[36] Debug and Trace: Access port gets locked in WFI and WFE

This anomaly applies to Revision 1, build codes LACA-A0A.

It was inherited from the previous IC revision Engineering A.

Symptoms

The registers in the NRF_APPROTECT peripheral are not retained in SYSTEM ON IDLE mode.

Conditions

The device enters SYSTEM ON IDLE mode after configuring access port protection to be open.

Consequences

The access port is closed.

Workaround

If current consumption is not critical during debugging, enter constant latency mode at startup to force retention of the AP-protect registers by implementing the following code.


#ifdef DEBUG
    NRF_POWER->TASKS_CONSTLAT = 1;
#endif

For power-aware troubleshooting, reapply the AP-protect settings when the system should be ready for debugging after waking from WFI/WFE. This prevents the attaching of a debugger in SYSTEM ON IDLE mode, so an external wakeup might be needed. The AP-protect settings can be reloaded using the following MDK-provided method.

nrf91_handle_approtect();

Debuggers and programmers can do a CTRL-AP reset to reload the AP-protect settings to unlock the device. If the device remains locked, performing a CTRL-AP eraseall opens the AP-protection.