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Groups > comp.arch.embedded > #31030

MCU reset and internal SRAM starting values

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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?

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Thread

MCU reset and internal SRAM starting values pozz <pozzugno@gmail.com> - 2022-04-06 13:37 +0200
  Re: MCU reset and internal SRAM starting values Stef <me@this.is.invalid> - 2022-04-06 14:07 +0200
    Re: MCU reset and internal SRAM starting values pozz <pozzugno@gmail.com> - 2022-04-06 15:45 +0200

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