ADM690–ADM695

REV. A

–6–

POWER FAIL RESET OUTPUT

RESET

is an active low output which provides a RESET signal

low. The nominal reset voltage threshold is 4.65 V (ADM690/

ADM691/ADM694/ADM695) or 4.4 V (ADM692/ADM693).

t

1

t

1

= RESET TIME.

V1 = RESET VOLTAGE THRESHOLD LOW

V2 = RESET VOLTAGE THRESHOLD HIGH

HYSTERESIS = V2–V1

V2

V2

V1

V1

V

CC

LOW LINE

RESET

Figure 2. Power Fail Reset Timing

On power-up RESET will remain low for 50 ms (200 ms for

reset threshold. This allows time for the power supply and mi-

croprocessor to stabilize. On power-down, the RESET output

microprocessor is held in a stable shutdown condition.

This RESET active time is adjustable on the ADM691/ADM693/

ADM695 by using an external oscillator or by connecting an

external capacitor to the OSC IN pin. Refer to Table I and

Figure 4.

The guaranteed minimum and maximum thresholds of the

ADM690/ADM691/ADM694/ADM695 are 4.5 V and 4.73 V,

while the guaranteed thresholds of the ADM692/ADM693 are

4.25 V and 4.48 V. The ADM690/ADM691/ADM694/ADM695

is, therefore, compatible with 5 V supplies with a +10%, –5%

tolerance while the ADM692/ADM693 is compatible with 5 V

± 10% supplies. The reset threshold comparator has approxi-

mately 50 mV of hysteresis. The response time of the reset volt-

age comparator is less than 1

µs. If glitches are present on the

should be decoupled close to the device.

In addition to RESET the ADM691/ADM693/ADM695 con-

tain an active high RESET output. This is the complement of

RESET

and is intended for processors requiring an active high

RESET signal.

Watchdog Timer RESET

The watchdog timer circuit monitors the activity of the micro-

processor in order to check that it is not stalled in an indefinite

loop. An output line on the processor is used to toggle the

Watchdog Input (WDI) line. If this line is not toggled within the

selected timeout period, a RESET pulse is generated. The

nominal watchdog timeout period is preset at 1.6 seconds on the

ADM690/ADM692/ADM694. The ADM691/ADM693/ADM695

may be configured for either a fixed “short” 100 ms or a “long”

1.6 second timeout period or for an adjustable timeout period.

If the “short” period is selected, some systems may be unable to

service the watchdog timer immediately after a reset, so the

ADM691/ADM693/ADM695 automatically selects the “long”

timeout period directly after a reset is issued. The watchdog

timer is restarted at the end of reset, whether the reset was

reset threshold.

The normal (short) timeout period becomes effective following

the first transition of WDI after RESET has gone inactive. The

watchdog timeout period restarts with each transition on the

WDI pin. To ensure that the watchdog timer does not time out,

either a high-to-low or low-to-high transition on the WDI pin

must occur at or less than the minimum timeout period. If WDI

remains permanently either high or low, reset pulses will be

issued after each “long” timeout period (1.6 s). The watchdog

monitor can be deactivated by floating the Watchdog Input

(WDI) or by connecting it to midsupply.

t

2

RESET

WDO

WDI

t

1

= RESET TIME.

t

2

= NORMAL (SHORT) WATCHDOG TIMEOUT PERIOD.

t

3

= WATCHDOG TIMEOUT PERIOD IMMEDIATELY FOLLOWING A RESET.

t

1

t

1

t

1

t

3

Figure 3. Watchdog Timeout Period and Reset Active

Time