DS1238
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debounce circuit connected to the RST pin. An internal watchdog timer can also force the reset outputs to
the active state if the strobe input is not driven low prior to watchdog timeout. Oscillator control pins
OSCSEL and OSCIN provide either external or internal clock timing for both the reset pulse width and
the watchdog timeout period. The Watchdog Status and Reset Voltage Threshold are provided via WDS
and RVT , respectively. A block diagram of the DS1238 is shown in Figure 1.
PIN DESCRIPTION
PIN NAME DESCRIPTION
V
BAT
+3V battery input provides nonvolatile operation of control functions.
V
CCO
V
CC
output for nonvolatile SRAM applications.
V
CC
+5V primary power input.
GND System ground.
PF
Power-fail indicator, active high, used for external power switching as shown in
Figure 9.
RVT
Reset Voltage Threshold. Indicates that V
CC
is below the reset voltage threshold.
OSCIN Oscillator input or timing capacitor. See Table 1.
OSCSEL Oscillator Select. Selects internal or external clock functions. See Table 1.
IN
Early warning power-fail input. This voltage sense point can be tied (via resistor
divider) to a user-selected voltage.
NMI
Non-maskable interrupt. Used in conjunction with the IN pin to indicate an impending
power failure.
ST
Strobe input. A high-to-low transition will reset the watchdog timer, indicating that
software is still in control.
CEO
Chip enable output. Write protected. Used with nonvolatile SRAM applications.
CEI
Chip enable input.
WDS
Watchdog Status. Indicates that a watchdog timeout has occurred.
RST
Active low reset output.
RST Active high reset output.
POWER MONITOR
The DS1238 employs a band gap voltage reference and a precision comparator to monitor the 5-volt
supply (V
CC
) in microprocessor-based systems. When an out-of-tolerance condition occurs, the RVT ,
RST, and RST outputs are driven to the active state. The V
CC
trip point (V
CCTP
) is set for 10% operation
so that the RVT , RST and RST outputs will become active as V
CC
falls below 4.5 volts (4.37 typical).
The V
CCTP
for the 5% operation option (DS1238-5) is set for 4.75 volts (4.62 typical). The RST and RST
signals are excellent for microprocessor reset control, as processing is stopped at the last possible moment
of in-tolerance V
CC
. On power up, RVT will become inactive as soon as V
CC
rises above V
CCTP
. However,
the RST and RST signals remain active for a minimum of 50 ms (100 ms typical) after V
CCTP
is reached
to allow the power supply and microprocessor to stabilize.
