The first case to consider is the boot from an external 16-bit NOR flash memory which is an eXecute In Place (XIP) memory. Different XIP memories can be used for booting purpose. At power up, the System Clock is the Slow Clock (32kHz) with the first instruction executed at the address 0x0. So at this step, no boot program has been executed so no device initializations have been performed. That means the user application running out of this memory must perform different initializations such as clock setups, timing configurations, and more.