FMC: How to get full memory access on a 32 bit SDRAM via 16 bit data bus?

[BHell.1]

1)STM32H745I-DISCO, the bus is 16bit and the memory is 32bit.

In the spec there is 128M-bit SDRAM.

Question is if full memory 1(28M-bit) is available for usage on this kit?

2) If answer is yes on first question. We can only access half of the memory and we don't understand why. Adress pin A11 never goes high. We have similar SDRAM ( issi 32-bit ) and STM32H750 via 16bit data bus. Question is how to configure FMC to access 32 bit SDRAM via 16 bit bus.

[Tesla DeLorean]

I believe it's one of the designs which sacrifices half the capacity for pricing, volume and long-term availability. ​

[waclawek.jan]

ST should probably make this clear in the related UMs.

@Imen DAHMEN​ , could this please be forwarded to the respective team?

JW

[Imen.D]

Hello,

You are right.

I escalated your request internally to the appropriate team to update the docs.

Imen

[waclawek.jan]

Thanks, Imen!

Jan

[Tesla DeLorean]

Even with the BGA packages ST is pin constrained, due to the kitchen sink of functionality they try and cram into the DISCO and EVAL boards. FMC External memory buses being a particular hog of pins, and least flexible in the terms of alternative selection / muxing.

If you can afford 32-bit memory buses in you own design this will certainly improve the bandwidth situation significantly.

Using half a part is often a strategically strong / sound move, people tend to box themselves into overly limiting choices.