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:[...]
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?
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