Dallas Semiconductor DS1236AS-5N, DS1236AS-5, DS1236AS-10N, DS1236AS-10, DS1236A-5N Datasheet

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FEATURES

 Holds microprocessor in check during power

transients

 Halts and restarts an out-of-control

microprocessor

 Monitors pushbutton for external override
 Warns microprocessor of an impending power

failure

 Converts CMOS SRAM into nonvolatile

memory

 Unconditionally write protects memory when

power supply is out of tolerance

 Consumes less than 100 nA of battery current

at 25°C

 Controls external power switch for high

current applications

 Accurate 10% power supply monitoring
 Optional 5% power supply monitoring

designated DS1236A-5

 Provides orderly shutdown in nonvolatile

microprocessor applications

 Supplies necessary control for low-power

“stop mode” in battery operated hand-held
applications

 Standard 16-pin DIP or space-saving 16-pin

SOIC

 Optional industrial temperature range -40°C

to +85°C

PIN ASSIGNMENT

PIN DESCRIPTION

V

BAT

- +3-Volt Battery Input

V

CCO

- Switched SRAM Supply Output
VCC - +5-Volt Power Supply Input
GND - Ground
PF - Power-Fail (Active High)

PF - Power-Fail (Active Low)

WC/

SC - Wake-Up Control (Sleep)

RC - Reset Control
IN - Early Warning Input

NMI - Non-Maskable Interrupt
ST - Strobe Input

CEO - Chip Enable Output
CEI - Chip Enable Input
PBRST - Pushbutton Reset Input
RST - Reset Output (Active Low)

RST - Reset Output (Active High)

DESCRIPTION

The DS1236A MicroManager Chip provides all the necessary functions for power supply monitoring,
reset control, and memory backup in microprocessor-based s ystems. 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 DS1236A 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

DS1236

A

MicroManager Chip

www.dalsemi.com

16-Pin SOIC (300-mil)

See Mech. Drawings Section

VBAT

VCCO

VCC

RST
RST
PBRST

1
2
3

16
15
14

GND CEI413

PF
PF

WC/SC

CEO
ST
NMI

5
6
7

12
11
10

RC IN

89

16-Pin DIP (300-mil)

See Mech. Drawings Section

VBAT

VCCO

VCC

RST
RST
PBRST

1
2
3

16
15
14

GND CEI413

PF
PF

WC/SC

CEO
ST
NMI

5
6
7

12
11
10

RC IN89

DS1236A

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input which is debounced and activates reset outputs. 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. Reset control
and wake-up/sleep control inputs also provide the necessary signals for orderly shutdown and start-up in
battery backup and battery operated applications. A block diagram of the DS1236A is shown in NO TAG.

PIN DESCRIPTION

PIN NAME DESCRIPTION

V

BAT

+3V battery input provides nonvolatile operation of control functions.

V

CCO

VCC output for nonvolatile SRAM applications.

V

CC

+5V primary power input.

PF Power-fail indicator, active high, used for external power switching as shown in NO

TAG.

PF

Power-fail indicator, active low.

WC/SC

Wake-up and Sleep control. Invokes low-power mode.

RC

Reset control input. Determines reset output . Normally low for NMOS processors and
high for battery backed CMOS processors.

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. Used with nonvolatile SRAM applications.

CEI

Chip enable input.

PBRST

Pushbutton reset input.

RST

Active low reset output.

RST Active high reset output.

PROCESSOR MODE

A distinction is often made between CMOS and NMOS processor systems. In a CMOS system, power
consumption may be a concern, and nonvolatile operation is possible by battery backing both the SRAM
and the CMOS processor. All resources would be maintained in the absence of VCC. A power-down reset
is not issued since the low-power mode of most CMOS processors (Stop) is terminated with a Reset. A

pulsed interrupt (

NMI ) is issued to allow the CMOS processor to invoke a sleep mode to save power. For

this case, a power-on reset is desirable to wake up and initialize the processor. The CMOS mode is
invoked by connecting RC to V

CCO

.

An NMOS processor consumes more power, and consequently ma y not be battery backed. In this case, it
is desirable to notify the processor of a power-fail, then keep it in reset during the loss of VCC. This avoids
intermittent or aberrant operation. On power-up, the processor will continue to be reset until V

CC

reaches

an operational level to provide an orderly start. The NMOS mode is invoked by connecting RC to ground.

