The second function the DS1232LP/LPS performs is pushbutton reset control. The DS1232LP/LPS
debounces the pushbutton input and guarantees an active reset pulse width of 250 ms minimum. The third
function is a watchdog timer. The DS1232LP/LPS has an internal timer that forces the reset signals to the
active state if the strobe input is not driven low prior to timeout. The watchdog timer function can be set
to operate on timeout settings of approximately 150 ms, 600 ms, and 1.2 seconds.
OPERATION - POWER MONITOR
The DS1232LP/LPS detects out-of-tolerance power supply conditions and warns a processor-based
system of impending power failure. When VCC falls below a preset level as defined by TOL, the VCC
comparator outputs the signals RST and RST . When TOL is connected to ground, the RST and RST
signals become active as VCC falls below 4.75 volts. When TOL is connected to VCC, the RST and RST
signals become active as VCC falls below 4.5 volts. The RST and RST are excellent control signals for a
microprocessor, as processing is stopped at the last possible moments of valid VCC. On power-up, RST
and RST are kept active for a minimum of 250 ms to allow the power supply and processor to stabilize.
OPERATION - PUSHBUTTON RESET
The DS1232LP/LPS provides an input pin for direct connection to a pushbutton (Figure 1). The
pushbutton reset input requires an active low signal. Internally, this input is debounced and timed such
that RST and RST signals of at least 250 ms minimum are generated. The 250 ms delay starts as the
pushbutton reset input is released from low level.
OPERATION - WATCHDOG TIMER
The watchdog timer function forces RST and RST signals to the active state when the ST input is not
stimulated for a predetermined time period. The time period is set by the TD input to be typically 150 ms
with TD connected to ground, 600 ms with TD left unconnected, and 1.2 seconds with TD connected to
VCC. The watchdog timer starts timing out from the set time period as soon as RST and RST are inactive.
If a high-to-low transition occurs on the ST input pin prior to timeout, the watchdog timer is reset and
begins to timeout again. If the watchdog timer is allowed to timeout, then the RST and RST signals are
driven to the active state for 250 ms minimum. The ST input can be derived from microprocessor address
signals, data signals, and/or control signals. When the microprocessor is functioning normally, these
signals would, as a matter of routine, cause the watchdog to be reset prior to timeout. To guarantee that
the watchdog timer does not timeout, a high-to-low transition must occur at or less than the minimum
shown in Table 1. A typical circuit example is shown in Figure 2.
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