External Reset

Figure 26. MCU Start-up, RESET Tied to VCC

VPOT

VCC

VRST

RESET

INTERNAL

RESET

Figure 27. MCU Start-up, RESET Controlled Externally

VPOT

VCC

VRST

RESET

tTOUT

TIME-OUT

INTERNAL

RESET

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than 500 ns 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 timer starts the MCU after the Time-out period tTOUT has expired.

Figure 28. External Reset During Operation

VCC

RESET

VRST

t TOUT

TIME-OUT

Brown-out Detection

Watchdog Reset

ATtiny26(L) has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level during the operation. The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled (BODEN programmed), and VCC decreases below the trigger level, the Brown-out Reset is immediately activated. When VCC increases above the trigger level, the Brown-out Reset is deactivated after a delay. The delay is defined by the user in the same way as the delay of POR signal, in Table 29. The trigger level for the BOD can be selected by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL programmed). The trigger level has a hysteresis of 50 mV to ensure spike free Brown-out Detection.

The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than tBOD given in Table 16.

Figure 29. Brown-out Reset During Operation

RESET

INTERNAL

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. Refer to page 80 for details on operation of the Watchdog.

Figure 30. Watchdog Time-out

1 CK Cycle

MCU Status Register –

MCUSR

1477J–AVR–06/07

ATtiny26(L)

• Bit 7..4 – Res: Reserved Bits

These bits are reserved bits in the ATtiny26(L) and always read as zero.

• Bit 3 – WDRF: Watchdog Reset Flag

This bit is set (one) if a Watchdog Reset occurs. The bit is reset (zero) by a Power-on Reset, or by writing a logic zero to the flag.

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set (one) if a Brown-out Reset occurs. The bit is reset (zero) by a Power-on Reset, or by writing a logic zero to the flag.

• Bit 1 – EXTRF: External Reset Flag

This bit is set (one) if an External Reset occurs. The bit is reset (zero) by a Power-on Reset, or by writing a logic zero to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set (one) if a Power-on Reset occurs. The bit is reset (zero) by writing a logic zero to the flag.

To make use of the reset flags to identify a reset condition, the user should read and then reset (zero) the MCUSR as early as possible in the program. If the register is cleared before another reset occurs, the source of the reset can be found by examining the reset flags.

37

Power Management

and Sleep Modes

MCU Control Register –

MCUCR

Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power. The AVR provides various sleep modes allowing the user to tailor the power consumption to the application’s requirements.

To enter any of the four sleep modes, the SE bit in MCUCR must be written to logic one and a SLEEP instruction must be executed. The SM1, and SM0 bits in the MCUCR Register select which sleep mode (Idle, ADC Noise Reduction, Power Down, or Standby) will be activated by the SLEEP instruction. See Table 17 for a summary. If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for four cycles in addition to the start-up time, it executes the interrupt routine, and resumes execution from the instruction following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes up from sleep. If a Reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.

Table 19 on page 40 presents the different clock systems in the ATtiny26, and their distribution. The figure is helpful in selecting an appropriate sleep mode.

The MCU Control Register contains control bits for general MCU functions.

• Bits 7 – Res: Reserved Bit

This bit is a reserved bit in the ATtiny26(L) and always reads as zero.

• Bit 6 – PUD: Pull-up Disable

When this bit is set (one), the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Configuring the Pin” on page 44 for more details about this feature.

• Bit 5 – SE: Sleep Enable

The SE bit must be set (one) to make the MCU enter the Sleep mode when the SLEEP instruction is executed. To avoid the MCU entering the Sleep mode unless it is the programmers purpose, it is recommended to set the Sleep Enable SE bit just before the execution of the SLEEP instruction.

• Bits 4,3 – SM1/SM0: Sleep Mode Select Bits 1 and 0

These bits select between the four available Sleep modes, as shown in the following table.

For details, refer to the paragraph “Sleep Modes” below.

• Bit 2 – Res: Reserved Bit

This bit is a reserved bit in the ATtiny26(L) and always reads as zero.