ATmega128 datasheet(52/386 Pages) ATMEL

52 / 386 page

2467S–AVR–07/09

ATmega128

2. VBOT 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 V

CC drops to a voltage where correct

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

BODLEVEL=1for ATmega128L and BODLEVEL=0 for ATmega128. BODLEVEL=1 is not

applicable forATmega128

Power-on Reset

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

is defined in Table 19. The POR is activated whenever V

CC 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. Reaching the

Power-on Reset threshold voltage invokes the delay counter, which determines how long the

device is kept in RESET after V

CC rise. The RESET signal is activated again, without any delay,

when V

CC decreases below the detection level.

Figure 23. MCU Start-up, RESET Tied to V

CC.

Figure 24. MCU Start-up, RESET Extended Externally

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 19) 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.

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

VCC

ATmega128 Datasheet(PDF) 52 Page - ATMEL Corporation

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