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System control and reset, 1 resetting the avr, 2 reset sources – Rainbow Electronics ATmega64M1 User Manual

Page 43

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43

8209A–AVR–08/09

ATmega16M1/32M1/64M1

11. System Control and Reset

11.1

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 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 11-1

shows the reset

logic.

“System and Reset Characteristics” on page 313

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 28

.

11.2

Reset Sources

The ATmega16M1/32M1/64M1 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 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.