Applications reportAPPLICATION OF CMOS TECHNOLOGY TO PROCESS INSTRUMENTATION:SOME CASE STUDIES

Indexing term: Instrumentation

Abstract: The paper considers the application of CMOS technology to process instrumentationand gives examples of the use of CMOS in design.

Introduction

This paper discusses the reasons whyCMOS is a desirable technology forsome process instrumentation applica-tions. Some examples are describedwhich use CMOS exclusively in thedesign, giving the reasons why CMOSwas used and the problems that werefound while developing these instru-ments. All these examples are jointdevelopments carried out by Sira Ltd.and a manufacturing company.

CMOS technology can be appliedto advantage in process instrumenta-tion, but the designer who is morefamiliar with NMOS and TTL devicesmust take into account the lowerspeed of CMOS components ascompared with LS and NMOS equiva-lents, must avoid 'latch-up' conditions,and should apply all the normal designrules for LS and NMOS to the CMOSdesign.

Why choose CMOS for process instru-mentation?

The growing use of CMOS componentsfor process instruments is due primarilyto the low-power requirement of thetechnology. The advantage of usinglow-power components is that it is fareasier to design intrinsically safe andbattery-operated instrumentation.Intrinsically safe instruments areessential for the energy industries (oil,gas, coal etc.) and for some processindustries. The reliability of processcontrol and measuring instruments isincreased by being able to site theinstrumentation in the hazardousenvironment, as opposed to relying onsensors connected by long lines to aremote instrument house. There arealso ergonomic advantages in beingable to install intelligent instrumentsclose to the process under control. Themain advantage of battery-operatedinstruments over traditional instru-ments which require a local supply isthat they are portable. Portable test

equipment can be carried onto site andused for on-site maintenance. Portabledata logging and remote monitoringand control equipment can be installedin hostile environments for long periodsof time, and collected only whennecessary.

Some product examples

Sira has worked with a number ofdifferent companies on the collabora-tive development of various CMOS-based instruments over the last fewyears. The product examples discussedbelow are:

Gas flow volume corrector (Fig. 1)BS Instruments Ltd.

System for tanker instrumentationand control (Fig. 2)

Drum Engineering Ltd.

Temperature readout for a hand-held thermal imager (Fig. 3)

Lasergage Ltd.

Gas flow volume corrector

The model 800 volume correctorjointly developed by BS Instrumentsand Sira is a battery-operated portableinstrument that is now in large-scaleproduction. The corrector is intendedfor use by industrial consumers ofnatural gas. Uncorrected flow rate ismeasured using a turbine meter. Thestatic pressure and temperature aretransduced and used to determine thegas compressibility factor, from whichthe true corrected volume flow can becalculated at a standard temperatureand pressure. The corrector is intrinsi-cally safe, and can be sited by the gassupply inlet.

The processor used is the Motorola146805. This processor, as well ashandling the analogue and pulsedturbine input, is also capable of per-forming the calculations required todetermine corrected volume flow. A

particular advantage of this device isthe 'go to sleep until interrupt'command, which is used to prolongthe battery life.

System for tanker instrumentation andcontrol

Development of the DRUM-STIC wasachieved through a four-way collabora-tion between Drum Engineering,Marconi, Sira and The City University.The product is a multiprocessor systemconsisting of satellite processors thatmeasure the fluid volume in theseparate compartments of a roadtanker, and a central controller.

The level transducer is a vibratingrod developed by Marconi. The localintelligence that converts the fre-quency, density and temperature datainto volume is a Motorola 146805-based single-board computer,developedby Sira and Drum Engineering, sitedabove each tank. The data from thesesatellite volume monitors are trans-mitted to a central controller based ona CMOS 8085. The central controlleris responsible for interrogating thevolume monitors, monitoring the frontpanel, driving the displays and con-trolling the tank valves.

Fig. 1 Gas flow volume corrector[BS Instruments Ltd.]

Software & Microsystems, Vol. 3, No. 3, June 1984 75

The complete system is intrinsicallysafe, and operates from the road

Fig. 2 System for tanker instrumenta-tion and control

[Drum Engineering Ltd.]

tanker's on-board battery. This applica-tion demonstrates another advantageof CMOS technology, in that it willtolerate a wide range of operatingvoltages and does not necessarily re-quire highly stable supply voltages.

Temperature readout for a hand-heldthermal imager

Lasergage manufactures a range ofportable hand-held thermal imagers. Athermal image of process plant is ofgreat advantage for locating possiblefailures in the plant. For example,a weak point in a furnace or a high-voltage transmission line will be clearlyseen as a 'hot spot', and preventativemaintenance can be carried out. Theimager must be portable and thereforeruns off batteries. Hot spots are amajor hazard in the gas, coal and oilindustries, and an intrinsically safe,portable thermal imager will help tomake their process plant safer tooperate.

Sira assisted with the developmentof a temperature readout facility forthe imager. By measuring the signalfrom the infra-red detector, and areference temperature, and then apply-ing a complex algorithm, the tempera-ture of the target being viewed can bedetermined. The target temperature isdisplayed in the lower band of thethermal image. The readout must.beintrinsically safe and battery operated,and was therefore developed using anNSC 800 and CMOS components.

Problems arising during development

Four main problems arose as a resultof using CMOS components: speed,power surges, 'latch-up' and failure topower on reset.

