RAM variables exchange between bootloader and application

Question

Hello, 
 I wish to exchange variables (16 bytes) between bootloader and application in RAM. 
 What i have to modify please ? 
 Both LD file ? 
 Thank you

ketil · Answer

Hi, 
 There are three options: 
 
 1) using SVC interface 
 Either use the Buttonless DFU as an example of using SuperVisor Calls to exchange data between app and bootloader. This would require storage in the Bootloader Settings page which has automatic deletion available when you go back to the application 
 This implementation is however too complex to write anything about in this answer and it may have some security considerations since you need to update the format of the bootloader settings page to do so. 
 
 2) using a named section 
 The alternative method is just allocating a page to use for data exchange and use a named section to refer to this from code. 
 Edit linker script for main app and bootloader 
 MEMORY 
 { 
 FLASH (rx) : ORIGIN = 0x78000, LENGTH = 0x6000 
 RAM (rwx) : ORIGIN = 0x200057b8, LENGTH = 0xa848 
 uicr_mbr_params_page (r) : ORIGIN = 0x10001018, LENGTH = 0x4 
 mbr_params_page (r) : ORIGIN = 0x0007E000, LENGTH = 0x1000 
 bootloader_settings_page (r) : ORIGIN = 0x0007F000, LENGTH = 0x1000 
 dfu_exchange_page(r) : ORIGIN = 0x00067000, LENGTH = 0x1000 // Add this and set to what page to use 
 uicr_bootloader_start_address (r) : ORIGIN = 0x10001014, LENGTH = 0x4 
 } 
 
 Add this in linker script sections 
 SECTIONS 
 { 
 .dfu_exchange_page(NOLOAD) : 
 { 
 PROVIDE(__start_ dfu_exchange_page = .); 
 KEEP(*(SORT(.dfu_exchange_page *))) 
 PROVIDE(__end_dfu_exchange_page = .); 
 } > dfu_exchange_page 
 } 
 
 Create a structure type 
 typedef struct 
 { 
 uint32_t version; // Ensure you aren’t handling illegal version 
 uint32_t crc; // CRC of the rest of the structure to ensure to not act on invalid data 
 uint32_t some_shared_data; 
 uint32_t some_other_shared_data; 
 } dfu_exchange_data_t; 
 
 Create an instance of this type in the named section in both projects, set it to used to ensure that it is not optimized away 
 dfu_exchange_data_t m_dfu_exchange 
 __attribute__((section(".bootloader_settings_page"))) 
 __attribute__((used)); 
 Use Flash API to write/erase from app/bootloader (don’t know if you need to go both ways) 
 Notes: 
 
 It is generally simplest to just use a structure type to read out in the bootloader, as this does not cost a lot of code space If you needed to add flash API and erase and so forth it just adds to the code-size (of which you may be limited) 
 We allocate a full page in memory since this is the smallest to erase 
 Section must be no-load on both app and bootloader to ensure it is not part of the hex-files to program the device 
 You must use flash API to clear out and set this structure 
 You can define your own structure type if you know what to exchange. Then you don't need to copy it out 
 Use a CRC-32 value to ensure that the exchanged data is correct 
 Use a version (uint32_t) in the structure in the beginning to ensure that you can update the format later on

3) Using an absolute address – The simplest, brute force method 
 
 As long as you have a structure type known to both bootloader and application it is also a possibility to skip the linker scripts altogether and just make an absolute address into a structure type 
 
 typedef struct 
 { 
 uint32_t version; // Ensure you aren’t handling illegal version 
 uint32_t crc; // CRC of the rest of the structure to ensure to not act on invalid data 
 uint32_t some_shared_data; 
 uint32_t some_other_shared_data; 
 } dfu_exchange_data_t; 
 
 dfu_exchange_data_t * p_dfu_exchange = (dfu_exchange_data_t*)0x67000; 
 
 Note: This is the simplest one but it will not be protected against the code growing into this area. You don’t have any assurances what will be in this page so you my end up erasing your code if you aren’t keeping track of what is at the address you are casting (and potentially are erasing). Method 2 is much safer in that regard. 
 
 Ketil

Fabien Comte · Answer

Thank you, i will try that asap.