[nRF54LM20A] Does nRF54LM20A Support Automatic WDT Start via UICR Before CPU Boot (like nRF54H20)?

[strongman]

Platform: nRF54LM20A SDK Version: nRF Connect SDK v3.3.1 MCUboot Version: Included in NCS v3.3.1

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Background

On the nRF54H20, the IronSide Secure Enclave provides a mechanism to automatically start a local watchdog timer before the application core boots, configured via UICR.WDTSTART. This is controlled by the following Kconfig options:

• CONFIG_GEN_UICR_WDTSTART — Enable automatic WDT start
• CONFIG_GEN_UICR_WDTSTART_INSTANCE_WDT0 / _WDT1 — Select WDT instance
• CONFIG_GEN_UICR_WDTSTART_CRV — Set the Counter Reload Value (timeout period)

This provides early boot protection, ensuring the system can recover from early boot failures without requiring the application or bootloader to explicitly initialize the watchdog.

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Observation on nRF54LM20A

Reviewing the nRF54LM20A product specification, the UICR register map does not appear to include a WDTSTART field. The documented UICR registers for nRF54LM20A include:

• APPROTECT, SECUREAPPROTECT, AUXAPPROTECT
• ERASEPROTECT
• BOOTCONF
• USER.ROT.PUBKEY
• OTP

There is no WDTSTART entry listed.

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Observation in MCUboot WDT Feed Code

In the MCUboot port for Nordic devices, when CONFIG_BOOT_WATCHDOG_FEED_NRFX_WDT is selected, the code handles nRF54LM20A's WDT instances correctly for feeding:

#elif defined(NRF_WDT30) && defined(NRF_WDT31)
#define MCUBOOT_WATCHDOG_FEED()   \
    do {                          \
        FEED_WDT_INST(NRF_WDT30); \
        FEED_WDT_INST(NRF_WDT31); \
    } while (0)

However, MCUBOOT_WATCHDOG_SETUP() is not defined in the nrfx WDT path, meaning it expands to a no-op. This implies MCUboot assumes the WDT has been initialized and started externally before MCUboot runs — which on nRF54H20 is handled by IronSide via UICR.

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Questions

1. Does the nRF54LM20A support any mechanism equivalent to UICR.WDTSTART to automatically start the WDT before the CPU boots?

2. If not, what is the recommended approach to ensure early boot WDT protection on nRF54LM20A when using MCUboot with CONFIG_BOOT_WATCHDOG_FEED_NRFX_WDT? Should the WDT be initialized in MCUboot itself using the Zephyr WDT driver path (DT_ALIAS(watchdog0)) instead?

3. Is the nrfx WDT feed path on nRF54LM20A only intended for scenarios where the WDT has already been started by a prior boot stage?

Any clarification or pointer to relevant documentation would be greatly appreciated. Thank you.