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lines changed Original file line number Diff line number Diff line change @@ -138,7 +138,14 @@ __attribute__((used, naked)) void Reset_Handler(void) {
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__DMB (); /* ARM says to use a DMB instruction before relocating VTOR */
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SCB -> VTOR = 0x90000000u ; /* We relocate vector table to the QSPI sector 1 */
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__DSB (); /* ARM says to use a DSB instruction just after relocating VTOR */
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- /* SystemInit call is not necessary, initializations are done in the H750 special bootloader */
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+ /*
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+ Since the STM32H750 microcontroller has only 128kB internal flash,
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+ CircuitPython has to run from an external flash (QSPI or FMC).
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+ This means a custom bootloader, like tinyUF2, is needed on the internal
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+ flash to initialize the clocks, PLLs and (QSPI) controller, and to
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+ start execution of the firmware from the external flash.
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+ It also makes the SystemInit() call not necessary for this chip.
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+ */
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#else
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SystemInit ();
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#endif
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