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NE56625-20

System reset with Watchdog timer

Philips Electronics 

Philips Semiconductors

System reset with Watchdog timer

Product data

NE56625-20

TECHNICAL DESCRIPTION

The NE56625-20 combines a Watchdog timer, a Battery Check and

an Undervoltage Reset functions in a single SO8 surface mount

package. This provides a space-saving solution for maintaining

proper operation of typical 2.0 volt cordless telephone and other low

voltage portable, handheld communication and industrial equipment.

While the Watchdog monitors the microprocessor operation, the

Battery Check and Undervoltage Reset monitor the supply voltage

to the microprocessor. If the microprocessor clock signal ceases or

becomes erratic, the NE56625-20 outputs a reset signal to the

microprocessor. If the microprocessor supply voltage sags to

2.0 volts or less, the NE56625-20 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 RESET output, the

NE56625-20 has a Battery Check output for system use. If the

supply voltage sags below 2.2 volts or less, the Battery Check

output goes LOW and remains LOW until the supply voltage

recovers. Both the undervoltage detection threshold and battery

check detection threshold incorporate 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, pin 6

is not connected (open) or an external resistor (RCT) of 1 MΩ or

greater is connected from Pin 6 to VCC and an external capacitor

(CT) is placed from Pin 1 to ground. The recommended range for CT

capacitor is 2.2 nF to 2.2 µF. The external RCT resistor and CT

capacitor establish the required minimum frequency of Watchdog

input signal for the device to not output a reset signal. The RCT

resistor establishes, in part, the rate of charge of the CT capacitor. 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.

The Watchdog function can be disabled by grounding Pin 6 without

affecting 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

–25 °C to +100 °C. Some degradation in performance will be

experienced at the temperature extremes and the system designer

should take this into account.

VCC

C

5 VCC

RCT

2 BC 6 RCT

230 kΩ

VS 7

C

250 kΩ

1.25 V (typ.)

CLK 3

PULSE

GENERATOR

SQ

R

0.1 V

0.2 V

SQ

R

SQ

R

1 CT

4 GND

Figure 16. Functional diagram.

1 µA

(typ.)

8

RESET

SL01594

2003 Oct 15

9

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