DS1236A

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POWER MONITOR

The DS1236A employs a band gap voltage reference and a precision comparator to monitor the 5-volt
supply (VCC) in microprocessor-based systems. When an out-of-tolerance condition occurs, t he 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

RST and RST outputs will become active as VCC falls below 4.5 volts (4.37 typical). The V

CCTP

for the

5% operation option (DS1236A-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, the RST and RST signals are held active for a minimum of 25 ms (100 ms

typical) after V

CCTP

is reached to allow the power supply and microprocessor to stabilize. Note: The
operation described above is obtained with the reset control pin (RC) connected to GND (NMOS mode).
Please review the reset control section for more information.

WATCHDOG TIMER

The DS1236A provides a watchdog timer function which forces the 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 400

ms typically with a maximum timeout of 600 ms. 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 time out again. The ST input timing is shown in NO TAG. To guarantee the
watchdog timer does not time out, a high-to-low transition on ST must occur at or less than 100 ms
(minimum timeout) from a reset. If the watchdog timer is allowed to time out, the RST and RST outputs
are driven to the active state for 25 ms minimum. The ST input can be derived from microprocessor

address, data, and/or control signals. Under normal operating conditions, these signals would routinely
reset the watchdog timer prior to timeout. If the watchdog timer is not required, two methods have been
provided to disable it.

Permanently grounding the IN pin in the CMOS mode (RC=1) will disable the watchdog. In normal
operation with RC=1, the watchdog is disabled as soon as the IN pin is below VTP. With IN grounded, an

NMI output will occur only at power-up, or when th e ST pin is strobed. As shown in the NO TAG, a

falling edge on ST will generate an NMI when IN is below VTP. This allows the processor to verify that
power is between VTP and V

CCTP

, as an NMI will be returned immediately after the ST strobe. The

watchdog timer is not affected by the IN pin when in NMOS mode (RC=0).

If the NMI signal is required to monitor supply voltages, the watchdog may also be disabled by leaving
the ST input open. Independent of the state of the RC pin, the watchdog is also disabled as soon as V

CC

falls to V

CCTP

.

PUSHBUTTON RESET

An input pin is provided on the DS1236A for direct connection to a pushbutton. The pushbutton reset
input requires an active low signal. Internally, this input is pulled high by a 10k resistor wheneve r VCC is

greater than V

BAT

. The PBRST pin is also debounced and timed such that the RST and RST outputs are
driven to the active state for 25 ms minimum. This 25 ms delay begins as the pushbutton is released from
a low level. A typical example of the power monitor, watchdog timer, and pushbutton reset connections

are shown in NO TAG. The

PBRST input is disabled wheneve r the IN pin voltage level is less than V

TP

and the reset control (RC) is tied high (CMOS mode). The PBRST input is also disabled whenever VCC is
below V

BAT

. Timing of the PBRST -generated RST is illustrated in Figure 1.

DS1236A

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NON-MASKABLE INTERRUPT

The DS1236A generates a non-maskable interrupt NMI for early warning of power failure to a
microprocessor. A precision comparator monitors the voltage level at the IN pin relative to a reference
generated by the 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 TA G) is used t o int erface wi th hi gh 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 VTP is 2.54 volts, the proper values
for R1 and R2 can be determined by the equation as shown in NO TAG. Proper operation of the
DS1236A requires that the voltage at the IN pin be limited to VIN. 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 high enough to ke ep 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 DS1236A pulses the NMI output
to the active state for a minimum of 200 µs. The NMI power-fail detection circuitry also has 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 a ctivate NMI . Therefore, the supply

must be below the voltage sense point for approximately 100 µs o r the comparator will reset. In this way,
power supply noise is removed from the monitoring function, preventing false trips. During a power-up,

any detected IN pin levels be low VTP by the comparator are disabled from reaching the NMI pin until
VCC rises to V

CCTP

. As a result, any potential NMI pulse will not be initiated until VCC reaches V

CCTP

.

