5 years, 12 months ago.

How does MBED set aside RAM for the NORDIC softdevice?

Hi, I am trying to connect multiple Peripherals to one Central using the MBED BLE librarys. However, when i try to change the maximum allowed clients, my app wont run anymore (hangs in BLE::Instance();).

I have tried following in sdk_config.h:

NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 1 ; NRF_SDH_BLE_CENTRAL_LINK_COUNT 3 ; NRF_SDH_BLE_TOTAL_LINK_COUNT 4 ; this and anything below works! NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 1 ; NRF_SDH_BLE_CENTRAL_LINK_COUNT 4 ; NRF_SDH_BLE_TOTAL_LINK_COUNT 5 ; this and anything above does not work! NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 1 ; NRF_SDH_BLE_CENTRAL_LINK_COUNT 1 ; NRF_SDH_BLE_TOTAL_LINK_COUNT 5 ; this does also not work!

I assume this has something to do with RAM reservation for the softdevice, because it needs a reserved area in RAM. I looked at the linker script (NRF52832.ld) and noticed following:

/* If app_start is 0, do not set aside space for the softdevice */
#if MBED_APP_START == 0
  #define MBED_RAM_START  0x20000000
  #define MBED_RAM_SIZE   0x10000
#else
  #define MBED_RAM_START  0x200031D0
  #define MBED_RAM_SIZE   0xCE30
#endif

if I am correct, that means that if the softdevice is Enabled MBED sets aside a predefined amount of RAM, but that may not be enough. Debugging shows that sd_ble_enable(uint32_t * p_app_ram_base) sets 0x20007410 as app_ram_base. So one can see that the value in the linker script isnt even close to what is needed by the Softdevice. I tried changing the MBED_RAM_START address manually, but that leads to strange behaviour on my device, I think i am missing something.

So what am I missing here?

EDIT: I just experimented with the linker settings, and i got it to work! I put a breakpoint to nrf_sdh_ble_enable in nrf_sdh_ble.c and read the *p_app_ram_start, app_ram_start_link then I used the value of *p_app_ram_start as MBED_RAM_START and ram_end_address_get() - (*p_app_ram_start) as MBED_RAM_SIZE in the linker script. and voila it works! (although i had to perform a full clean for the changes to take effect)

then I checked if the values of *p_app_ram_start and app_ram_start_link would match now, but they dont. then I did some calculating and found out, that its not the MBED_RAM0 address thats important, but MBED_RAM1, so I have to subtract MBED_RAM0_SIZE from *p_app_ram_start and get the real intendet ram start address. again, I tried to verify it, and now the ram start address and the ram_start_link address differ by only 0x4. I guess thats because the ram start address is set to the next Memory page instead of the almost full one. anyways, I can accept wasting 4 bytes of memory and it now works! BUT i Highly suggest to change the linker script so that it calculates these values automatically, because its a real hassle to go through this. maybe I wrote this text a bit confusing, so if you have questions please ask, as i really would like to know if all of my assumptions were true.

thanks in advance, Jonas

this is a follow-up question regarding an issue discussed here: https://os.mbed.com/questions/83165/why-cant-i-connect-multiple-peripherals-/