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NE56605-42

System reset with built-in watchdog timer

Philips Electronics 

Philips Semiconductors

System reset with built-in watchdog timer

Product data

NE56605-42

TECHNICAL DESCRIPTION

General discussion

The NE56605-42 combines a watchdog timer and an undervoltage

reset function in a single SO8 surface mount package. This provides

a space-saving solution for maintaining proper operation of typical

5.0 volt microprocessor-based logic systems. Either function, or

both, can force the microprocessor into a reset.

While the watchdog monitors the microprocessor operation, the

undervoltage reset monitors the supply voltage to the

microprocessor. If the microprocessor clock signal ceases or

becomes erratic, the NE56605-42 outputs a reset signal to the

microprocessor. If the microprocessor supply voltage sags to

4.2 volts or less, the NE56605-42 outputs a reset signal for the

duration of the supply voltage deficiency. The undervoltage reset

signal allows the microprocessor to shut down in an orderly manner

to avoid system corruption. In addition to a single reset output, the

NE56605-42 has complementary RESET and RESET outputs for

system use. The undervoltage detection threshold incorporates

hysteresis to prevent generating erratic resets.

The watchdog timer requires a pulse input. Normally this signal

comes from the system microprocessor’s clock. For operation, an

external capacitor (CT) must be connected from Pin 1 to ground.

Normally a 0.1 µF capacitor is used for CT. The CT capacitor and a

fixed internal resistance establish the required minimum frequency

of watchdog input signal for the device to not output a reset signal.

In the absence of a watchdog input pulse, the CT capacitor charges

to the 0.2 volt threshold of the internal comparator, causing a reset

signal to be output. If microprocessor clock signals are received

within the required interval, no watchdog reset signal will be output.

Grounding the watchdog control pin (WDC, Pin 6) disables the

watchdog function. Removing the ground from Pin 6, allowing it to

float, enables the watchdog function. Enabling or disabling the

watchdog function has no effect on the undervoltage detection

function.

Although the temperature coefficient of detection threshold is

specified over a temperature of –20 °C to +70 °C, the device will

support operation in excess of this temperature range. See the

supporting curves for performance over the full temperature range of

–30 °C to +85 °C. Some degradation in performance will be

experienced at the temperature extremes and the system designer

should take this into account.

VCC

5

CP

RR

54 kΩ

7

R

C

26 kΩ

3

PULSE

GENERATOR

WDC

6

1.2 µA

1.2 µA

0.1 V

SQ

R

R

R

8.2 kΩ

0.2 V

SQ

R

SQ

R

R

47 kΩ

12 µA

R

R

1 CT

4 GND

Figure 17. Functional diagram.

2

RESET

8

RESET

SL01293

2001 Aug 22

9

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