WDT documentation clarification

[JeanMBoones]

On page 69 of nRF52840 Product Specification 1.0 4413_417 v1.0 / 2018-03-16:

The table in section 5.3.6.8 shows that the WDT is NOT reset from a Wakeup from System OFF mode reset.

On page 520 of nRF52840 Product Specification 1.0 4413_417 v1.0 / 2018-03-16:

When the device starts running again, after a reset, or waking up from OFF mode, the watchdog configuration registers will be available for configuration again.

This sentence implies that waking up from System Off mode is like a reset: WDT registers are available for configuration. However, the table in section 5.3.6.8  shows that the WDT is not reset. This implies that the registers are available for read/write and the WDT is active. That means the WDT could be reconfigured. However if it is not reconfigured, and the WDT was previously configured and enabled, it will cause a WDT reset. These two points are not contradictory, however it can leave the WDT in a compromised state. What exactly happens to the WDT when the system leaves Wakeup from System Off mode?

[bjorn-spockeli]

Hi Jean, 

FYI: The nRF52840 Product Specification has been updated, the latest version is v1.1

In SystemOFF the power domain to the LFCLK source as well as the power domain that supplies the WDT itself is disabled. Hence, the following footnote in the 5.3.6.8 Reset behavior section

       9) Watchdog reset is not available in System OFF.

When entering System OFF the WDT registers are retained and the WDT is stopped.  This is also the state when waking up from SystemOFF, which is the reason for the WDT not beeing marked with the 'x' in the Reset behavior table for "Wakeup from System OFFmode reset",

I agree that should be stated in the WDT section of the Product Specification and will report this internally so that the documentation may be updated. 

Best regards

Bjørn

[JeanMBoones]

Bjørn,

Thanks for the reply. There is one question that is not answered in your reply. According to 4413_417 v1.1 / 2019-02-28 page 570:

When the device starts running again, after a reset, or waking up from OFF mode, the watchdog configuration registers will be available for configuration again.

In your answer, you state that WDT registers are retained across the SystemOFF mode. This implies that these registers are not available for configuration.

Can you clarify: Are these registers available for configuration after a SystemOFF?

Thanks,

-J

[bjorn-spockeli]

They are retained, i.e. not reset. They are as stated in the WDT section of the product specification,available for configuration when waking up from System OFF mode. 

So they will retain the values configured when the device was put in System OFF mode and the registers are no longer blocked, i.e. they can be modified after waking up from System OFF.