MCU reset and internal SRAM starting values

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?

On 2022-04-06 pozz wrote in comp.arch.embedded:
> 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?

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.

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.