10. System Control and Reset

10.1Resetting the AVR

During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset Vector. For the ATmega168, the instruction placed at the Reset Vector must be a JMP – Absolute Jump – instruction to the reset handling routine. For the ATmega48 and ATmega88, the instruction placed at the Reset Vector must be an RJMP – Relative 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 (ATmega88/168 only). The circuit diagram in Figure 10-1 shows the reset logic. Table 28-3 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 “Clock Sources” on page 29.

10.2Reset Sources

The ATmega48/88/168 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 System Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog System Reset mode 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.

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ATmega48/88/168

10.3Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in “System and Reset Characteristics” on page 308. The POR is activated whenever VCC is below the detection level. The POR circuit can be used to trigger the start-up Reset, as well as to detect a failure in supply voltage.

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset threshold voltage invokes the delay counter, which determines how long the device is kept in RESET after VCC rise. The RESET signal is activated again, without any delay, when VCC decreases below the detection level.

Figure 10-2. MCU Start-up, RESET Tied to VCC

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Figure 10-3. MCU Start-up, RESET Extended Externally

VPOT

VCC

VRST

RESET

tTOUT

TIME-OUT

INTERNAL

RESET

10.4External Reset

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum pulse width (see “System and Reset Characteristics” on page 308) will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches the Reset Threshold Voltage – VRST – on its positive edge, the

delay counter starts the MCU after the Time-out period – tTOUT – has expired. The External Reset can be disabled by the RSTDISBL fuse, see Table 27-6 on page 288.

Figure 10-4. External Reset During Operation

CC

10.5Brown-out Detection

ATmega48/88/168 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detection level should be interpreted as VBOT+ =

VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.When the BOD is enabled, and VCC decreases to a value below the trigger level (V in Figure 10-5), the Brown-out Reset is immediately acti-

vated. When VCC increases above the trigger level (VBOT+ in Figure 10-5), the delay counter starts the MCU after the Time-out period tTOUT has expired.

The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than tBOD given in “System and Reset Characteristics” on page 308.

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