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Application Note 100
DS1670 Portable System Controller
www.dalsemi.com
FEATURES
There is an increasing trend in the industry today towards portable battery-operated products. These
products typically require extremely small form factors and low power. The DS1670 Portable System
Controller is designed to help the portable system designer in meeting these two requirements by
integrating key functions typically needed in these systems while consuming very little power.
Key functions frequently required in portable products include a real time clock; nonvolatile RAM
control to insure that data is not lost in the event of a power-down or battery-fail; microprocessor
monitoring to insure that the microprocessor does not “run out of control”; and analog-to-digital
converters for interfacing with external environmental sensors, battery monitoring, etc. The DS1670
Portable System Controller replaces as many as four separate integrated circuits by incorporating all of
these functions in a single device. The DS1670 provides a very accurate real time clock, nonvolatile
SRAM controller, microprocessor monitor, and 3-channel 8-bit analog-to-digital converter.
Communication with the device is established through a simple 3-wire interface.
USING THE DS1670
The DS1670 can be directly interfaced with any microprocessor or microcontroller through a 3-wire serial
interface. The serial interface gives the microprocessor access to the DS1670 registers, which consist of
the Real Time Clock Registers, Control Register, Status Register, Watchdog Register, and ADC (Analog-
to-Digital Converter) Register. Figures 1 and 2 illustrate a block diagram and address map of the DS1670,
respectively. The following paragraphs briefly describe the operation of the various components of the
DS1670 as well as the functions of the various control and status registers. For more detailed information
please consult the DS1670 datasheet.
DS1670 BLOCK DIAGRAM
Figure 1
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APPLICATION NOTE 100
TYPICAL APPLICATION
Figure 2
REAL TIME CLOCK
The Real Time Clock (RTC) provides seconds, minutes, hours, day, date, month, and year information
with leap year compensation. The RTC has a back-up power supply input (V
BAT
) that enables it to
continue to keep time even when the primary supply connected to V
CC
is removed. The RTC also
provides an alarm interrupt which is functional both when the DS1670 is powered by the system power
supply or when in battery back-up operation. Functionality while in battery back-up operation allows the
alarm to “wake up” a system that is powered down.
NONVOLATILE SRAM CONTROL
The nonvolatile SRAM controller of the DS1670 provides two important functions. First, it insures a
constant uninterrupted power supply to the system SRAMs. This is provided by the V
CC
Output pin
(V
CCO
). V
CCO
is internally connected to V
CC
when V
CC
is within nominal limits. However, when V
CC
is
below nominal limits, V
CCO
is connected to the power source connected to V
BAT
(typically a lithium
battery, rechargeable battery, or super capacitor). The second important feature of the nonvolatile
controller is that it gates the chip enable signals to the SRAMs. This insures that the microprocessor can
access the SRAMs only when the primary power supply is within nominal limits, thus eliminating the
corruption of data during power transients. It should also be noted that the DS1670 provides nonvolatile
control for two separate SRAMs. The two inputs from the microprocessor are Byte High Enable (
BHE
)
and Byte Low Enable (
BLE
). These inputs correspond to the two output Chip Enable Output High
(
CEOH
) and Chip Enable Output Low (
CEOL
).
MICROPROCESSOR MONITOR
The microprocessor monitor circuitry of the DS1670 provides three basic functions.
1. 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
reset to the active state. When V
CC
returns to an in-tolerance condition, the reset signals are kept in the
active state for 250 ms to allow the power supply and processor to stabilize.
2. Pushbutton reset control: The DS1670 debounces a pushbutton input and guarantees an active reset
pulse width of 250 ms.
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APPLICATION NOTE 100
3. Watchdog timer: The DS1670 has an internal timer that forces the reset signals to the active state if
the strobe input is not driven low prior to watchdog time-out.
ANALOG-TO-DIGITAL CONVERTER
The DS1670 also provides a 3-channel 8-bit successive approximation analog-to-digital converter. The
converter has an internal 2.55 volt (typical) reference voltage generated by an on-board band-gap circuit.
The A/D converter is monotonic (no missing codes) and has an internal analog filter to reduce high
frequency noise.
An A/D conversion is the process of assigning a digital code to an analog input voltage. This code
represents the input value as a fraction of the full scale voltage (FSV) range. Thus the FSV range is then
divided by the A/D converter into 256 codes (8 bits). The FSV range is bounded by an upper limit equal
to the reference voltage and the lower limit, which is ground. The DS1670 has a FSV of 2.55V (typically)
which provides a resolution of 10 mV. An input voltage equal to the reference voltage converts to FFh
while an input voltage equal to ground converts to 00h. The relative linearity of the A/D converter is
±0.5
LSB.
The analog input (AIN0, AIN1, or AIN2) for an analog-to-digital conversion is selected by the condition
of the Analog Input Select (AIS) bits in the Control Register. Conversions occur every 10 ms and the
result is placed in the ADC Register.
TYPICAL APPLICATION
Figure 3 illustrates a typical application using the DS1670. In this application, the DS1670 is shown
interfacing with a 386EX or 80186 microprocessor (microprocessors designed with embedded
applications in mind). Communication between the microprocessor and the DS1670 is accomplished
easily through three port pins (P1.0-P1.2) which are connected to the 3-wire serial interface pins of the
DS1670 Serial Clock (SCLK), Chip Select (
CS
), and serial input/output (I/O). This serial interface
provides access to the real time clock, ADC, control, and status registers of the DS1670.
