DS1673 Datasheet, PDF (8/18 Pages) Dallas Semiconductor – Portable System Controller

DS1673
EXTERNAL SRAM INTERFACE (WORD-WIDE) TO THE DS1673 Figure 4
MICROPROCESSOR MONITOR
The DS1673 monitors three vital conditions for a microprocessor: power supply, software execution, and
external override.
First, a precision temperature-compensated reference and comparator circuit monitors the status of V_CC.
When an out-of-tolerance condition occurs, an internal power-fail signal is generated which forces the
RST pin to the active state, thus warning a processor-based system of impending power failure. When
V_CC returns to an in-tolerance condition upon power-up, the reset signal is kept in the active state for
250ms (typical) to allow the power supply and microprocessor to stabilize. Note, however, that if the
EOSC bit is set to a logic 1 (to disable the oscillator during battery-backup mode), the reset signal will be
kept in an active state for 250 ms plus the start-up time of the oscillator.
The second monitoring function is push-button reset control. The DS1673 provides for a pushbutton
switch to be connected to the RST output pin. When the DS1673 is not in a reset cycle, it continuously
monitors the RST signal for a low going edge. If an edge is detected, the DS1673 will debounce the
switch by pulling the RST line low. After the internal 250ms timer has expired, the DS1673 will continue
to monitor the RST line. If the line is still low, the DS1673 will continue to monitor the line looking for a
rising edge. Upon detecting release, the DS1673 will force the RST line low and hold it low for 250ms.
The third microprocessor monitoring function provided by the DS1673 is a watchdog timer. The
watchdog timer function forces RST to the active state when the ST input is not stimulated within the
predetermined time period. The time period is set by the Time Delay (TD) bits in the Watchdog Register.
The time delay can be set to 250ms, 500ms, or 1000ms (see Figure 5). If TD0 and TD1 are both set to
zero, the watchdog timer is disabled. When enabled, the watchdog timer starts timing out from the set
time period as soon as RST is inactive. The default setting is for the watchdog timer to be enabled with
1000ms time delay. If a high-to-low 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 RST
signal is driven to the active state for 250ms (typical). The ST input can be derived from microprocessor
address signals, data signals, and/or control signals. To guarantee that the watchdog timer does not time-
out, a high-to-low transition must occur at or less than the minimum period.
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