Speed

Standard 4000 series and 74C seriesdevices have much longer switchingtimes than their TTL equivalents. Fig. 4shows a circuit which demonstratesthe problems that can arise. The de-coder part of the circuit is used toselect the I/O map of an NSC 800,CMOS 8085 or equivalent. The outputlatch is only selected during a writeoperation. This circuit was found to beadequate on a processor systemrunning at up to 4 MHz, but anyattempt to run it faster failed. A quickexamination of the worst-case gatedelays gives the reason. Each NANDgate has a typical delay of 125 ns each,the OR gate takes 160 ns, the decodertakes 550 ns to settle and the set-uptime for the latch is 220 ns, giving atotal typical delay from address set-upto chip select of 1180 ns. The writecycle of an 8085/NSC800 is only990 ns when running at 6 MHz. Thesolution to this problem, and othermore subtle timing problems, is to usehigh-speed CMOS devices, such as theSignetics HEF series, or the 74PC

Fig. 3 Hand-held thermal imagerfLasergage Ltd.]

(National) and 74HC (National,Motorola) devices which approachTTL speeds.

Power supply spikes

Just because CMOS technology takesless power than other equivalenttechnologies, it is incorrect to assumethat it does not generate current surgesand therefore voltage drops on supplylines. Fig. 5 shows two traces takenfrom a 27C32 CMOS EPROM undermaximum switching load. One trace isthe output enable input signal; theother is a data output line. The bus isthe multiplexed address/data bus of aCMOS 8085. The address is FF hexand the data is 00 hex; therefore theoutput buffers of the 27C32 are re-quired to perform the maximumswitching load pulling a set of bus linesall set high to their low states. Despitethe fact that the device is CMOS, thecurrent surge generated when the out-put is enabled is sufficient to put a 1 Vspike on the supply lines. As a resultthe device switches off and the datadisappear. This problem was cured inthe final prototype by the addition ofa more substantial power griddingnetwork on the PCB. This is of coursegood engineering practice. However,the lesson to be learnt is that, despiteCMOS's low power consumption, it isstill capable of generating large switch-ing spikes.

Latch-up

CMOS, owing to its construction,inherently has parasitic SCR junctionsthat in normal operation are reversebiased. If input signals exceed thepower supply then these junctions be-come activated, causing SCR latch-upand allowing high currents to passwhich will easily destroy the device.Sensible design can ensure that a latch-up does not occur, the easiest methodbeing to employ level-shifting devicesat input interfaces. A particularproblem arose with a fast rising powersupply that put some ringing on thesupply lines. As a result, on power up

I0/M.

selectWR

Fig. 4 Address decoding for CMOS 8085

76 Software & Microsystems, Vol. 3, No. 3, June 1984

the CMOS processor invariably wentinto a latch-up mode. This was curedby adding extra decoupling to thesupply lines.

Power on reset

A phenomenon that was discoveredwith a variety of processors was theirfailure to start their clocks oscillatingon power up. As yet the cause has notbeen firmly established. The solutionis to use an active reset device, such asthe 4541, with a long time constant,and not rely on an RC network.

Conclusion

CMOS-technology components can beused in the engineering of processinstruments. The design rules forCMOS-based instruments are morecomplex than those for NMOS andTTL instruments. This is because theengineer has not only to apply thesame rules required for NMOS andTTL, but has also to take into accountthe additional rules that apply toCMOS.

E.N.GOODYER 3rd May 1984

Electronics & Computing SystemsDepartment, Sira Ltd., South Hill,Chislehurst, Kent, England

Fig. 5A Data output from 27C32 showing 1V switching spike on RD line

SM86 Fig. 5B Data output from 27C32 with correct PCB power gridding

Book reviewDesigning systems with microprocessors — a systematicapproachD. M. Freedman and L. B. EvansPrentice-Hall 1983, 332pp., £28.75ISBN: 0-13-201350-9

About a decade ago, in the early days of the microprocessor,it was the custom for logic designers to attend courses tointroduce them to the new device. If the subject of top-down design was mentioned at all it was touched upononly briefly. Indeed, the object of the courses was to allowthe participants to start producing code as quickly aspossible, with a little perfunctory testing if time permitted.It is sobering to consider how much randomly designedcode (with its attendant bugs) must exist to this day inassorted control systems around the world.

Fortunately, the past few years have seen an increasedawareness of the need for a more structured approach tosystem design. Today, the terms 'top-down design' and'software engineering' are commonplace. In keeping withthis trend, the book by Freedman and Evans provides awelcome new variation upon the 'introduction to micro-processors' theme.

This attractively presented book is aimed at under-graduates and at practising engineers. The structure ofthe book reflects the 'top-down' nature of its subject.Anyone raised on more conventional microprocessortexts will be intrigued that the first five chapters of thisbook contain no mention of arithmetic logic unit. How-ever, it is this lack of obsession with minute detail in theearly chapters which makes this text so refreshing.Similarly, whereas most books dive directly into assemblylanguage, the emphasis here is on the high-level languagePL/M. Details of hardware and assembly language arecovered later, only after the more abstract concepts havebeen established.

In keeping with its subject matter, the book itself iswell structured: the authors present a diagram showingthe relationship between the system design cycle andthe chapters of the text. The reader is led from re-quirements definition, functional specification and systemdesign, through hardware and software design and im-plementation, to system integration and evaluation. Con-tinuity is provided by the use of a running example —a burglar alarm system.

Chapter 1 leads in gently with a discussion on the

Software & Microsystems, Vol. 3, No. 3, June 1984 77