5.14.2 Power Sequencing
No requirements are placed on the power up/down sequence of the various power pins to ensure the correct reset state for all the modules. However, if the 3.3-V transistors in the level shifting output buffers of the I/O pins are powered prior to the 1.8-V transistors, it is possible for the output buffers to turn on, causing a glitch to occur on the pin during power up. To avoid this behavior, power the VDD (core voltage) pins prior to or simultaneously with the VDDIO (input/output voltage) pins, ensuring that the VDD pins have reached 0.7 V before the VDDIO pins reach 0.7 V.
There are some requirements on the XRS pin:
- During power up, the XRS pin must be held low for tw(RSL1) after the input clock is stable (see Table 5-8). This is to enable the entire device to start from a known condition.
- During power down, the XRS pin must be pulled low at least 8 μs prior to VDD reaching 1.5 V. This is to enhance flash reliability.
No voltage larger than a diode drop (0.7 V) above VDDIO should be applied to any digital pin (for analog pins, it is 0.7 V above VDDA) prior to 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/2. Since 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. This explains why XCLKOUT = OSCCLK/8 during this phase.
B. 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.
C. See
Section 5.14.2 for requirements to ensure a high-impedance state for GPIO pins during power-up.
Figure 5-6 Power-on Reset