WHY DDR PINS OF DISCO BOARDS and EVAL BOARDS ARE DIFFERENT

[Gencay]

I was analyzing stm32mp15 disco and eval board schematics for my project. I noticed that DDR pinning are not the same between them. Disco boards have 512MB and EVAL boards have 2x512MB. My project also going to have two variants. I wanted to know why pinouts of these two schematics are different and can I use same pcb for both of my variants with one 1x512 and other with 2x512. (1x512 will have two locations on the pcb but only one chip. Is it possible?)

[PatrickF]

Hi @Gencay​ ,

When you talk about difference, I think you are pointing bit swapping within bytes or byte swapping, right ?

On DDR3/DDR3L, this is allowed to optimize routing. See AN5122 for more details.

Note that DK2 and EV1 boards uses different STM32MP15 package, so best routing is not identical.

You could build a PCB with 2xDDR3L 16-bits and only populate one on DQ0-DQ15. This is possible.

Nevertheless, when using 2 x DDR3L, the use of fly-by and Address/command termination resistors to VTT voltage is mandatory, even if you only populate one of the two memory on your PCB. See AN5122 for more details.

Regards.

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[Gencay]

So I believe this is okay then?

[PatrickF]

Yes, this is ok.

Regards.