13 system control and reset, 1 resetting the avr, 2 reset sources – Rainbow Electronics ATmega128RFA1 User Manual
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8266A-MCU Wireless-12/09
ATmega128RFA1
13 System Control and Reset
13.1 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
shows the reset logic.
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
.
13.2 Reset Sources
The ATmega128RFA1 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 RSTN 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 EVDD 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