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