Файл: ATmega8.pdf

Brown-out Detector

If the Brown-out Detector is not needed in the application, this module should be turned

off. If the Brown-out Detector is enabled by the BODEN Fuse, it will be enabled in all

sleep modes, and hence, always consume power. In the deeper sleep modes, this will

contribute significantly to the total current consumption. Refer to “Brown-out Detection”

on page 38 for details on how to configure the Brown-out Detector.

Internal Voltage Reference

The Internal Voltage Reference will be enabled when needed by the Brown-out Detec-

tor, the Analog Comparator or the ADC. If these modules are disabled as described in

the sections above, the internal voltage reference will be disabled and it will not be con-

suming power. When turned on again, the user must allow the reference to start up

before the output is used. If the reference is kept on in sleep mode, the output can be

used immediately. Refer to “Internal Voltage Reference” on page 40 for details on the

start-up time.

Watchdog Timer

If the Watchdog Timer is not needed in the application, this module should be turned off.

If the Watchdog Timer is enabled, it will be enabled in all sleep modes, and hence,

always consume power. In the deeper sleep modes, this will contribute significantly to

the total current consumption. Refer to “Watchdog Timer” on page 41 for details on how

to configure the Watchdog Timer.

Port Pins

When entering a sleep mode, all port pins should be configured to use minimum power.

The most important thing is then to ensure that no pins drive resistive loads. In sleep

modes where the both the I/O clock (clkI/O) and the ADC clock (clkADC) are stopped, the

input buffers of the device will be disabled. This ensures that no power is consumed by

the input logic when not needed. In some cases, the input logic is needed for detecting

wake-up conditions, and it will then be enabled. Refer to the section “Digital Input

Enable and Sleep Modes” on page 53 for details on which pins are enabled. If the input

buffer is enabled and the input signal is left floating or have an analog signal level close

to VCC/2, the input buffer will use excessive power.

ATmega8(L)

System Control and

Reset

Resetting the AVR

During Reset, all I/O Registers are set to their initial values, and the program starts exe-

cution from the Reset Vector. If the program never enables an interrupt source, the

Interrupt Vectors are not used, and regular program code can be placed at these loca-

tions. 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 14

shows the Reset Logic. Table 15 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

CKSEL Fuses. The different selections for the delay period are presented in “Clock

Sources” on page 24.

Reset Sources

The ATmega8 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.

ATmega8(L)

Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detec-

tion level is defined in Table 15. 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. Reach-

ing 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 15. MCU Start-up,

Tied to VCC

RESET

VCC

VPOT

VRST

RESET

TIME-OUT

tTOUT