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DS1238A
MicroManager
www.dalsemi.com
FEATURES
Holds microprocessor in check during power
transients
Halts and restarts an out-of-control
microprocessor
Warns microprocessor of an impending power
failure
Converts CMOS SRAM into nonvolatile
memory
Unconditionally write protects memory when
power supply is out of tolerance
Delays write protection until completion of
the current memory cycle
Consumes less than 200 nA of battery current
Controls external power switch for high
current applications
Debounces pushbutton reset
Accurate 10% power supply monitoring
Optional 5% power supply monitoring
designated DS1238A-5
Provides orderly shutdown in microprocessor
applications
Pin-for-pin compatible with MAX691
Standard 16-pin DIP or space-saving 16-pin
SOIC
Optional industrial temperature range -40°C
to +85°C
PIN ASSIGNMENT
VBAT
VCCO
VCC
GND
PF
RVT
OSCIN
OSCSEL
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
RST
RST
WDS
CEI
CEO
ST
NMI
IN
VBAT
VCCO
VCC
GND
PF
RVT
OSCIN
OSCSEL
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
RST
RST
WDS
CEI
CEO
ST
NMI
IN
16-Pin DIP (300-mil)
See Mech. Drawings Section
16-Pin SOIC (300-mil)
See Mech. Drawings Section
PIN DESCRIPTION
V
BAT
V
CCO
V
CC
GND
PF
RVT
OSCIN
OSCSEL
IN
NMI
ST
CEO
CEI
WDS
RST
RST
- +3-Volt Battery Input
- Switched SRAM Supply Output
- +5-Volt Power Supply Input
- Ground
- Power-Fail
- Reset Voltage Threshold
- Oscillator In
- Oscillator Select
- Early Warning Input
- Non-Maskable Interrupt
- Strobe Input
- Chip Enable Output
- Chip Enable Input
- Watchdog Status
- Reset Output (active low)
- Reset Output (active high)
DESCRIPTION
The DS1238A MicroManager provides all the necessary functions for power supply monitoring, reset
control, and memory backup in microprocessor-based systems. A precise internal voltage reference and
comparator circuit monitor power supply status. When an out-of-tolerance condition occurs, the
microprocessor reset and power-fail outputs are forced active, and static RAM control unconditionally
write protects external memory. The DS1238A also provides early warning detection of a user-defined
threshold by driving a non-maskable interrupt. External reset control is provided by a pushbutton reset
1 of 13
111899
DS1238A
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 DS1238A is shown in NO TAG.
PIN DESCRIPTION
PIN NAME
V
BAT
V
CCO
V
CC
PF
RVT
DESCRIPTION
+3V battery input provides nonvolatile operation of control functions.
V
CC
output for nonvolatile SRAM applications.
+5V primary power input.
Power-fail indicator, active high, used for external power switching as shown in NO
TAG.
Reset Voltage Threshold. Indicates that V
CC
is below the reset voltage threshold.
Oscillator input or timing capacitor. See NO TAG.
Oscillator Select. Selects internal or external clock functions. See NO TAG.
Early warning power-fail input. This voltage sense point can be tied (via resistor
divider) to a user-selected voltage.
Non-maskable interrupt. Used in conjunction with the IN pin to indicate an impending
power failure.
Strobe input. A high-to-low transition will reset the watchdog timer, indicating that
software is still in control.
Chip enable output. Used with nonvolatile SRAM applications.
Chip enable input.
Watchdog Status. Indicates that a watchdog timeout has occurred.
Active low reset output.
Active high reset output.
OSCIN
OSCSEL
IN
NMI
ST
CEO
CEI
WDS
RST
RST
POWER MONITOR
The DS1238A employs a bandgap 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 (DS1238A-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.
