This change has the following effects:
- Reduces the resolution of the RTC sub-second counter from 30.52us to
122.07us.
- Allows RTC.calibration() to now support positive values (as well as
negative values).
- Reduces VBAT current consumption in standby mode by a small amount.
For general purpose use 122us resolution of the sub-second counter is
good enough, and the benefits of full range calibration and minor reduction
in VBAT consumption are worth the change.
Entering a bootloader (ST system bootloader, or custom mboot) from software
by directly branching to it is not reliable, and the reliability of it
working can depend on the peripherals that were enabled by the application
code. It's also not possible to branch to a bootloader if the WDT is
enabled (unless the bootloader has specific provisions to feed the WDT).
This patch changes the way a bootloader is entered from software by first
doing a complete system reset, then branching to the desired bootloader
early on in the start-up process. The top two words of RAM (of the stack)
are reserved to store flags indicating that the bootloader should be
entered after a reset.
Previously the end of the heap was the start (lowest address) of the stack.
With the changes in this commit these addresses are now independent,
allowing a board to place the heap and stack in separate locations.
