nRF52840 SDK16 S140 - How to put in RAM retention

Question

Hi everyone, 
 I am working on a project where I have to enter the system into System OFF mode and RAM retention. What I actually want is to retain the value of some variables. 
 How could I do that in terms of software? Are there any specific functions I have to use? I also read that I have to make a no-init/zero_init section in order to ensure that the firmware does not clear the values at boot up but I don't even know what is this and how to implement it. 
 Could someone guide me? Or is there a similar example I could follow? 
 
 Thanks in advance 
 Nick

haakonsh · Accepted Answer

Nikosant03 said: Ok, what I have understood so far is that in order to keep retention in RAM you have to write on RAM[n].POWERSET register and if you are using a softdevice you have to use the sd_power_ram_power_set() API. right? 
 Yes, that's correct. 
 Nikosant03 said: 
 By default the POWER field (bits A-P) is set while the RETENTION field (bits Q-f) of RAM[n].POWERSET register is unset. So in my case I have to configure only the RETENTION field (as I want RAM ON on system ON mode). What I have understand is that the RAM has 9 sections and you can select which of the sections to retain during system OFF.. 
 How can I find the memory section (n=0..8) that my variable is located? 
 How do I know what bit positions (Q-f) I have to set? 
 
 See Memory : RAM0-9 and the sections within each RAM block. The sections correspond to the Q-F bit positions. ( S[i]RETENTION (i=0..15)). F.ex. if the variable is at address 0x2002 8420 that will be RAM8, section 3. #Define RAM_SECTION_3 (1 << 19)
#Define RAM_INSTANCE 8
#Define RAM_ON_SYSTEMON 0xFFFF //All sections ON in SystemON

err_code = sd_power_ram_power_set(RAM_INSTANCE, RAM_ON_SYSTEMON | RAM_SECTION_3);

//There's also bitmasks and bitfield definitions in nrf_power.h:
err_code = sd_power_ram_power_set(RAM_INSTANCE, NRF_POWER->RAM[RAM_INSTANCE].POWER | NRF_POWER_RAMPOWER_S3RETENTION_MASK);

Nikosant03 said: Taking for example the "state" variable, which of the following expression is correct? 
 From https://developer.arm.com/documentation/dui0472/m/compiler-specific-features/--attribute----zero-init---variable-attribute : 
 uint8_t state __attribute__((zero_init)); will place the variable state in the .bss section. uint8_t state __attribute__((section("zero_init"), zero_init)); will place the variable state in the user-defined .zero_init section 
 If you declare a user-defined section then you will most likely need to add it to the flash placement file in order to let the linker know that such a section exists.

haakonsh · Answer

Nikosant03 said: The address of a global variable during program execution and when I rebuild the application (say that I did some code changes) it remains constant ? 
 I don't think that's a safe assumption. I assume the linker can choose freely wherein the given Section a variable will be placed. Probably it depends on when the variable is processed by the build system, ie if it is the 4th variable to be placed in a section it will probably be placed on top of the previous three, so if you change the number of (or size of) the variables the addresses changes. 
 Nikosant03 said: Should I declare the variable with a special type in order to retain its address? 
 If you declare a user-defined section that only contains your special variable, and the address of that section is constant, then I think it's safe to assume the address of the variable is constant across builds. see https://www.google.com/search?newwindow=1&sxsrf=ALeKk03Rot_MBgmYBmeqylePyUzOOpxPoQ:1597063079918&q=arm+gcc+section+placement&spell=1&sa=X&ved=2ahUKEwjCvr_M05DrAhVHxIsKHaPUDDEQBSgAegQIDBAo&biw=1080&bih=1777

nRF52840 SDK16 S140 - How to put in RAM retention

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