9. System Control and Reset

9.0.1Resetting 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 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. The circuit diagram in Figure 9-1 on page 41 shows the reset logic. Table 22-3 on page 183 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 27.

9.0.2Reset Sources

The ATtiny24/44/84 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 when RESET function is enabled.

•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.

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Figure 9-1. Reset Logic

Power-on Reset

Circuit

BODLEVEL [1..0]

Brown-out

Reset Circuit

Pull-up Resistor

SPIKE

FILTER

Watchdog

Oscillator

DATA BUS

MCU Status

Register (MCUSR)

PORF

BORF

EXTRF

WDRF

Clock	CK	Delay Counters
Generator			TIMEOUT

CKSEL[1:0]

SUT[1:0]

9.0.3Power-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 Characterizations” on page 183. 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 9-2. MCU Start-up, RESET Tied to VCC

	VCC					VPOT
						VRST
	RESET
TIME-OUT							tTOUT
INTERNAL
RESET

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Figure 9-3. MCU Start-up, RESET Extended Externally

VPOT

VCC

VRST

RESET

tTOUT

TIME-OUT

INTERNAL

RESET

9.0.4External Reset

An External Reset is generated by a low level on the RESET pin if enabled. Reset pulses longer than the minimum pulse width (see ”System and Reset Characterizations” on page 183) 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.

Figure 9-4. External Reset During Operation

CC

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9.0.5Brown-out Detection

ATtiny24/44/84 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 (VBOT- in Figure 9-5 on page 43), the Brown-out Reset is immediately activated. When VCC increases above the

trigger level (VBOT+ in Figure 9-5 on page 43), the delay counter starts the MCU after the Timeout 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 Characterizations” on page 183.

Figure 9-5. Brown-out Reset During Operation

VCC	VBOT+
	VBOT-
RESET
TIME-OUT	tTOUT
INTERNAL
RESET

9.0.6Watchdog Reset

When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On

the falling edge of this pulse, the delay timer starts counting the Time-out period tTOUT. See ”Watchdog Timer” on page 44 for details on operation of the Watchdog Timer.

Figure 9-6. Watchdog Reset During Operation

CC

CK

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