The 16k FreeRTOS heap originally had all TCBs and stacks dynamically
allocated within it (plus semaphores and some other things). Now that
xTaskCreateStatic is used instead of xTaskCreate, the TCBs and stacks
are allocated statically and no longer use any of the FreeRTOS heap.
Therefore, the FreeRTOS stack can be shrunk by the amount that has been
made static. Furthermore, the TCBs and stack that are now static should
be placed in the .rtos_heaps section of RAM because this RAM is treated
specially by the bootloader (the bootloader executes from the first 16k
of RAM and loads the firmware into the section starting after the 16k).
After this patch the FreeRTOS heap (ucHeap) is 7200 bytes. The memory
available for the MicroPython heap is 54936 bytes (including GC overhead).
This allows to statically allocate the TCB (thread control block) and
thread stack in the BSS segment, reducing the need for dynamic memory
allocation.
py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for. A
port will also provide mphalport.h with further HAL declarations.
Such functions are never used after MicroPython has started, and they
remain in RAM wasting space. Now they are placed in a special section
named "boot" which sits just before the heap, allowing us to extend
the effective heap area up to the new boot section. Right now, this
gives us back ~1K, but in the future, more functions might end up in
there as well.
The safe boot pin, when pulled high during reset rolls back the
firmware to the "factory" image and skips execution of 'boot.py'
and 'main.py'. This is useful to recover from a crash condition.
The system led is used mostly to signal errors.
The port currently implements support for GPIO, RTC, ExtInt and the WiFi
subsystem. A small file system is available in the serial flash. A
bootloader which makes OTA updates possible, is also part of this initial
implementation.
Damien George