ATMEGA8535 datasheet(41/321 Pages) ATMEL

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ATmega8535(L)

2502K–AVR–10/06

Internal Voltage

Reference

ATmega8535 features an internal bandgap reference. This reference is used for Brown-

out Detection, 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 reference 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 Com-

parator 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.

Note:

1. Values are guidelines only.

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 V

CC levels. By controlling the Watchdog Timer prescaler, the Watchdog Reset

interval can be adjusted as shown in Table 18 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 ATmega8535 resets and executes from the Reset Vector. For tim-

ing details on the Watchdog Reset, refer to page 40.

To prevent unintentional disabling of the Watchdog or unintentional change of Time-out

period, three different safety levels are selected by the Fuses S8535C and WDTON as

shown in Table 17. Safety level 0 corresponds to the setting in AT90S8535. There is no

restriction on enabling the WDT in any of the safety levels.

Table 16. Internal Voltage Reference Characteristics

(1)

Symbol

Parameter

Min

Typ

Max

Units

VBG

Bandgap reference voltage

1.15

1.23

1.35

tBG

Bandgap reference start-up time

µs

IBG

Bandgap reference current consumption

µA

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