Added clarification about firmware execution from external flash

snkYmkrct · snkYmkrct · commit fe55cb78490b · 2025-03-18T20:53:27.000+01:00
diff --git a/ports/stm/supervisor/port.c b/ports/stm/supervisor/port.c
@@ -138,7 +138,14 @@ __attribute__((used, naked)) void Reset_Handler(void) {
     __DMB(); /* ARM says to use a DMB instruction before relocating VTOR */
     SCB->VTOR = 0x90000000u; /* We relocate vector table to the QSPI sector 1 */
     __DSB(); /* ARM says to use a DSB instruction just after relocating VTOR */
-    /* SystemInit call is not necessary, initializations are done in the H750 special bootloader */
+    /*
+       Since the STM32H750 microcontroller has only 128kB internal flash,
+       CircuitPython has to run from an external flash (QSPI or FMC).
+       This means a custom bootloader, like tinyUF2, is needed on the internal
+       flash to initialize the clocks, PLLs and (QSPI) controller, and to
+       start execution of the firmware from the external flash.
+       It also makes the SystemInit() call not necessary for this chip.
+    */
     #else
     SystemInit();
     #endif
