Functional description

The power and clock management system automatically ensures maximum power efficiency.

The core of the power and clock management is the power management unit (PMU) illustrated in the image below.

Power management unit (PMU)

The PMU automatically tracks the power and clock resources required by the different components in the system. It then starts/stops and chooses operation modes in supply regulators and clock sources, without user interaction, to achieve the lowest power consumption possible.

Power management

The power management unit (PMU) handles two system-wide power modes - System ON and System OFF.

Internal blocks of the device are automatically powered by the PMU as they are required by the application.

System ON mode

System ON is the power mode entered after a power-on reset.

While in System ON, the system can reside in one of two sub modes:
  • Low power
  • Constant latency

The low power mode is default after power-on reset.

In low power mode, whenever no application or wireless activity takes place, function blocks like the application CPU, LTE modem and all peripherals are in IDLE state. That particular state is referred to as System ON IDLE. In this state, all function blocks retain their state and configuration, so they are ready to become active once configured by the CPU.

If any application or modem activity occurs, the system leaves the System ON IDLE state. Once a given activity in a function block is completed, the system automatically returns to IDLE, retaining its configuration.

As long as the system resides in low power mode, the PMU ensures that the appropriate regulators and clock sources are started or stopped based on the needs of the function blocks active at any given time.

This automatic power management can be overridden by switching to constant latency mode. In this mode, the CPU wakeup latency and the PPI task response are constant and kept at a minimum. This is secured by keeping a set of base resources that are always enabled. The advantage of having a constant and predictable latency will be at the cost of having significantly increased power consumption compared to the low power mode. The constant latency mode is enabled by triggering the CONSTLAT task (TASKS_CONSTLAT).

While the system is in constant latency mode, the low power mode can be enabled by triggering LOWPWR task (TASKS_LOWPWR).

To reduce power consumption while in System ON IDLE, RAM blocks can be turned off in System ON mode while enabling the retention of these RAM blocks in RAM[n].POWER registers in VMC. RAM[n].POWER are retained registers, see Reset behavior. Note that these registers are usually overwritten by the startup code provided with the nRF application examples.

System OFF mode

System OFF is the deepest power saving mode the system can enter.

In this mode, the core system functionality is powered down and ongoing tasks terminated, and only the reset and the wakeup functions are available and responsive.

The device is put into System OFF mode using the REGULATORS register interface. When in System OFF mode, one of the following signals/actions will wake up the device:

  1. DETECT signal, generated by the GPIO peripheral
  2. RESET
  3. start of debug session

When the device wakes up from System OFF mode, a system reset is performed.

One or more RAM blocks can be retained in System OFF mode depending on the settings in the RAM[n].POWER registers in VMC. RAM[n].POWER are retained registers, see Reset behavior. Note that these registers are usually overwritten by the startup code provided with the nRF application examples.

Before entering System OFF mode, the user must make sure that all on-going EasyDMA transactions have completed. This can be accomplished by making sure that EasyDMA enabled peripherals have stopped and END events from them received. The LTE modem also needs to be stopped, by issuing a command through the modem API, before entering System OFF mode. Once the command is issued, one should wait for the modem to respond that it actually has stopped, as there may be a delay until modem is disconnected from the network.

Emulated System OFF mode

If the device is in debug interface mode, System OFF will be emulated to secure that all required resources needed for debugging are available during System OFF.

See Overview chapter for more information. Required resources needed for debugging include the following key components: Overview, CLOCK — Clock control, POWER — Power control, NVMC — Non-volatile memory controller, CPU, flash, and RAM. Since the CPU is kept on in emulated System OFF mode, it is required to add an infinite loop directly after entering System OFF, to prevent the CPU from executing code that normally should not be executed.