ATMEGA644PV ATMEL [ATMEL Corporation], ATMEGA644PV Datasheet - Page 50

• Manufacturer Part Number: ATMEGA644PV
• Description: 8-bit Microcontroller with 16/32/64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

9. System Control and Reset
9.0.1
9.0.2
50
ATmega164P/324P/644P
Resetting the AVR
Reset Sources
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
shows the reset logic.
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
The ATmega164P/324P/644P has five sources of reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset
• External Reset. The MCU is reset when a low level is present on the RESET pin for longer than
• Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the
• Brown-out Reset. The MCU is reset when the supply voltage V
• JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one
threshold (V
the minimum pulse length.
Watchdog is enabled.
threshold (V
of the scan chains of the JTAG system. Refer to the section
scan” on page 264
POT
BOT
).
) and the Brown-out Detector is enabled.
for details.
”System and Reset Characteristics” on page 329
”Clock Sources” on page
”IEEE 1149.1 (JTAG) Boundary-
CC
is below the Brown-out Reset
Figure 9-1 on page 51
defines the electrical
30.
8011D–AVR–02/07