Datasheet

37
2466T–AVR–07/10
ATmega16(L)
System Control
and Reset
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 JMP – absolute
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. This is also the case if the Reset Vector is in the Application section while the Interrupt
Vectors are in the Boot section or vice versa. The circuit diagram in Figure 15 shows the reset
logic. Table 15 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 CKSEL Fuses. The different selec-
tions for the delay period are presented in “Clock Sources” on page 25.
Reset Sources The ATmega16 has five 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 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.
• JTAG AVR Reset.
The MCU is reset as long as there is a logic one in the Reset Register, one of the scan
chains of the JTAG system. Refer to the section “IEEE 1149.1 (JTAG) Boundary-scan” on
page 228 for details.