My platform is LPC546xx, Cortex-M4 by NXP, and the build environment is
based on GCC ARM toolchain (MCUXpresso IDE). However the question is
generic.
The default linker script instructs linker to put bss sections (zero initialized data) in internal RAM. During startup, before main, bss
sections, described in a table on Flash, are reset to zero:
__attribute__ ((section(".after_vectors.init_bss")))
void bss_init(unsigned int start, unsigned int len) {
unsigned int *pulDest = (unsigned int*) start;
unsigned int loop;
for (loop = 0; loop < len; loop = loop + 4)
*pulDest++ = 0;
}
extern unsigned int __bss_section_table;
extern unsigned int __bss_section_table_end;
void ResetISR(void) {
...
// At this point, SectionTableAddr = &__bss_section_table;
// Zero fill the bss segment
while (SectionTableAddr < &__bss_section_table_end) {
ExeAddr = *SectionTableAddr++;
SectionLen = *SectionTableAddr++;
bss_init(ExeAddr, SectionLen);
}
...
main();
...
}
This works well until I add a new RAM memory section on external bus (it
is a SDRAM). Suppose I want to allocate a big variable on SDRAM. This
can be done in different ways (I use some facilities of MCUXpresso IDE).
Now the startup code is broken, because SDRAM memory can be accessed
only after it is configured (during main), but the bss initialization is
done before main.
I know I can rewrite ResetISR() or bss_init() to avoid accessing
external bus addresses or I can configure SDRAM during ResetISR() before calling bss_init().
I suspect there's a better and more cleaner way. Noinit sections?
On 18/03/2022 08:56, pozz wrote:
My platform is LPC546xx, Cortex-M4 by NXP, and the build environment is
based on GCC ARM toolchain (MCUXpresso IDE). However the question is
generic.
The default linker script instructs linker to put bss sections (zero
initialized data) in internal RAM. During startup, before main, bss
sections, described in a table on Flash, are reset to zero:
__attribute__ ((section(".after_vectors.init_bss")))
void bss_init(unsigned int start, unsigned int len) {
unsigned int *pulDest = (unsigned int*) start;
unsigned int loop;
for (loop = 0; loop < len; loop = loop + 4)
*pulDest++ = 0;
}
extern unsigned int __bss_section_table;
extern unsigned int __bss_section_table_end;
void ResetISR(void) {
...
// At this point, SectionTableAddr = &__bss_section_table;
// Zero fill the bss segment
while (SectionTableAddr < &__bss_section_table_end) {
ExeAddr = *SectionTableAddr++;
SectionLen = *SectionTableAddr++;
bss_init(ExeAddr, SectionLen);
}
...
main();
...
}
This works well until I add a new RAM memory section on external bus (it
is a SDRAM). Suppose I want to allocate a big variable on SDRAM. This
can be done in different ways (I use some facilities of MCUXpresso IDE).
Now the startup code is broken, because SDRAM memory can be accessed
only after it is configured (during main), but the bss initialization is
done before main.
I know I can rewrite ResetISR() or bss_init() to avoid accessing
external bus addresses or I can configure SDRAM during ResetISR() before
calling bss_init().
I suspect there's a better and more cleaner way. Noinit sections?
There are several ways to handle this. You can set up the sdram before clearing the bss - I think you'll find a weak function called
SytemInitHook or similar, that you can override with your own version.
If you are putting a lot of different stuff into the sdram, this is
probably the best.
You can fiddle with either the linker script or the ResetISR function to avoid clearing the sdram bss (and copying the sdram initialised data)
during ResetISR, and do it manually after initialising the ram.
Often, however, it is a good idea /not/ to use sdram for bss or data.
Use the internal ram (DTC ram if possible, or OCRAM - depending on the particular chip) for that. Put your heap in sdram, along with other big buffers such as network buffers, FreeRTOS heap, etc. These can all be initialised later once the ram is configured. And you can mark any
large statically allocated arrays or structures with a section attribute
for a "noinit" section of the sdram.
I know I can rewrite ResetISR() or bss_init() to avoid accessing
external bus addresses or I can configure SDRAM during ResetISR() before calling bss_init().
I suspect there's a better and more cleaner way. Noinit sections?
Am 18.03.2022 um 08:56 schrieb pozz:
I know I can rewrite ResetISR() or bss_init() to avoid accessing
external bus addresses or I can configure SDRAM during ResetISR() before
calling bss_init().
I suspect there's a better and more cleaner way. Noinit sections?
When I did bootloaders, I did the SDRAM initialisation there, before the application starts. This also means there's a nice place to deal with
board varieties, i.e. if I have a board variant with a different SDRAM
chip, I only have to change the bootloader. And it allows loading parts
of the application code into SDRAM, if that is relevant for you.
Otherwise, I'd probably use a noinit section.
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