ATTINY9_14 Datasheet, PDF (28/170 Pages) ATMEL Corporation – Atmel 8-bit AVR Microcontroller

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset
threshold voltage invokes the delay counter, which determines how long the device is kept in reset after V_CC rise.
The reset signal is activated again, without any delay, when V_CC decreases below the detection level.
Figure 8-2.
MCU Start-up, RESET Tied to V_CC
V _CC
V _POT
RESET
V _RST
TIME-OUT
t _TOUT
INTERNAL
RESET
Figure 8-3. MCU Start-up, RESET Extended Externally
V _CC
V _POT
RESET
> t _TOUT
V _RST
TIME-OUT
t _TOUT
INTERNAL
RESET
8.2.2
V_CC Level Monitoring
ATtiny4/5/9/10 have a V_CC Level Monitoring (VLM) circuit that compares the voltage level at the V_CC pin against
fixed trigger levels. The trigger levels are set with VLM2:0 bits, see “VLMCSR – VCC Level Monitoring Control and
Status register” on page 33.
The VLM circuit provides a status flag, VLMF, that indicates if voltage on the V_CC 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 V_CC 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 Figure 17-48 on page 145. To save power
the VLM circuit can be turned off completely, or it can be switched on and off at regular intervals. However, detec-
tion takes some time and it is therefore recommended to leave the circuitry on long enough for signals to settle.
See “VCC Level Monitor” on page 118.
ATtiny4/5/9/10 [DATASHEET] 28
8127F–AVR–02/2013