ATMEGA640 datasheet(59/436 Pages) MICROCHIP

59 / 436 page

ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Figure 12-3. MCU Start-up, RESET Extended Externally

12.2.2

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 Characteristics” on page 360) 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 Volt-

age – VRST – on its positive edge, the delay counter starts the MCU after the Time-out period – tTOUT – has expired.

Figure 12-4. External Reset During Operation

12.2.3

Brown-out Detection

ATmega640/1280/1281/2560/2561 has an On-chip Brown-out Detection (BOD) circuit for monitoring the AVCC

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

When the BOD is enabled, and AVCC decreases to a value below the trigger level (VBOT- in Figure 12-5 on page

60), the Brown-out Reset is immediately activated. When AVCC increases above the trigger level (VBOT+ in Figure

12-5 on page 60), the delay counter starts the MCU after the Time-out period tTOUT has expired.

The BOD circuit will only detect a drop in AVCC if the voltage stays below the trigger level for longer than tBOD given

in “System and Reset Characteristics” on page 360.

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

VCC

ATMEGA640 датащи(PDF) 59 Page - Microchip Technology