Datasheet

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 18-2. 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
18.6.4 Interrupts
The WDT has the following interrupt source:
• Early Warning (EW)
Each interrupt source has an interrupt flag associated with it. The interrupt flag in the Interrupt Flag Status
and Clear (INTFLAG) register is set when the interrupt condition occurs. Each interrupt can be
individually enabled by writing a '1' to the corresponding bit in the Interrupt Enable Set (INTENSET)
register, and disabled by writing a '1' to the corresponding bit in the Interrupt Enable Clear (INTENCLR)
register.
An interrupt request is generated when the interrupt flag is set and the corresponding interrupt is enabled.
The interrupt request remains active until the interrupt flag is cleared, the interrupt is disabled, or the
WDT is reset. See the INTFLAG register description for details on how to clear interrupt flags.
The WDT has one common interrupt request line for all the interrupt sources. The user must read the
INTFLAG register to determine which interrupt condition is present.
Note: Interrupts must be globally enabled for interrupt requests to be generated.
The Early Warning interrupt behaves differently in normal mode and in window mode. In normal mode,
the Early Warning interrupt generation is defined by the Early Warning Offset in the Early Warning Control
register (EWCTRL.EWOFFSET). The Early Warning Offset bits define the number of GCLK_WDT clocks
before the interrupt is generated, relative to the start of the watchdog time-out period. For example, if the
WDT is operating in normal mode with CONFIG.PER = 0x2 and EWCTRL.EWOFFSET = 0x1, the Early
Warning interrupt is generated 16 GCLK_WDT clock cycles from the start of the watchdog time-out
period, and the watchdog time-out system reset is generated 32 GCLK_WDT clock cycles from the start
of the watchdog time-out period. The user must take caution when programming the Early Warning Offset
bits. If these bits define an Early Warning interrupt generation time greater than the watchdog time-out
SAM D21 Family
WDT – Watchdog Timer
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40001882D-page 243