topic Re: .m_data_2 not initialized in Kinetis Microcontrollers

.m_data_2 not initialized

kat1 — Wed, 10 Jan 2024 16:31:06 GMT

Hi!

Long story short: .m_data_2 memory segment is correctly initialized right after flashing, but is not getting initialized after powercycling.

This is with Kinetis MK22FN512VLH12 ARM Cortex-M4.

Long story:

I have inherited the responsibility for dealing with issues at https://github.com/UltimateHackingKeyboard/firmware, while not having much actual experience setting toolchains for embedded/bare-metal C...

The MCU in question (Kinetis MK22FN512VLH12) has RAM divided into two data segments: m_data and m_data_2.

Assume following code:

__attribute__((section (".m_data_2"))) uint8_t foo = 123; int main(void) { PRINT(foo); }

On first run after flashing, it correctly prints 123
If the board is powercycled, it randomly prints either 108 or 124.
If NVIC_SystemReset() is issued the same value gets printed as in the last run.

Here are datasheets, in case they are of any help:

Edit: fixed the MCU part number

Re: .m_data_2 not initialized

xiangjun_rong — Mon, 08 Jan 2024 06:27:23 GMT

Hi,

I suppose that the line has issue.

__attribute__((section (".m_data_2"))) uint8_t foo = 123;

The above line declares a variable uint8_t foo, and save it in SRAM section .m_data_2. But as you know that the data in SRAM will be lost after power off, so the value 123 is lost after power off.

If you want to keep the value 123, the 123 must be saved in flash and copy to RAM.

1)you can define:

__attribute__((section (".m_data_2"))) uint8_t foo;

int main(void) { foo=123; PRINT(foo); }

2)you can use the line:

uint8_t foo=123;

int main(void) { PRINT(foo); }

3)const uint8_t var=123;

__attribute__((section (".m_data_2"))) uint8_t foo;

int main(void) { foo=var; PRINT(foo); }

Hope it can help you

XiangJun Rong

Re: .m_data_2 not initialized

kat1 — Mon, 08 Jan 2024 08:44:59 GMT

Thanks for reply!

So I take it that the .m_data_2 cannot be initialized on bootup automatically just as .m_data is?

I am still confused as to why that is so. I would expect that to be the default behavior, automatically inserted by the compiler somewhere before the main is called.

Re: .m_data_2 not initialized

xiangjun_rong — Tue, 09 Jan 2024 02:56:31 GMT

Hi,

The basic theory is that the RAM data is lost when the chip power off and on, that is why that the constant must be saved in flash. If it is a variable saved in RAM with an initialized constant, there is a copy from flash to RAM.

You can not declare a variable in RAM with initialized constant data.

XiangJun Rong

Re: .m_data_2 not initialized

kat1 — Tue, 09 Jan 2024 11:04:13 GMT

Yeah, but how/why is that different from simply declaring it as follows?

uint8_t foo = 123;

This foo is also declared in RAM, yet after power on it gets automatically assigned to 123. (If I am not mistaken, compiler inserts additional code that either assigns the value as an immediate, or copies the entire segment from flash.)

Re: .m_data_2 not initialized

xiangjun_rong — Wed, 10 Jan 2024 03:50:14 GMT

Hi,

For the global variable

uint8_t foo = 123;

The compiler will save the 123 in flash, the foo is allocated in RAM, before calling main(), the 123 in flash is copied to foo in RAM by initializing code.

Hope it can help you

XiangJun Rong

Re: .m_data_2 not initialized

kat1 — Wed, 10 Jan 2024 09:43:58 GMT

Hi!

Would you be so kind as to elaborate on the reasons of this difference in handling of those two RAM regions?

Re: .m_data_2 not initialized

bobpaddock — Wed, 10 Jan 2024 13:36:33 GMT

This gets traced back to the 'C' standard.
Initialized variables that are global in scope or marked as 'static' in a function will be placed in a segment typically called 'data' placed in Flash. The start up code knows that it needs to load those 'data' variables.

As there can be any number of segments with arbitrary names, the standard start up code does nothing with them. So the start up code and linker scripts need modified to treat other segments the way they treat the 'data' segment.

Re: .m_data_2 not initialized

kat1 — Wed, 10 Jan 2024 15:39:14 GMT

Ah, thank you very much!

> So the start up code and linker scripts need modified

Any pointers on how to do that?

By "start up code", you mean a compiler-generated code, or an actual file which I need to modify by hand? (How do I indentify it?)

Re: .m_data_2 not initialized

bobpaddock — Wed, 10 Jan 2024 16:08:22 GMT

The answer depends on the tool set you are using and its age.

What you are looking for is the code that happens right out of 'reset'.
Older code would be in assembler and end with .S.

Newer code is often .c file.

GCC linker scripts generally end with .ld.

I can't be more detailed because each tool, and generation of the tool, calls them something different in different locations.

Re: .m_data_2 not initialized

kat1 — Wed, 10 Jan 2024 22:57:23 GMT

You are the best!

Took a while of further searching, experimenting and digging, but following two commits seem to have done the trick (at least nothing seems broken so far ), in case anyone stumbles across this thread in the future..

- https://github.com/UltimateHackingKeyboard/KSDK_2.0_MK22FN512xxx12/commit/b1f3446d5e380a3bb851a97d07c731125f232769
- https://github.com/UltimateHackingKeyboard/firmware/commit/1b1306efa6924ecec4ad9f31f9a47a7f9633abf4