In S110 SoftDevice, NRF_POWER-SYSTEMOFF is written. Why?

Question

Hello. 
 I'm now writing a software based on nrf51_sdk_v6_1_0/nrf51822/Board/nrf6310/s110/ble_app_hrs. The SoftDevice I'm using is s110_nrf51822_7.1.0_softdevice.hex. Why I use the version is that the target device is IC revision 2. Exactly speaking, it is nRF51822 QFAAG20 (0x0057). 
 When I run the code, it is working for a while (just advertising without connection). However after a few minutes later, the software crashes by HardFault. When I checked the stack area, I noticed that the RAM Block 0 and 1 looked all filled with zero. I suspected that NRF_POWER->RAMON had a wrong value but it was 0x00000003. It looked fine. However, I also noticed that the memory area would be preserved when I reset the target. Finally I noticed that writing 1 to NRF_POWER->SYSTEMOFF caused a similar phenomenon. 
 So I had a watchpoint on the NRF_POWER->SYSTEMOFF (0x40000500) and reran the code. By the watchpoint, I found that a code in the SoftDevice was writing 1 to the SYSTEMOFF register. It is the following code. 
 0x00011328: ldr r1, [pc, #424] ; 0x114d4
0x0001132a: movs r0, #1
0x0001132c: str r0, [r1, #0]
 
 I also confirmed that the code was called in the IRQ #11 context (RTC0_IRQ). 
 I guess that there is an intention that SoftDevice triggers the SYSTEMOFF, but I don't imagine the reason. 
 FYI, I suspected that heap or stack area was flooded but it looked okay. 
 Could you please let me know when and why SoftDevice triggers the SYSTEMOFF? (Do we still have a way to access RAM even though SYSTEMOFF == 1?) 
 Best Regards, 
 Atsushi

Ulrich Myhre · Accepted Answer

The SYSTEMOFF will be triggered at that point in the SoftDevice if you call sd_power_system_off(). This happens at line 709 in the app you are using, in the on_ble_evt handler: 
 case BLE_GAP_EVT_TIMEOUT:
 if (p_ble_evt->evt.gap_evt.params.timeout.src == BLE_GAP_TIMEOUT_SRC_ADVERTISEMENT)
 {
 nrf_gpio_pin_clear(ADVERTISING_LED_PIN_NO);

 // Go to system-off mode (this function will not return; wakeup will cause a reset).
 err_code = sd_power_system_off();
 APP_ERROR_CHECK(err_code);
 }
 break;
 
 Is this what you are seeing? The purpose for this example is to enable advertisements for some time after startup, then shut down and wait for a keypress if no connection is established within the time limits.

In S110 SoftDevice, NRF_POWER->SYSTEMOFF is written. Why?

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