Low Power Operation

In most examples when your device goes idle, it will go into a sleep mode automatically. In the best case, v4 boards draw about 35µA via 3.3V Vbat at room temp.

Optimizing for low power does take some extra work to make sure all sources of current draw are turned off. Here are some important steps to getting low power.

  1. Remove all connections to SWD programmer

  2. Set your prj.conf to match these values

    # Stacks and heaps
    CONFIG_MAIN_STACK_SIZE=4096
    CONFIG_HEAP_MEM_POOL_SIZE=16384
    
    CONFIG_GPIO=y
    CONFIG_I2C=y
    CONFIG_SPI=y
    CONFIG_DEBUG=n
    
    # Add the accelerometer
    CONFIG_SENSOR=y
    CONFIG_LIS2DH=y
    
    CONFIG_CONSOLE=n
    CONFIG_UART_CONSOLE=n
    CONFIG_SERIAL=n
    
    # Zephyr Device Power Management
    CONFIG_DEVICE_POWER_MANAGEMENT=y
    CONFIG_PM=y
    
    # Network
    CONFIG_NETWORKING=y
    CONFIG_NET_SOCKETS=y
    CONFIG_NET_NATIVE=n
    CONFIG_NET_SOCKETS_POSIX_NAMES=y
    
    # Modem 
    CONFIG_NRF_MODEM_LIB=y
    
    # Enable Zephyr application to be booted by MCUboot
    CONFIG_BOOTLOADER_MCUBOOT=y
    

    Importantly the accelerometer needs to be enabled and CONFIG_SERIAL MUST be set to =n. This turns off all console/debug output to the USB to UART chip.

  3. In an boards/circuitdojo_feather_nrf9160ns.overlay define the following:

    &i2c1 {
      lis2dh@18 {
          compatible = "st,lis2dh";
          label = "LIS2DH";
          reg = <0x18>;
          irq-gpios = <&gpio0 29 GPIO_ACTIVE_HIGH>, <&gpio0 30 GPIO_ACTIVE_HIGH>;
          disconnect-sdo-sa0-pull-up;
      };
    };
    

    disconnect-sdo-sa0-pull-up saves about 130µA during sleep as it disables a Pull-Up within the LIS2DH12TR that is connected to the grounded SDO pin.

If you only have one thread running you can put the device into idle this way:

while (1)
{
k_cpu_idle();
}

Examples like the Asset Tracker will manage this state for you.