MCU reset and internal SRAM starting values

[pozz]

I often use with success a simple way to implement a bootloader:


#define START_MAGIC 0xAA55AA55
uint32_t start_application __attribute__((section=".noinit"));

void main(void) {
...
if (start_application == START_MAGIC) {
start_application = ~START_MAGIC;
launch_application();
}
/* Stay in bootloader and wait for new application from
* USART, SPI, USB, whatever. */
...
start_application = START_MAGIC;
ResetMCU(); /* It's NVIC_SystemReset() */
}


I don't use launch_application() after download, because I think it
isn't good to start the application with some already configured
peripherals (USART, SPI, USB). I know I can deinitialize peripherals and
clocks before launch_application(), but the technique above it's much
more simple.
Of course, start_application variable shouldn't be touched after a
reset, so I put it in noinit section.

Recently I tried to implement a bootloader on a LPC546XX from NXP and I
noticed it didn't work. After some investigation I found that
start_application variable is reset to zero immediately after ResetMCU().
After other investigation I noticed that the first 4kB space of SRAM
doesn't change after a reset, while the next 4kB space is really put to
zero.
I changed the location of start_application variable to the first 4kB
space and the bootloader started working perfectly.

This is the first time I see this behaviour, so my question.

When can you trust that internal SRAM content isn't changed when the MCU
core is reset?

[Stef]

The LPC546XX seems to have a quite complicated boot process that
validates boot images. It may use (and afterwards clear?) some ram for
that?

--
Stef

Happiness isn't having what you want, it's wanting what you have.

[pozz]

Il 06/04/2022 14:07, Stef ha scritto:
> On 2022-04-06 pozz wrote in comp.arch.embedded:

[...]

>> When can you trust that internal SRAM content isn't changed when the MCU
>> core is reset?
>
> The LPC546XX seems to have a quite complicated boot process that
> validates boot images. It may use (and afterwards clear?) some ram for
> that?

Good point, LPC546XX has a ROM bootloader that always starts at the
beginning (and I think even after a software reset like NVIC_SystemReset).

However I couldn't find any info about the SRAM it uses, but this could
explain the behaviour I observed.