NSC800D datasheet(69/76 Pages) NSC

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130 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 AD 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 AD 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 AD 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 AD 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 32768 Hz

clock for the MM58167 and a clock within the operating

range of the AD 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) andor

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-

NSC800D Datasheet(PDF) 69 Page - National Semiconductor (TI)

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