How is the M4 booted with the Vybrid with M4 as primary? What's the difference between M4 as a primary and A5 as a primary in the hardware? Via fuse setting.
The boot core is determined by which part number you buy. The device on the Vybrid tower board is an A5 primary device.
What that means is that the A5 core runs all the BootROM code, and then jump to your user code. You have to explicitly enable the M4 core in your user code to begin using it.
Likewise on an M4 primary part, the M4 core would run all the BootROM code and then jump to user code. The A5 core would have to be explicitly enabled in your user code to begin using it.
See this thread for more information on emulating an M4 primary chip on the tower system: M4 Primary Emulation
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