peripherals to ensure that doesn't happen, or use Cache IP to eliminate such accesses (this can lead to cache coherence problems, so make sure you know what you are doing and have a plan on how to deal with these issues before they manifest themselves, otherwise you will be in a world of pain).

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