NSC800 Datasheet, PDF (69/76 Pages) National Semiconductor (TI) – NSC800TM High-Performance Low-Power CMOS Microprocessor

13 0 Data Acquisition System
A natural application for the NSC800 is one that requires
remote operation Since power consumption is low if the
system consists of only CMOS components the entire
package can conceivably operate from only a battery power
source In the application described herein the only source
of power will be from a battery pack composed of a stacked
array of NiCad batteries (see Figure 20 )
The application is that of a remote data acquisition system
Extensive use is made of some of the other LSI CMOS com-
ponents manufactured by National notably the ADC0816
and MM58167 The ADC0816 is a 16-channel analog-to-
digital converter which operates from a 5V source The
MM58167 is a microprocessor-compatible real-time clock
(RTC) The schematic for this system is shown in Figure 20
All the necessary features of the system are contained in six
integrated circuits NSC800 NSC810A NSC831 HN6136P
ADC0816 and MM58167 Some other small scale integra-
tion CMOS components are used for normal interface re-
quirements To reduce component count linear selection
techniques are used to generate chip selects for the
NSC810A and NSC831 Included also is a current loop com-
munication link to enable the remote system to transfer data
collected to a host system
In order to keep component count low and maximize effec-
tiveness many of the features of the NSC800 family have
been utilized The RAM section of the NSC810A is used as
a data buffer to store intermediate measurements and as
scratch pad memory for calculations Both timers contained
in the NSC810A are used to produce the clocks required by
the A D converter and the RTC The Power-Save feature of
the NSC800 makes it possible to reduce system power con-
sumption when it is not necessary to collect any data One
of the analog input channels of the A D is connected to the
battery pack to enable the CPU to monitor its own voltage
supply and notify the host that a battery change is needed
In operation the NSC800 makes readings on various input
conditions through the ADC0816 The type of devices con-
nected to the A D input depends on the nature of the re-
mote environment For example the duties of the remote
system might be to monitor temperature variations in a large
building In this case the analog inputs would be connected
to temperature transducers If the system is situated in a
process control environment it might be monitoring fluid
flow temperatures fluid levels etc In either case operation
would be necessary even if a power failure occurred thus
the need for battery operation or at least battery backup At
some fixed times or at some particular time durations the
system takes readings by selecting one of the analog input
channels commands the A D to perform a conversion
reads the data and then formats it for transmission or the
system checks the readings against set points and trans-
mits a warning if the set points are exceeded With the addi-
tion of the RTC the host need not command the remote
system to take these readings each time it is necessary
The NSC800 could simply set up the RTC to interrupt it at a
previously defined time and when the interrupt occurs make
the readings The resultant values could be stored in the
NSC810A for later correlation In the example of tempera-
ture monitoring in a building it might be desired to know the
high and low temperatures for a 12-hour period After com-
piling the information the system could dump the data to
the host over the communications link Note from the sche-
matic that the current for the communication link is supplied
by the host to remove the constant current drain from the
battery supply
The required clocks for the two peripheral devices are gen-
erated by the two timers in the NSC810A Through the use
of various divisors the master clock generated by the
NSC800 is divided down to produce the clocks Four exam-
ples are shown in the table following Figure 20
All the crystal frequencies are standard frequencies The
various divisors listed are selected to produce from the
master clock frequency of the NSC800 an exact 32 768 Hz
clock for the MM58167 and a clock within the operating
range of the A D converter
The MM58167 is a programmable real-time clock that is
microprocessor compatible Its data format is BCD It allows
the system to program its interrupt register to produce an
interrupt output either on a time of day match (which in-
cludes the day of the week the date and month) and or
every month week day hour minute second or tenth of a
second With this capability added to the system precise
time of day measurements are possible without having the
CPU do timekeeping The interrupt output can be connect-
ed through the use of one port bit of the NSC810A to put
the CPU in the power-save mode and reenable it at a preset
time The interrupt output is also connected to one of the
hardware restart inputs (RSTB) to enable time duration
measurements This power-down mode of operation would
not be possible if the NSC800 had the duties of timekeep-
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