nrf9160 modem not responding after switching to offline mode

Question

My application uses the nrf9160 in eDRX mode. I regularly have the problem, that when switching the modem to OFFLINE mode (same for POWER-OFF mode), the modem is not responding anymore to any AT commands. I need to switch to OFFLINE mode for battery saving and for switching between Cat-M1 and Cat-NB1.

I wrote a minimalistic application for the demo board (PCA10090 0.8.5, modem Firmware 1.0.0), where I can reproduce the problem.

First the modem is configured to LTE Cat-M1 mode with an eDRX intervall of 82 seconds. Then it is set so ONLINE mode. After a connection is established or a timeout of 100 seconds expires, the modem is set to OFFLINE mode. Then this sequence is repeated.

For the application to run, the AT Command Driver and Logging with synchronous processing must be configured.

Here is the application code for the demo board:

#include <zephyr.h>
#include <stdio.h>
#include <stdlib.h>
#include <uart.h>
#include <string.h>
#include <logging/log.h>
#include <at_cmd.h>

LOG_MODULE_REGISTER(main, LOG_LEVEL_DBG);

volatile int regStatus = 0;

static void modem_NotificationHandler(char* msg)
{
  LOG_DBG("%s", msg);
  if (memcmp(msg, "+CEREG:", strlen("+CEREG:")) == 0)
  {
    char* p = msg + strlen("+CEREG:");
    regStatus = strtol(p, NULL, 10);
  }
}

void main(void)
{
  const u32_t timeout = 100;
  bool connected;
  u32_t t = 0;

	LOG_DBG("application started");

  at_cmd_set_notification_handler(modem_NotificationHandler);
  at_cmd_write("AT+CEREG=3", NULL, 0, NULL);

  while (1)
  {
    regStatus = 0;
    LOG_DBG("configuring modem");
    at_cmd_write("AT%XSYSTEMMODE=1,0,0,0", NULL, 0, NULL); // LTE Cat-M1
    at_cmd_write("AT+CEDRXS=2,4,\"0101\"", NULL, 0, NULL); // 81.92 seconds
    LOG_DBG("modem configured");
    at_cmd_write("AT+CFUN=1", NULL, 0, NULL);
    LOG_DBG("connecting . . .");
    t = 0;
    connected = false;
    while (!connected && t++ < timeout)
    {
      k_sleep(1000);
      connected = regStatus == 1 || regStatus == 5;
    }
    if (connected)
    {
      LOG_DBG("connected");
      k_sleep(10000);
    }
    else
    {
      LOG_DBG("connection timed out");
    }
    LOG_DBG("setting modem to OFFLINE");
    at_cmd_write("AT+CFUN=4", NULL, 0, NULL);
    LOG_DBG("modem set to OFFLINE");
  }
}

After the second connection cycle, when setting the modem to OFFLINE mode, the modem is not responding anymore.

Here is the output:

***** Booting Zephyr OS build v1.14.99-ncs3-snapshot2-1276-g4493a423a645 *****
[00:00:00.365,661] <dbg> main.main: application started
[00:00:00.371,734] <dbg> main.main: configuring modem
[00:00:00.379,455] <dbg> main.main: modem configured
[00:00:00.415,435] <dbg> main.main: connecting . . .
[00:00:30.800,292] <dbg> main.modem_NotificationHandler: +CEREG: 2,"0328","010D2B08",7,0,0

[00:01:05.904,235] <dbg> main.modem_NotificationHandler: +CEREG: 5,"0328","010D2B08",7

[00:01:05.912,994] <dbg> main.modem_NotificationHandler: +CEDRXP: 4,"0101","0101","0000"

[00:01:06.426,910] <dbg> main.main: connected
[00:01:16.431,854] <dbg> main.main: setting modem to OFFLINE
[00:01:16.592,407] <dbg> main.modem_NotificationHandler: +CEREG: 0,"0328","010D2B08",7,0,0

[00:01:18.002,746] <dbg> main.main: modem set to OFFLINE
[00:01:18.008,575] <dbg> main.main: configuring modem
[00:01:18.022,521] <dbg> main.main: modem configured
[00:01:18.064,727] <dbg> main.main: connecting . . .
[00:01:19.749,694] <dbg> main.modem_NotificationHandler: +CEREG: 2,"0328","01158B05",7,0,0

[00:02:58.079,345] <dbg> main.main: connection timed out
[00:02:58.085,144] <dbg> main.main: setting modem to OFFLINE

Any suggestions?