at90pwm81-16se ATMEL Corporation, at90pwm81-16se Datasheet - Page 49

• Manufacturer Part Number: at90pwm81-16se
• Description: 8-bit Avr Microcontroller With 8k Bytes In- System Programmable Flash
• Manufacturer: ATMEL Corporation

8.
8.1
8.1.1
8.1.2
7734M–AVR–03/10
System Control and Reset
System Control overview
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 dia-
gram in
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
The AT90PWM81 has four sources of reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold
• External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the
• Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is
• Brown-out Reset. The MCU is reset when the supply voltage V
(V
minimum pulse length. The external reset pin can be disabled in 2 ways:
enabled.
threshold (V
POT
– By the RSTDISBL fuse. In this case , the SPI programming is disabled
– By software using the RSTDIS bit in MCUCR register. In this case , the SPI programming is
Figure 8-1
still active at power up time.
).
BOT
shows the reset logic.
) and the Brown-out Detector is enabled.
“Clock Sources” on page
Table 8-1
28.
defines the electrical parameters of the reset circuitry.
CC
is below the Brown-out Reset
AT90PWM81
49