55
9151AINDCO07/09
ATA8742
16. System Control and Reset
16.0.1
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 16-1 on page 56 shows the reset logic. Table 16-1 on
page 57 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 dif-
ferent selections for the delay period are presented in Clock Sources on page 42.
16.0.2
Reset Sources
The ATtiny24/44/84 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 when RESET
function is enabled.
" 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 supply voltage V
CC
is below the Brown-out Reset
threshold (V
BOT
) and the Brown-out Detector is enabled.
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