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embeddedstm32cortex-mmemory-mapping

Cortex-M3 External RAM Region

I'm currently researching topics such as RAM/ROM/Stack/Heap and data segments etc. I was looking at the ARM Cortex-M3 memory map and saw the region labeled "External RAM".

According to the data sheet of a random Cortex-M3 STM32 MCU the external RAM region is mapped from 0x60000000- 0x9FFFFFFF, so it is quite large! Memory map

I couldn't find a definitive answer about how this region is actually used.

I imagine you would have an external SRAM and you would choose between two options.

  • (1) Read via the SPI interface and place into a local buffer(stack), then load that local buffer into the external ram region. This option seems to have a lot of negative consequences, such as hogging the CPU and increasing the stack temporarily if the requested data is very large.
  • (2) Utilize a DMA and transfer from the SPI interface into the external ram region.

Now I can't understand, why you would map the data to this specific address range, what are the advantages, why don't you just place the data directly in that huge memory region?

Now I'm asking this question because I have a slight feeling I have completely missed the point of what the External RAM region really is.

-Edit- In the data sheet that is linking to the STM32 device, the memory region "External RAM" is marked as reserved. It is my conclusion that the memory regions listed by ARM is showing the full potential of a 32bit MCU, as I incorrectly state that the external RAM region "is quite large!" does not necessarily mean that this is "real" size of that region, if it is even used, it depends on what the vendor can physically achieve within the MCU hardware, and I imagine they would limit hardware capabilities to be competitive on price, power consumption etc.

Solution

  • I imagine you would have an external SRAM and you would choose between two options.

    • (1) Read via the SPI interface and place into a local buffer(stack), then load that local buffer into the external ram region. This option seems to have a lot of negative consequences, such as hogging the CPU and increasing the stack temporarily if the requested data is very large.
    • (2) Utilize a DMA and transfer from the SPI interface into the external ram region.

    None of the above. External memory on an SPI bus is not memory mapped. If you have an SPI memory, it is not mapped to that region, it is simply an SPI device, and the "address" is simply an offset from the start of the memory device itself. MCUs with a Quad or Octo-SPI controller are memory mapped. QSPI RAM is not that common and relatively expensive. QSPI is more commonly used for flash memory.

    The external memory region can be used by STM32 parts with an FSMC (Flexible Static Memory Controller) or an FMC (Flexible Memory Controller), or as mentioned a QPSI interface. The latter FMC SDRAM, and is generally available on the higher end parts. Apart from the QSPI and NAND flash, these interfaces require using the GPIO EMIF (external memory interface) alternate function to create an address and data bus. So it generally requires parts with high pin count to accommodate. The EMIF can be configured for 8, 16 or 32bit data bus for reduced pin count (and slower access).

    Now I can't understand, why you would map the data to this specific address range, what are the advantages, why don't you just place the data directly in that huge memory region?

    Since it was precipitated by your earlier misconception this question is perhaps redundant, but memory that exists in the memory map can be used to store data accessed as regular variables rather than transferring to an from internal buffers and it can be used as an execution region - code can loaded to and be executed directly from such memory.

    Now I'm asking this question because I have a slight feeling I have completely missed the point of what the External RAM region really is.

    Self awareness is a skill. That is known as conscious incompetence and is a motivator for learning.

    It is my conclusion that the memory regions listed by ARM is showing the full potential of a 32bit MCU, as I incorrectly state that the external RAM region "is quite large!" does not necessarily mean that this is "real" size of that region, if it is even used, it depends on what the vendor can physically achieve within the MCU hardware, and I imagine they would limit hardware capabilities to be competitive on price, power consumption etc.

    No, it is largely about the number of pins available for an address bus (except for QSPI). The external memory is a matter for the board design - it is not something the MCU vendor decides must be present. The constraint is a maximum, not a required amount of physical memory. The STM32 FMC supports the following memory sizes/types:

    enter image description here

    So you can have up to 512Mb of SDRAM for example. The space available for static memories (NOR/PSRAM/SRAM) is significantly larger than the than the typical size of such memories.