7.
System Control and Reset
7.1
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 7-1 on page 38 shows the reset logic. Table 7-1 on page 39 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
Section 5.2 “Clock Sources” on page 26.
7.2
Reset Sources
The Atmel ATmega16/32/64/M1/C1 has four sources of reset:
●
●
Power-on reset. The MCU is reset when the supply voltage is below the power-on reset threshold (VPOT).
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 VCC is below the brown-out reset threshold (VBOT) and the
brown-out detector is enabled.
Figure 7-1. Reset Logic
DATA BUS
MCU Status
Register (MCUSR)
Power-on Reset
Circuit
VCC
Brown-out
BODLEVEL [2.. 0]
RESET
Reset Circuit
Pull-up Resistor
INTERNAL
RESET
Q
Spike
Filter
Reset Circuit
S
R
Watchdog
Timer
Watchdog
Oscillator
Delay Counters
CK
Clock
Generator
TIMEOUT
CKSEL[3:0]
SUT[1:0]
38
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
