ATmega325P datasheet(47/364 Pages) ATMEL

47 / 364 page

8023F–AVR–07/09

ATmega325P/3250P

Figure 10-3. MCU Start-up, RESET Extended Externally

10.2.2

External Reset

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

minimum pulse width (see ”System and Reset Characterizations” on page 308) 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.

Figure 10-4. External Reset During Operation

10.2.3

Brown-out Detection

ATmega325P/3250P 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.When the BOD is enabled, and VCC decreases to a

value below the trigger level (V

BOT- in Figure 10-5), the Brown-out Reset is immediately acti-

vated. When V

CC increases above the trigger level (VBOT+ in Figure 10-5), the delay counter

starts the MCU after the Time-out period t

TOUT has expired.

The BOD circuit will only detect a drop in V

CC if the voltage stays below the trigger level for lon-

ger than t

BOD given in ”System and Reset Characterizations” on page 308.

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

VCC

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