unified_firmware.md

Unified firmware

This file contains info about using the same firmware for both cores on an STM32H745 chip.

Core detection

A core can be detected by looking at the SCB->CPUID register. Bits [15:4] hold the part number, which will bewhich will be 0xC27 for a CM7 core and 0xC24 for a CM4 core.

We can test for this immediately within the ResetHandler using assembly language

Reset_Handler:
  /* set M4 stack pointer */
  ldr   sp, =_estack_cm4

  /* on M4 core, skip past initialization */
  ldr r2, =_scb_cpuid
  ldr r0, [r2]
  lsr r0, #4
  and r0, #0xF
  cmp r0, #4
  beq CoreM4

CoreM7:
  /* set M7 stack pointer */
  ldr   sp, =_estack_cm7

Implementing printf

It would be nice to have printf implemented so that it can be used as a debug log. However, there are some issues:

• Increase in code size by ~20kB
  • This is generally not a problem as we have the space
• printf calls malloc
  • This goes against our desire to have no dynamically allocated memory
• Some implementations are not thread-safe
  • For a dual-core device, this can be a problem if they are part of the same compilation unit, which is the goal of the unified firmware.

One option would be to implement printf. This project does it:

• https://github.com/mpaland/printf

I took this last option which seems to be working well. To use it, one needs to include printf.h prior to using any printf calls.