17.6.3.1 Always-On Mode

The Always-On mode is enabled by setting the Always-On bit in the Control register (CTRLA.ALWAYSON=1). When the Always-On mode is enabled, the WDT runs continuously, regardless of the state of CTRL.ENABLE. Once written, the Always-On bit can only be cleared by a power-on reset. The Configuration (CONFIG) and Early Warning Control (EWCTRL) registers are read-only registers while the CTRL.ALWAYSON bit is set. Thus, the time period configuration bits (CONFIG.PER, CONFIG.WINDOW, EWCTRL.EWOFFSET) of the WDT cannot be changed.

Enabling or disabling Window mode operation by writing the Window Enable bit (CTRLA.WEN) is allowed while in Always-On mode, but note that CONFIG.PER cannot be changed.

The CTRL.ALWAYSON bit must never be set to one by software if any of the following conditions is true:
  1. The GCLK_WDT is disabled
  2. The clock generator for the GCLK_WDT is disabled
  3. The source clock of the clock generator for the GCLK_WDT is disabled or off

The Interrupt Clear and Interrupt Set registers are accessible in the Always-On mode. The Early Warning interrupt can still be enabled or disabled while in the Always-On mode, but note that EWCTRL.EWOFFSET cannot be changed.

Table 17-2. WDT Operating Modes With Always-On
WEN Interrupt enable Mode
0 0 Always-on and normal mode
0 1 Always-on and normal mode with Early Warning interrupt
1 0 Always-on and window mode
1 1 Always-on and window mode with Early Warning interrupt