Reset

There are multiple reset sources that may trigger a reset of the system. After a reset the CPU can query the RESETREAS (reset reason register) to find out which source generated the reset.

Power-on reset

The power-on reset generator initializes the system at power-on. The system is held in reset state until the supply has reached the minimum operating voltage and the internal voltage regulators have started.

Pin reset

A pin reset is generated when the physical reset pin (nRESET) on the device is pulled low.

To ensure that reset is issued correctly, the reset pin should be held low for time given in .

nRESET pin has an always-on internal pull-up resistor connected to VDD. The value of the pull-up resistor is given in .

Wakeup from System OFF mode reset

The device is reset when it wakes up from System OFF mode.

The Debug access port is not reset following a wake up from System OFF mode if the device is in debug interface mode, see Overview chapter for more information.

Soft reset

A soft reset is generated when the SYSRESETREQ bit of the application interrupt and reset control register (AIRCR register) in the ARM® core is set.

Watchdog reset

A watchdog reset is generated when the watchdog timer (WDT) times out.

See WDT — Watchdog timer chapter for more information.

Brownout reset

The brownout reset generator puts the system in reset state if the supply voltage drops below the brownout reset threshold.

Retained registers

A retained register is a register that will retain its value in System OFF mode, and through a reset depending on reset source. See individual peripheral chapters for information of which registers are retained for the different peripherals.

Reset behavior

Reset behavior depends on the reset source.

The reset behavior is summarized in the table below.

Table 1. Reset behavior for the main components
Reset source Reset target
CPU Modem Debug1 SWJ-DP Not retained RAM2 Retained RAM2 WDT RESETREAS
CPU lockup 3 x x            
Soft reset x x            
Wakeup from System OFF mode reset x x x 4   x x  
Watchdog reset 5 x x x   x   x  
Pin reset x x x x x   x  
Brownout reset x x x x x x x x
Power-on reset x x x x x x x  
Note: The RAM is never reset but its content may be corrupted after reset in the cases given in the table above.
Table 2. Reset behavior for the retained registers
Reset source Reset target
Regular peripheral registers GPIO, SPU NVMC WAITSTATENUM NVMC IFCREADDELAY REGULATORS, OSCILLATORS POWER.GPREGRET
CPU lockup3 x x x      
Soft reset x x x      
Wakeup from System OFF mode reset x   x      
Watchdog reset5 x x x   x  
Pin reset x x x   x  
Brownout reset x x x x x x
Power-on reset x x x x x x

Electrical specification

Pin reset

Symbol Description Min. Typ. Max. Units
tHOLDRESET

Hold time for reset pin when doing a pin reset

.. .. .. µs
RPULL-UP

Value of the internal pull-up resistor

.. .. ..
1 All debug components excluding SWJ-DP. See Overview chapter for more information about the different debug components in the system.
2 RAM can be configured to be retained using registers in VMC — Volatile memory controller
3 Reset from CPU lockup is disabled if the device is in debug interface mode. CPU lockup is not possible in System OFF.
4 The debug components will not be reset if the device is in debug interface mode.
5 Watchdog reset is not available in System OFF.