NXP Semiconductors K32 L2A Series, Reference Manual, Page: 132 / 1379

6.3.3 Boot sequence
At power up, the on-chip regulator holds the system in a POR state until the input supply
exceeds 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 Reset Controller logic then controls a sequence to exit reset.
1. A system reset is held on internal logic, the RESET pin is driven out low, and the
SCG is enabled in its default clocking mode.
2. Required clocks are enabled (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 pin out low.
4. Early in reset sequencing, the NVM option byte is read and stored to the FOPT
register of the Flash Memory module (FTFA_FOPT). If the bits associated with
FTFA_FOPT[LPBOOT] are programmed for an alternate clock divider reset value,
the system/core clock is switched to a slower clock speed. If
FTFA_FOPT[FAST_INIT] is programmed clear, the flash initialization switches to
slower clock resulting longer recovery times.
5. When flash Initialization completes, the RESET pin is released. If RESET continues
to be asserted (an indication of a slow rise time on the RESET pin or external drive
in low), the system continues to be held in reset. Once the RESET 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. The next sequence of events depends on the NMI/BOOTCFG0 input
and FTFA_FOPT[NMI_DIS] and FTFA_FOPT[BOOTSRC_SEL] and
FTFA_FOPT[BOOTPIN_OPT] as well as RCM_MR[BOOTROM](See
Table 6-3
and RCM block guide) :
• If the NMI/BOOTCFG0 input is high or the NMI function is disabled in
FTFA_FOPT, the CPU fetches the reset vector from flash address 0x0000_0004
and code execution begins from this address.
• If the NMI/BOOTCFG0 input is low and the NMI function is enabled in
FTFA_FOPT, 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.
• When FTFA_FOPT[BOOTPIN_OPT] = 0, it forces boot from ROM if NMI/
BOOTCFG0 pin asserted.
Boot
K32 L2A Reference Manual, Rev. 2, 01/2020
132 NXP Semiconductors