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ATtiny20 [DATASHEET]
8235E–AVR–03/2013
8.
System Control and Reset
8.1
Resetting the AVR
During reset, all I/O registers are set to their initial values, and the program starts execution from the Reset Vector. The
instruction placed at the Reset Vector must be a RJMP – Relative Jump – instruction to the reset handling routine. If the
program never enables an interrupt source, the interrupt vectors are not used, and regular program code can be placed
at these locations. The circuit diagram in
Figure 8-1 shows the reset logic. Electrical parameters of the reset circuitry are
Figure 8-1.
Reset Logic
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This does not require
any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This allows the power
8.2
Reset Sources
The ATtiny20 has four sources of reset:
Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold (VPOT)
External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the minimum pulse
length
Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is enabled
Brown Out Reset. The MCU is reset when the Brown-Out Detector is enabled and supply voltage is below the
brown-out threshold (VBOT)
8.2.1
Power-on Reset
A Power-on Reset (POR) pulse is generated by an on-chip detection circuit. The detection level is defined in section
POR circuit can be used to trigger the Start-up Reset, as well as to detect a failure in supply voltage.
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 VCC rise. The reset signal
is activated again, without any delay, when VCC decreases below the detection level.
DATA BUS
RESET FLAG REGISTER
(RSTFLR)
POWER-ON
RESET CIRCUIT
PULL-UP
RESISTOR
BODLEVEL2...0
VCC
CC
SPIKE
FILTER
RESET
EXTERNAL
RESET CIRCUIT
BROWN OUT
RESET CIRCUIT
RSTDISBL
WATCHDOG
TIMER
DELAY
COUNTERS
S
R
Q
WATCHDOG
OSCILLATOR
CLOCK
GENERATOR
BORF
PORF
EXTRF
WDRF
INTERNAL
RESET
CK
TIMEOUT
COUNTER RESET