Removal of an active low level on the NMI pin is controlled by either an internal timeout (when IN pin is
less than VTP) or by the subsequent rise of the IN pin above VTP. The initiation and removal of the NMI
signal during power-up results in an NMI pulse of from 0 µs minimum to 500 µs maximum, depending

on the relative voltage relationship between VCC and the IN pin voltage. As an example, when the IN pin
is tied to ground during power-up, the internal timeout will result in a pulse of 200 µs minimum to 500 µs
maximum. In contrast, if the IN pin is tied to V

CCO

during power-up, NMI will not produce a pulse on

power-up. 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.

DS1236A

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DS1236A FUNCTIONAL BLOCK DIAGRAM Figure 1

DS1236A

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If the IN pin is connected to V

CCO

, the NMI output will pulse low as VCC decays to V

CCTP

in the NMOS

mode (RC=0). In the CMOS mode (RC=V

CCO

) the power-down of VCC out of tolerance at V

CCTP

will not
produce a pulse on the NMI pin. Given that an y NMI pulse has been completed by the time VCC decays
to V

CCTP

, the NMI pin will remain high. The NMI voltage will follow VCC down until VCC decays to

V

BAT

. Once VCC decays to V

BAT

, the NMI pin will either remain at V

OHL

or enter tri-state mode as

determined by the RC pin (see “Reset Control” section).

MEMORY BACKUP

The DS1236A provides all of the necessary functions required to battery back a static RAM. First, a
switch is provided to direct SRAM power from the incoming 5-volt supply (VCC) or from an external
battery (V

BAT

), whichever is greater. This switched supply (V

CCO

) can also be used to battery back a
CMOS microprocessor. For more information about nonvolatile processor applications, review the “Reset
Control” and “Wake Control” sections. Second, the same power-fail detection described in the power

monitor section is used to hold the chip enable output (

CEO ) to within 0.3 volts of V

CC

or to within 0.7

volts of V

BAT

. This write protection mechanism occurs as VCC falls below V

CCTP

as specified. If CEI is

low at the time power-fail detection occurs, CEO is held in its present state until CEI is returned high or
the period t

CE

expires. This delay of write protection until the current memory cycle is completed prevents
the corruption of data. If CEO is in an inactive state at the time of VCC-fail detection, CEO will be
unconditionally disabled within tCF. During nominal supply conditions CEO will follow CEI with a

maximum propagation delay of 20 ns. NO TAG shows a typical nonvolatile SRAM application. The
DS1236A unlike the DS1236 can be operated without a battery. In this method of op eration the V

BAT

, pin
1, must be grounded. In general, it would also be expected to have the RC, pin 8, grounded (NMOS
mode) since no battery backup is available.

FRESHNESS SEAL

In order to conserve battery capacity during initial construction of an end system, the DS1236A provides
a freshness seal that electrically disconnects the battery. This means that upon battery attach, the V

CCO

output will remain inactive until VCC is applied. This prevents V

CCO

from powering other devices when
the battery is first attached, and VCC is not present. Once VCC is applied, the freshness seal is broken and
cannot be invoked again without subsequent removal and reattachment of the battery.

POWER SWITCHING

When larger operating currents are required in a battery backed system, the 5-volt supply and battery
supply switches internal to the DS1236A may not be large enough to support the required load through
V

CCO

with a reasonable voltage drop. For these applications, the PF and PF outputs are provided to gate

external power switching devices. As shown in Figure 8, power to the load is switched from V

CC

to
battery on power-down, and from battery to VCC on power-up. The DS1336 is designed to use the PF
output to switch between V

BAT

and VCC It provides better leakage and switchover performance than
currently available discrete components. The transition threshold for PF and PF is set to the external
battery voltage V

BAT

, allowing a smooth transition between sources. The load applied to the PF pin from
the external switch will be supplied by the battery. Therefore, if a discrete switch is used, this load should
be taken into consideration when sizing the battery.

RESET CONTROL

As mentioned above, the DS1236A supports two modes of operation. The CMOS mode is used when the
system incorporates a CMOS microprocessor which is battery backed. The NMOS mode is used wh en a
non-battery backed processor is incorporated. The mode is selected b y the RC (Reset Control) pin. The

level of this pin distinguishes timing and level control on RST, RST , and NMI outputs for volatile
processor operation versus nonvolatile battery backup or battery-operated processor applications.