STM32 FMC SDRAM bus width 8/16/32 bit

[EasyNet]

Hello,

I want to add a SDRAM chip to my board. Processing the images with internal RAM is insuficient.

Reading the manual for FMC part I'm not sure how the bandwidth is working.

If I have a chip with 8/16/32 bit, is pretty clear: one chip and that's it.

But if the FMC has a bandwidth set to 16 for example and I have 2 chips with bandwidth of 8 bits, FMC can access the summed capacity of these 2 chips or it will see just capacity of 1 chip?

Example: 2x 512Mbit chip with 8 bit, FMC will see just capacity of 1 chip aka 512Mbit or capacity of 2 chips aka 1024Mbit if bus width is 16 bits and the chips are connected in parallel?

I was thinking if the FMC bandwidth is similar like in PC where the memory controller is using 64 bit bus width to access rhw whole 4x 16bit chips capacity.

Thank you.

[mƎALLEm]

Hello,

2x8-bit or 16-bit memories with the same characteristics are seen in the same way from FMC perspective.

So if you have the same memory timing characteristics for both configs and accessed in the same way by the CPU you will get the same bandwidth.

Hope that answered your question. 

[EasyNet]

Thanks for the answer. But my question was abiut the memory capacity.

I'm trying to give an example:
1x 512Mbit (64MB) with 16bit bus connected to 16 bit bus is the same as 2x 256Mbit (2x 32MB) in parallel with 2x 8 bits bus connected to 16 bit bus?
Or 2x256Mbit with 2x8 bit bus on 16 bit FMC bus will be seen by FMC as 1x256Mbit with 16 bit bus?

[mƎALLEm]

Sorry maybe I misunderstood your question I thought you are asking about the bandwidth because of thet statement:

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@EasyNet wrote:

But if the FMC has a bandwidth set to 16 for example and I have 2 chips with bandwidth of 8 bits, FMC can access the summed capacity of these 2 chips or it will see just capacity of 1 chip?

---

The answer to that question:

"Or 2x256Mbit with 2x8 bit bus on 16 bit FMC bus will be seen by FMC as 1x256Mbit with 16 bit bus?"

is Yes. And you need to connect NBL and NBH (I think to their Chip select) for each memory of 8 bit.

[mƎALLEm]

Sorry I noticed you are talking about SDRAM. 

So my answer now is I don't now :) .. My answer above is for Static RAM.

[EasyNet]

Hi @mƎALLEm ,

Indeed 16 bit and also I believe 32 bit chips have some additonal pins for low bytes and high bytes. Could be indeed the answer to summarize the capacity of the chips in parallel with higher bus on FMC.

Let me check this.