ATtiny4/5/9/10 have a VCC Level Monitoring (VLM) circuit that compares the voltage level at the VCC pin against fixed trigger levels. The trigger levels are set with VLM[2:0] bits, see VLMCSR – VCC Level Monitoring Control and Status register.
The VLM circuit provides a status flag, VLMF, that indicates if voltage on the VCC pin is below the selected trigger level. The flag can be read from VLMCSR, but it is also possible to have an interrupt generated when the VLMF status flag is set. This interrupt is enabled by the VLMIE bit in the VLMCSR register. The flag can be cleared by changing the trigger level or by writing it to zero. The flag is automatically cleared when the voltage at VCC rises back above the selected trigger level.
The VLM can also be used to improve reset characteristics at falling supply. Without VLM, the Power-On Reset (POR) does not activate before supply voltage has dropped to a level where the MCU is not necessarily functional any more. With VLM, it is possible to generate a reset earlier.
When active, the VLM circuit consumes some power, as illustrated in the figure of VCC Level Monitor Current vs. VCC in Typical Characteristics. To save power the VLM circuit can be turned off completely, or it can be switched on and off at regular intervals. However, detection takes some time and it is therefore recommended to leave the circuitry on long enough for signals to settle. See VCC Level Monitor.
When VLM is active and voltage at VCC is above the selected trigger level operation will be as normal and the VLM can be shut down for a short period of time. If voltage at VCC drops below the selected threshold the VLM will either flag an interrupt or generate a reset, depending on the configuration.
When the VLM has been configured to generate a reset at low supply voltage it will keep the device in reset as long as VCC is below the reset level. If supply voltage rises above the reset level the condition is removed and the MCU will come out of reset, and initiate the power-up start-up sequence.
If supply voltage drops enough to trigger the POR then PORF is set after supply voltage has been restored.