DS1831C Просмотр технического описания (PDF) - Dallas Semiconductor -> Maxim Integrated

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DS1831C

3.3V/2.5V Multi-Supply MicroMonitor

Dallas Semiconductor -> Maxim Integrated 

DS1831C/D/E

OPERATION - WATCHDOG TIMER

The watchdog timer function (DS1831D only) forces the WDS signal active (low) when the ST input

does not have a transition (high-to-low or low-to-high) within the predetermined time period. The

time-out period is determined by the condition of the TDWD pin. If TDWD is connected to ground the

minimum watchdog time-out would be 10 ms, TD floating would yield a minimum time-out of 100 ms,

and TDWD connected to VCC would provide a time-out of 1000 ms minimum. Time-out of the watchdog

starts when at least one of the RST outputs becomes inactive (high). If a transition 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, then the WDS output is pulsed active for a minimum of 100 µs.

The WDS output is an open-drain output and must be pulled up externally. In most applications this

output would be connected to one of the Pushbutton inputs and would not require an external pull-up

resistor. The value of the resistors is not critical in most cases but must be set low enough to pull the

output to a high state. A common value used is 10 kΩs. If a WDS output is connected to a pushbutton

input an additional pull-up resistor can be used (to improve speed of transitions) but is not required.

The ST input can be derived from many microprocessor outputs. The most typical signals used are the

microprocessor 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 transition must occur at or less than the minimum

times shown in Table 1. A typical circuit example is shown in Figure 10. The watchdog timing is shown

in Figure 11.

The DS1831A watchdog function cannot be disabled. The watchdog strobe input must be strobed to avoid

a watchdog time-out however the watchdog status output can be disconnected yielding the same result.

WATCHDOG CIRCUIT EXAMPLE Figure 10

VSENSE1

VCC

R1

PBRST2.5V

PBRST3.3V

IN1

NMI1

R2

µP

DS1831D

ST

WDS

GND

TDWD

10 kΩ

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