ATmega48 datasheet(44/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

8.0.4

External Reset

An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the

minimum pulse width (see Table 8-1 on page 43) 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 – V

RST – on its positive edge, the delay counter starts the

MCU after the Time-out period – t

TOUT – has expired. The External Reset can be disabled by the

RSTDISBL fuse, see Table 25-6 on page 281.

Figure 8-4.

External Reset During Operation

8.0.5

Brown-out Detection

ATmega48/88/168 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V

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 V

BOT+ =

BOT + VHYST/2 and VBOT- = VBOT - VHYST/2.

Notes:

1. V

BOT may be below nominal minimum operating voltage for some devices. For devices where

this is the case, the device is tested down to V

CC = VBOT during the production test. This guar-

antees that a Brown-Out Reset will occur before VCC drops to a voltage where correct

operation of the microcontroller is no longer guaranteed. The test is performed using

BODLEVEL = 110 and BODLEVEL = 101 for ATmega48V/88V/168V, and BODLEVEL = 101

and BODLEVEL = 101 for ATmega48/88/168.

2. Min/Max values applicable for ATmega48.

Table 8-2.

BODLEVEL Fuse Coding

(1)

BODLEVEL 2..0 Fuses

Min V

BOT

Typ V

BOT

Max V

BOT

Units

111

BOD Disabled

110

1.7

(2)

1.8

2.0

(2)

101

2.5

(2)

2.7

2.9

(2)

100

4.1

(2)

4.3

4.5

(2)

011

Reserved

010

001

000

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