The Analog and Digital Subsystems' each have a POR circuit that creates a clean reset throughout the device enabling glitchless GPIOs during the power-on procedure. The POR function keeps both ARS and XRS driven low during device power up. This functionality is always enabled, even when VREG is disabled.
While in most applications, the POR generated reset has a long enough duration to also reset other system ICs, some applications may require a longer lasting pulse. In these cases, the ARS and XRS reset pins (which are open-drain) can also be driven low to match the time the device is held in a reset state with the rest of the system.
When POR drives the ARS and XRS pins low, the POR also resets the digital logic associated with both subsystems and puts the GPIO pins in a high impedance state.
In addition to the POR reset, the Resets block of the Digital Subsystem also receives reset inputs from the NVIC, the Cortex-M3 Watchdogs (0, 1), and from the Cortex-M3 NMI Watchdog. The resulting reset output signal is then fed back to the XRS pin after being AND-ed with the POR reset (see Figure 6-6).
On a related note, only the Master Subsystem comes out of reset immediately following a device power up. The Control and Analog Subsystems continue to be held in reset until the Master Processor (Cortex-M3) brings them out of reset by writing a "1" to the M3RSNIN and ACIBRST bits of the CRESCNF Register (see Figure 6-6).