21.3.2 Boot sequence
At power up, the on-chip regulator holds the system in a POR state until the input supply
is above the POR threshold. The system continues to be held in this static state until the
internally regulated supplies have reached a safe operating voltage as determined by the
LVD. The Mode Controller reset logic then controls a sequence to exit reset.
1. A system reset is held on internal logic, the RESET_b pin is driven out low, and the
SCG is enabled in its default clocking mode.
2. Required clocks are enabled (Core Clock, System Clock, Flash Clock, and any Bus
Clocks that do not have clock gate control reset to disabled).
3. The system reset on internal logic continues to be held, but the Flash Controller is
released from reset and begins initialization operation while the Reset Control logic
continues to drive the RESET_b pin out low.
4. Early in reset sequencing the NVM option byte is read and stored to the Flash
Memory module's FOPT register.
5. When Flash Initialization completes, the RESET_b pin is released. If RESET_b
continues to be asserted (an indication of a slow rise time on the RESET_b pin or
external drive in low), the system continues to be held in reset. Once the RESET_b
pin is detected high, the Core clock is enabled and the system is released from reset.
6. When the system exits reset, the processor sets up the stack, program counter (PC),
and link register (LR). The processor reads the start SP (SP_main) from vector-table
offset 0. The core reads the start PC from vector-table offset 4. LR is set to
0xFFFF_FFFF. What happens next depends on the NMI input and the
FOPT[NMI_DIS] field in the Flash Memory module:
• If the NMI input is high or the NMI function is disabled in the NMI_DIS field,
the CPU begins execution at the PC location.
• If the NMI input is low and the NMI function is enabled in the NMI_DIS field,
this results in an NMI interrupt. The processor executes an Exception Entry and
reads the NMI interrupt handler address from vector-table offset 8. The CPU
begins execution at the NMI interrupt handler.
7. If FlexNVM is enabled, the flash controller continues to restore the FlexNVM data.
This data is not available immediately out of reset and the system should not access
this data until the flash controller completes this initialization step as indicated by the
EEERDY flag.
Subsequent system resets follow this same reset flow.
The following figure shows the boot sequence.
Chapter 21 Reset and Boot
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors
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