Device and power monitoring

This manager monitors the system. It generates a core system event when the monitoring timer elapses. The timer period is defined by the core_monitoring_period parameter. This manager is paused in OFF state.

Device manager

The device manager monitors the temperature.

The temperature monitoring process consists of the following steps:

1. Measure the current temperature
2. Compare it to the minimum and maximum values stored in the internal configuration parameter group
3. Update the associated internal parameter if a new minimum or a new maximum is detected
4. Compare the current temperature against the configuration parameters core_temp_high_threshold and core_temp_low_threshold and deduce whether we are in critical high, critical low or normal temperature state

Power manager

The power manager collects the consumption of the tracker components and calculates the total power consumption.

Based on the battery type a specific process takes place:

• Primary battery: Each time the total consumption exceeds by 5% the previous one, the manager saves the consumption in the associated internal configuration parameter (in flash).
• Rechargeable battery: The battery voltage is measured and converted into a remaining charge percentage. The manager does not store permanently the consumption in the internal configuration parameter. The manager also detects the presence of the USB cable connection. If connected, the battery is charging and the charge percentage is set to the special 0 value (reported via uplinks). Once the cable is disconnected, the manager resets the consumption of all consumers, so the power consumption reported is counted from last battery charge. Please note that the battery level reported may show less than 100% as it is estimated based on the voltage for rechargeable batteries. The charger operates with a voltage hysteresis to prevent continuous charging of a full battery: it allows the battery to discharge to a threshold before recharging begins again. If the cable is disconnected at the lowest voltage point, the battery may be reported as less than 100%.

When the battery monitoring is called the USB cable state is checked in case of rechargeable battery:

• If connected the system event type_power_charging is sent .
• If disconnected the system event type_power_not_charging is sent .

At the end of the processing, the power manager ensures that there is enough energy in the battery. If it is not the case it sends a type_power_low_battery to the system to signal a critically low level.

The condition to trigger the event is:

• Primary battery: If the last reset cause was a brown-out the event is sent. This will put the system to OFF state, preventing a potential condition of cyclic brownout resets followed by network activity (e.g. LoRaWAN startup message).
• Rechargeable battery: If the remaining battery charge is lower or equal to 5% the event is sent.

Managing low-power shut-down (brownout)

Rechargeable battery devices

When the main functional state machine processes a low battery system event (critically low-battery condition detected by the power manager), the following actions occur:

• A battery low power notification is sent via the network
• Geolocation is stopped
• The accelerometer is stopped
• BLE activities (connectivity, beaconing) are stopped
• The network is stopped
  The main FSM moves to the Shutting-down state, sends a core_stdn system event, and five seconds later transitions to the Off state, sending a core_off system event. This delay allows to send the residual network events.

To restart the system, one of the following actions must be performed:

• Press a button (if the start_device action is mapped to the button)
• Plug a USB cable into the tracker (recharging the battery)

Primary Battery devices

When the main functional state machine processes a low battery system event, the only action taken is sending a battery low notification to the network.

The system remains in the Running state until a brownout reset occurs (triggered by the MCU). If the voltage is sufficient, the MCU will attempt a restart, which can cause restart loops and useless network trafic. In order to prevent this, when it restarts after a brownout reset and in low battery condition, the firmware checks the tamper switch presence and state:

• Tamper switch absent

  • The application stays in the Off state and starts a 6-hour timer.
  • When the timer expires, the system transitions to the Running state and attempts to:
    • Connect to the network
    • Restart the accelerometer
    • Restart geolocation
    • Restart BLE activities

• Tamper switch present

  • If rebooting with tamper off (casing closed), the 6-hour timer is started.
  • If rebooting with tamper on (casing open), the 6-hour timer is not started and the system restarts immediately.
    Note: If tamper was off and the 6-hour timer was started, opening the casing (tamper on) forces a system restart.
    This mechanism is implemented because when batteries are replaced while in the Off state (due to low battery), the tracker may not reset immediately. This is caused by the slow discharge of the supercap, which drains very little current and takes a long time.