NONVOLATILE SRAM CONTROL
The DS1670 also provides nonvolatile control of external SRAMs in this circuit example. The Chip
Select (
CS
), Byte High Enable (
BHE
), and Byte Low Enable (
BLE
) outputs of the microprocessor are
gated
through the DS1670 to its Chip Enable Output Low (
CEOL
) and chip Enable Output High (
CEOH
)
output pins. The
CEOL
and
CEOH
pins can be driven to the active state only when V
CC
is within nominal
limits (>2.88V) thus preventing the system from receiving or writing corrupted data. Note that the
structure of the SRAM access controller allows the microprocessor to access data in worldwide format
(16 bits wide by enabling
CEOL
and
CEOH
simultaneously) or byte-wide format (8-bit wide by enabling
either
CEOL
or
CEOH
).
Power to the SRAMs is provided from the V
CCO
pin of the DS1670. When V
CC
is within nominal limits
(greater than 2.7 volts typical), V
CCO
is internally connected to V
CC
. However, when V
CC
goes below
nominal limits, V
CCO
is internally connected to V
BAT
,
thus insuring that the SRAM data is maintained in
the event of a primary power supply failure. V
BAT
,
is typically connected to a lithium battery, rechargeable
battery, or super capacitor. Figure 3 illustrates V
BAT
connected to a super capacitor. The super capacitor
is charged whenever the power source connected to V
CC
is a diode drop above V
BAT
plus the voltage drop
across resistor R1. R1 is used to decrease the charge current to the super capacitor. Note that in this
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APPLICATION NOTE 100
application, the primary power source connected to V
CC
is a battery. This could be a rechargeable battery
pack or perhaps a series of AA cells. Regardless of the type of battery used, the DS1670 allows the
battery to be changed without the loss of important data. Depending on the size and type of super
capacitor or battery connected to V
BAT
,
the data will be saved from several hours to several years without
replacing the main system battery. It should be noted also that when using a super capacitor, the limiting
factor is often the internal leakage of the capacitor itself. In other words, the battery back-up current
consumed by the DS1670 and SRAMs is negligible compared to the internal leakage of the capacitor.
MICROPROCESSOR MONITORING
The illustrated typical application also takes advantage of the microprocessor monitoring features of the
DS1670. The
RST
of the DS1670 is connected to the
RESET
pin of the microprocessor. The DS1670 will
generate a reset signal if V
CC
goes out of the nominal limits or if the watchdog timer is allowed to expire.
Toggling the strobe (
ST
) input before the watchdog timer expires will keep the watchdog from generating
a reset. The watchdog timer can be disabled or programmed by the user to be 250 ms, 500 ms, or 1000
ms. It is also possible to connect the
RST
pin to a monetary pushbutton switch to allow a manual reset of
the system. In this type of application the
RST
pin will produce a 250-ms active reset after being
momentarily grounded by an external momentary switch.
REAL TIME CLOCK
Another important feature of the DS1670 is the Real Time Clock (RTC). The RTC provides the system
with seconds, minutes, hours, day, date, month, and year information. The accuracy of the clock is
typically
±1
minute per month at room temperature. An especially powerful function of the RTC is the
alarm. The alarm gives the user flexibility to program events at a specific time of the day. The DS1670
can be programmed to generate a single alarm or, through the use of its alarm mask bits, can generate an
alarm once every second, minute, hour, or day. Added flexibility is provided in that the alarm is
functional both when the device is powered by V
CC
or when powered by V
BAT
. Generating an alarm while
powered by V
BAT
is especially useful because this allows the DS1670 to “wake up” a sleeping system. For
example, a portable datalogger may need to collect data once every hour. This system could be placed in
a low power “sleep mode” when idle. The alarm could be used to “wake up” the system at the appropriate
time to collect data. After the collection is completed, software could place the system back into sleep
mode where it would remain until the next alarm interrupt.
ANALOG-TO-DIGITAL CONVERTER
The DS1670 also has a 3-channel 8-bit analog-to-digital converter. In our typical application example,
one input of the ADC is used as a battery “gas gauge.” This input samples the voltage on the primary
power supply battery, which is first reduced through a voltage divider circuit. The microprocessor
monitors this voltage and generates a low battery warning if the voltage falls below a specified threshold,
thus signaling the end user that the battery needs to be changed. The additional ADC inputs could be
interfaced with environmental sensors or other analog signals.
LOW POWER
The functional features of the DS1670 are certainly a great benefit to many system designers, but equally
important in portable systems is low power to insure long battery life. It is probable that the majority of
the time the DS1670 will sit idle in a powered-up system. In this type of standby operation, the DS1670
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APPLICATION NOTE 100
will consume a maximum of 100
µA
(200
µA
maximum if the analog-to-digital converter is enabled).
During active operation, when the 3-wire interface is being driven, the DS1670 will typically consume 5
mA (10 mA maximum). The lowest power mode of the device is when the primary power supply is
removed and the device is being powered by V
BAT
.
In this mode, the device typically consumes 300 nA
(500 nA maximum).
SUMMARY
Overall, it has been shown that the DS1670 Portable System Controller provides many of the functions
necessary in battery backed portable products. The integration of these features onto a single piece of
silicon replaces as many as four separate integrated circuits and is available in an extremely small 20-pin
TSSOP package. The DS1670 is also ideally suited for battery backed products because of its low power
consumption, particularly when the device is in standby or battery backed mode.
DS1670 ADDRESS MAP
Figure 3
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