There is no power sequencing requirement needed to ensure the device is in the proper state after reset or to prevent the I/Os from glitching during power up or power down (GPIO19, GPIO26–27, GPIO34–38 do not have glitch-free I/Os). No voltage larger than a diode drop (0.7 V) above VDDIO should be applied to any digital pin (for analog pins, this value is 0.7 V above VDDA) before powering up the device. Voltages applied to pins on an unpowered device can bias internal p-n junctions in unintended ways and produce unpredictable results.
A. Upon power up, SYSCLKOUT is OSCCLK/4. Because the XCLKOUTDIV bits in the XCLK register come up with a reset state of 0, SYSCLKOUT is further divided by 4 before it appears at XCLKOUT. XCLKOUT = OSCCLK/16 during this phase.
B. Boot ROM configures the DIVSEL bits for /1 operation. XCLKOUT = OSCCLK/4 during this phase. XCLKOUT will not be visible at the pin until explicitly configured by user code.
C. After reset, the boot ROM code samples Boot Mode pins. Based on the status of the Boot Mode pin, the boot code branches to destination memory or boot code function. If boot ROM code executes after power-on conditions (in debugger environment), the boot code execution time is based on the current SYSCLKOUT speed. The SYSCLKOUT will be based on user environment and could be with or without PLL enabled.
D. Using the XRS pin is optional due to the on-chip POR circuitry.
E. The internal pullup or pulldown will take effect when BOR is driven high.Figure 5-5 Power-on Reset