DS1705/DS1706

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The second function the DS1705/DS1706 performs is

pushbutton reset control.

The DS1705/DS1706

debounces the pushbutton input and guarantees an

active reset pulse width of 130 ms minimum.

The third function is a watchdog timer.

The

DS1705/DS1706 has an internal timer that forces the

WDO signal to the active state if the strobe input is not

driven low prior to time–out.

OPERATION

Power Monitor

The DS1705/DS1706 detects out–of–tolerance power

supply conditions and warns a processor–based sys-

RST (or RST) signal. RST (or RST) is an excellent con-

trol signal for a microprocessor, as processing is

power–up, RST (or RST) are kept active for a minimum

of 130 ms to allow the power supply and processor to

stabilize.

Pushbutton Reset

The DS1705/DS1706 provides an input pin for direct

connection to a push–button reset (see Figure 2). The

pushbutton reset input requires an active low signal.

Internally, this input is debounced and timed such that a

RST (or RST) signal of at least 130 ms minimum will be

generated. The 130 ms delay commences as the push-

button reset input is released from the low level. The

push–button can be initiated by connecting the WDS or

NMI outputs to the PBRST input as shown in Figure 3.

Non–Maskable Interrupt

The DS1705/DS1706 generates a non–maskable inter-

rupt (NMI) for early warning of a power failure. A preci-

sion comparator monitors the voltage level at the IN pin

relative to an on–chip reference generated by an inter-

nal band gap. The IN pin is a high impedance input

allowing for a user–defined sense point. An external

resistor voltage divider network (Figure 5) is used to

interface with high voltage signals. This sense point

may be derived from a regulated supply or from a higher

DC voltage level closer to the main system power input.

ues for R1 and R2 can be determined by the equation as

shown in Figure 5.

Proper operation of the

DS1705/DS1706 requires that the voltage at the IN pin

be derived as shown in Figure 5. A simple approach to

solving the equation is to select a value for R2 high

enough to keep power consumption low, and solve for

R1. The flexibility of the IN input pin allows for detection

of power loss at the earliest point in a power supply sys-

tem, maximizing the amount of time for system shut–

down between NMI and RST (or RST).

When the supply being monitored decays to the voltage

sense point, the DS1705/DS1706 pulses the NMI out-

put to the active state for a minimum 200

µs. The NMI

power fail detection circuitry also has built–in hysteresis

of 100

µV. The supply must be below the voltage sense

point for approximately 5

µs before a low NMI will be

generated. In this way, power supply noise is removed

from the monitoring function, preventing false inter-

rupts. During a power–up, any detected IN pin levels

Connecting NMI to PBRST would allow non–maskable

interrupt to generate an automatic reset when an out–

of–tolerance condition occurred in a monitored supply.

An example is shown in Figure 3.

Watchdog Timer

The watchdog timer function forces WDS signals active

when the ST input is not clocked within the 1 second

time out period. Timeout of the watchdog starts when

RST (or RST) becomes inactive. If a high–to–low transi-

tion occurs on the ST input pin prior to time–out, the

watchdog timer is reset and begins to time–out again. If

the watchdog timer is allowed to time out, the WDS sig-

nal is driven active (low) for a minimum of 130 ms. The

ST input can be derived from many microprocessor out-

puts. The typical signals used are the microprocessors

address signals, data signals, or control signals. When

the microprocessor functions normally, these signals

would, as a matter of routine, cause the watchdog to be

reset prior to time–out. To guarantee that the watchdog

timer does not time–out, a high–to–low transition must

occur at or less than the minimum watchdog time–out of

1 second. A typical circuit example is shown in Figure 6.