4

U5021M

4756C–AUTO–09/04

Figure 3. Period t versus R

Figure 4. Power-on Reset and Switch-over Mode

Supply Voltage

Monitoring, Pin 5

During ramp-up of the supply voltage and in the case of supply-voltage drops the inte-

grated power-on reset (POR) circuitry sets the internal logic to a defined basic status

and generates a reset pulse at the reset output, pin 5. A hysteresis in the POR threshold

prevents the circuit from oscillating. During ramp-up of the supply voltage, the reset out-

put stays active for a specified period of time (t

its defined reset status (see Figure 4). Pin 5 has an open-drain output.

Switch-over Mode Time,

Pin 3

The switch-over mode time enables the synchronous operation of microcontroller and

watchdog. When the power-on reset time has elapsed, the watchdog has to be switched

to monitoring mode by the microcontroller by a “low” signal transmitted to the mode pin

is generated.

1.00

10.00

100.00

1000.00

1

10

100

1000

t (µs)

C

4.5 V

5.5 V

5.0 V

R

t

0

Reset Out

Mode

VDD

Pin 6

Pin 5

Pin 3

t

1

t

6