Serial LTE modem with CONFIG_SLM_START_SLEEP=y does not work after RESET or power cycle

[Michal Mühlpachr]

nRF9160 DK NCS v1.6.0, changes to SLM prj.conf:

CONFIG_SLM_START_SLEEP=y

CONFIG_SLM_GPIO_WAKEUP=y

CONFIG_SLM_INTERFACE_PIN=24

After west flash --hex-file build/zephyr/merged.hex or nRF9160 power cycle or nRF9160 RESET SLM can not be wakeup by pulse on pin 24.

SLM wakeup after sequence (sends Ready on UART):

nrfjprog -r

nrfjprog -p

Sometimes I got double Ready from SLM.

Wakeup after AT#XSLEEP=0 works as expected:

AT#XSLEEP=0
Ready

[Jun Qing ZOU]

This might be related the to bug fixed in https://github.com/nrfconnect/sdk-nrf/pull/5190

We cannot enter system off immediately after writing to registers. In above PR I added 100ms delay in between.

In NCSv1.5.1 we have 1s delay in between. Later after code re-structure the bug pops up.