ATmega16 datasheet(42/357 Pages) ATMEL

42 / 357 page

2466S–AVR–05/09

ATmega16(L)

Internal Voltage

Reference

ATmega16 features an internal bandgap reference. This reference is used for Brown-out Detec-

tion, and it can be used as an input to the Analog Comparator or the ADC. The 2.56V reference

to the ADC is generated from the internal bandgap reference.

Voltage Reference

Enable Signals and

Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. The

start-up time is given in Table 16. To save power, the reference is not always turned on. The ref-

erence is on during the following situations:

When the BOD is enabled (by programming the BODEN Fuse).

When the bandgap reference is connected to the Analog Comparator (by setting the

ACBG bit in ACSR).

When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user

must always allow the reference to start up before the output from the Analog Comparator or

ADC is used. To reduce power consumption in Power-down mode, the user can avoid the three

conditions above to ensure that the reference is turned off before entering Power-down mode.

Watchdog Timer

The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz. This is

the typical value at V

CC = 5V. See characterization data for typical values at other VCC levels. By

controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as

shown in Table 17 on page 43. The WDR – Watchdog Reset – instruction resets the Watchdog

Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.

Eight different clock cycle periods can be selected to determine the reset period. If the reset

period expires without another Watchdog Reset, the ATmega16 resets and executes from the

Reset Vector. For timing details on the Watchdog Reset, refer to page 41.

To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be fol-

lowed when the Watchdog is disabled. Refer to the description of the Watchdog Timer Control

Figure 21. Watchdog Timer

Table 16. Internal Voltage Reference Characteristics

Symbol

Parameter

Min

Typ

Max

Units

VBG

Bandgap reference voltage

1.15

1.23

1.4

tBG

Bandgap reference start-up time

µs

IBG

Bandgap reference current consumption

µA

WATCHDOG

OSCILLATOR

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