2 of 13
DS1238A
DS1238A FUNCTIONAL BLOCK DIAGRAM
Figure 1
WATCHDOG TIMER
The DS1238A provides a watchdog timer function which forces the
WDS
, RST, and
RST
signals to the
active state when the strobe input (
ST
) is not stimulated for a predetermined time period. This time period
is described below in NO TAG. The watchdog timeout period begins as soon as RST and
RST
are
inactive. If a high-to-low transition occurs at the
ST
input prior to timeout, the watchdog timer is reset
and begins to timeout again. The
ST
input timing is shown in NO TAG. In order to guarantee that the
watchdog timer does not timeout, a high-to-low transition on
ST
must occur at or less than the minimum
timeout of the watchdog as described in the AC Electrical Characteristics. If the watchdog timer is
allowed to time out, the
WDS
, RST, and
RST
outputs are driven to the active state.
WDS
is a latched
signal which indicates the watchdog status, and is activated as soon as the watchdog timer completes a
3 of 13
DS1238A
full period as outlined in NO TAG. The
WDS
pin will remain low until one of three operations occurs.
The first is to strobe the
ST
pin with a falling edge, which will both set the
WDS
as well as the watchdog
timer count. The second is to leave the
ST
pin open, which disables the watchdog. Lastly, the
WDS
pin is
active low whenever V
CC
falls below V
CCTP
and activates the
RVT
signal. The
ST
input can be derived
from microprocessor address, data, or control signals, as well as microcontroller port pins. Under normal
operating conditions, these signals would routinely reset the watchdog timer prior to timeout. The
watchdog is disabled by leaving the
ST
input open, or as soon as V
CC
falls to V
CCTP
.
NON-MASKABLE INTERRUPT
The DS1238A generates a non-maskable interrupt (
NMI
) for early warning of a power failure to the
microprocessor. A precision comparator monitors the voltage level at the IN pin relative to an on-chip
reference generated by an internal band gap. The IN pin is a high-impedance input allowing for a user-
defined sense point. An external resistor voltage divider network (NO TAG) is used to interface with high
voltage signals. This sense point may be derived from the regulated 5-volt supply, or from a higher DC
voltage level closer to the main system power input. Since the IN trip point V
TP
is 1.27 volts, the proper
values for R1 and R2 can be determined by the equation as shown in NO TAG. Proper operation of the
DS1238A requires that the voltage at the IN pin be limited to V
IH
. Therefore, the maximum allowable
voltage at the supply being monitored (V
MAX
) can also be derived as shown in NO TAG. A simple
approach to solving this equation is to select a value for R2 of high enough value to keep power
consumption low, and solve for R1. The flexibility of the IN input pin allows for detection of power loss
at the earliest point in a power supply system, maximizing the amount of time for microprocessor
shutdown between
NMI
and RST or
RST
.
When the supply being monitored decays to the voltage sense point, the DS1238A will force the
NMI
output to an active state. Noise is removed from the
NMI
power-fail detection circuitry using built-in
time domain hysteresis. That is, the monitored supply is sampled periodically at a rate determined by an
internal ring oscillator running at approximately 30 kHz (33
µs/cycle).
Three consecutive samplings of
out-of-tolerance supply (below V
SENSE
) must occur at the IN pin to active
NMI
. Therefore, the supply
must be below the voltage sense point for approximately 100
µs
or the comparator will reset. In this way,
power supply noise is removed from the monitoring function preventing false trips. During a power-up,
any IN pin levels below V
TP
detected by the comparator are disabled from reaching the
NMI
pin until
V
CC
rises to V
CCTP
. As a result, any potential active
NMI
will not be initiated until V
CC
reaches V
CCTP
.
Removal of an active low level on the
NMI
pin is controlled by the subsequent rise of the IN pin above
V
TP
. The initiation and removal of the
NMI
signal during power up depends on the relative voltage
relationship between V
CC
and the IN pin voltage. Note that a fast-slewing power supply may cause the
NMI
to be virtually nonexistent on power-up. This is of no consequence, however, since an RST will be
active. The
NMI
voltage will follow V
CC
down until V
CC
decays to V
BAT
. Once V
CC
decays to V
BAT
, the
NMI
pin will enter a tri-state mode.
ST INPUT TIMING
Figure 2
4 of 13
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