Datasheet

49
7728G–AVR–06/10
ATtiny87/ATtiny167
6. System Control and Reset
6.1 Reset
6.1.1 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 an 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 6-1 shows the reset circuit. Tables in Section 22.5
“RESET Characteristics” on page 246 defines the electrical parameters of the reset circuitry.
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 to reach a stable level before normal operation starts. The
time-out period of the delay counter is defined by the user through the SUT and CKSEL
Fuses. The different selections for the delay period are presented in Section 4.2 “Clock
Sources” on page 25.
6.1.2 Reset Sources
The ATtiny87/167 has four sources of reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset
threshold (V
POT
).
• External Reset. The MCU is reset when a low level is present on the RESET
pin for longer
than the minimum pulse length.
• Watchdog System Reset. The MCU is reset when the Watchdog Timer period expires and
the Watchdog System Reset mode is enabled.
• Brown-out Reset. The MCU is reset when the supply voltage Vcc is below the Brown-out
Reset threshold (V
BOT
) and the Brown-out Detector is enabled.