8751 EMULATORThis article describes hardware and

software that together torm apowerful development system tor the

popular 8751 microcontroller tromIntel.

Design by O. Bailleux

THE microcontroller emulator describedhere allows you to develop and debug

8751 application circuits with a minimum ofeffort. The emulator is has two main connec-tions: one to the serial port of a pe, and an-other to the Ie socket reserved for the 8751 inthe target system. The combination of thepe, the software that runs on it, and the emu-lator allows you to

download, modify, and upload 8751programs;erase and program a 8751;put breakpoints in programs;display regtster and memory contents;fW1 programs in slngle step mode;modify the contents of certain registers.

Clearly, this makes the emulator a powerndand indispensable tool for all of YOllwho, ata certain stage, are 'confronted' with a 8751application.

General remarksThis article does not aim at discussing oreven introducing a11the hardware and soft-ware aspects of the 8751 microcontroUer,since this Held is covered adequately by theMicrocontroller Handbook: from Intel. As re-gards practical programming of the 8751, the'8051/8032 assembler course' published inthis magazine will be very useful to fol1ow(the 8032, 8051 and 8751 are all devices fromIntel's MCS52 family of microcontrollers).

address bus

L-

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Fig. 1. Basic structure of a microcontroller system. rcs are available that combine all thefunctions shown here.

ELEKTOR ELECTRONICS MARCII1992

920019-11

None the less, the file READ.:tv1Eon the dis-kette supplied in relation to the present ernu-Iator contains some basic inforrnation on the8751.

Limitations:

• Monochip mode only (internalprogram memory, no externaldata memory)

• 1/0 bits P3.6 and P3.7 reservedlor system

• Register 0 banks only

• System software uses 12 stacklocatlons

• Masked interrupts duringbreakpoint processing

MAIN CHARACTERISTICS

• Real-time 8751 emulator

• Clock frequency: 8 MHz

• All 1/0 ports available

• All interna I interrupt sourcesavailable

• Powered by target circuit

• 9,600 baud seriallink to PC

Breakpoint analysis andsingle-step mode

• Internal register and internalRAM contents displayed andavailable for editing

• Symbolic assembler lor 8751

• Full-screen editor wlth errorlocation lacility

Binary or Intel-hex output liles

• Hard copy 01 lormatted sourceprogram

COMPUTERS AND MICROPROCESSORS

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Fig. 2. Circuit diagram of the 8751 emulator. Note that so me 01 the ports of the 80C451 are not used.

The 8751 is a microcontroller that inte-grates:

an 8-bit microprocessor;a RAM of 128 bytes;4 8-bit parallell/O ports;1 serial comrnunication port;2 programmable timers;2 external interrupf lines.

The 8-bit (256-byte) addressable range of the8751 is divided into two parts:

user RAM between OOHand 7FH: thiscontains registers ROto R7 (OOHto07H), and the system stack. A sub-range of 16addresses is bit-addressable.contro1 register range between 80H andFFH: this range can be accessed by di-reet addressing only, and contains the1/0ports, the timer control registers,and a number of other special functions.

Principle of operationAs illustrated by Fig. 1, a microprocessorsystem generally consists of microprocessor,

a program memory, a data memory, andinput/output devices. The elements thatform the system are interconnected by an ad-dress bus, a data bus and a control bus. Sincethe 8751 microcontroller works in 'mono-chip' mode, it has all of the above elementsintemally. On.ly the four S-bit ports can beaccessed by extern al devices. Emulating a8751thus calls for a device capable of:

running object code from RAMinsteadof ROM-trus is necessary to allowchanges to the program to be maderapidly;providing four input/ output ports aswell as the basic hardware environ-ment of the 8751;providtng an instruction set that is com-patible with that of the 8751.

The Type 80C451 meets all these require-ments beautifully, and is used here to emu-late a 8751. The pinning of this deviee isgiven in Fig. 3.

The emu1ator based on the 80C451 con-sists basically of:

an EPROM with a start-up program;a 32-KByte RAM to hold the systemroutines and the user program beingdeveloped;a memory switching device with twostates:

start state: the EPROM is located ataddress OOOH,and the RAM at ad-dress 8000Hexeeute state: the RAM is at OOOOHand the EPROM at 8000H;

a TIL compatible (pseudo-RS232) serialinterface for the connection the Pe.

The initialization procedure consists of thefollowing steps:

1. lnitialization (reset), and start phase.EPROM-based program executed from ed-dress OOOOH.

2. A routine in the stert-up EPROM thatsends the system program code frorn the PCto the ernu1ator RAM. This is done via theserial link, and with the aid a simplified

ELEKTOR ELECTRONICS MARCH 1992

920019_13

Fig. 3. Pinning of the 80C451 mierocon-troller (courtesy Signetics).

transfer protoco!.

3. A routine in the start-up EPROM actuatesa (reserved) 1/0 bit, which causes the systemto switch to switch to the 'run' (program ex-ecution] stage, and in addition generates areset.

4. The system program starts at addressOOOOH,it duplicates itself, and runs againfrom the upper part of the RAM at 7800H.From that IDeation, it arranges the comrnuni-cation with the pe with the aid of a datatransfer routine that allows the pe to

issue a software reset to the microcon-troll er;read the emulator RAM;modify the emulator RAM;modify the stack pointer;upload a new user program;start or continue the execution of a userprograrn.

5. The program that fWlS on the pe arrangesthe contraI of the emulator. Initially, it sendsthe user program object code to the emulatorfor the purpose of testing. This program willcontain at least one breakpoint, which callsup a subroutine in the system software thatstops the program execution.

6. The user program is executed until abreakpoint is encountered. At that location,

ELEKTOR ELECTRONICS MARCH 1992

8751 EMULATOR

920019-14

Fig. 4. Internal structure 01 the 80C451 microcontroller.

the system software coples the current con-tents of the registers and the internal RAM ofthe 8751 into a buffer formed by the externaJRAM of the emulator, at 7000H. Next, theremote control subroutine is called.

7. The system softwäre switches to the exter-nal emulator RAM, and causes the executionof the user program to continue, until thenext breakpoint is enccuntered. At thisstage, it is not possible to re-initialize the8751 via the pe, or run a new user program.

8. The contents of the buffer starting at7000H (wh ich may have been rnodified inthe mean time by the control program) arereloaded into the internal RAM and the reg-isters. and the breakpoint routine is ended.The user program Is continued up to the nextbreakpoint.

The key of the system is formed by theremote contrei routine, which enables thecontrol softwa.re to modify the user programand the contents of the registers in the 8751.This function makes it possfble to set break-points (temporarily), which are particularlyuseful in single-step mode.

SoftwareThe software for the emulator is supplied ona 5'II-inch 360 KByte MSDOS formatted dis-kette, and consists of the following files:

DEV.EXECONFIG.EMUSYS8751.C51SYS8751.A51

programming environmentconfiguration fileinitialization filesource code of initializationfile

READ.ME additional information

The user or the contral program can accessthe emulator only while a static or dynamicbreakpoint is being processed. The Iocationof a breakpoint is determined by the user bylnserttng it in the source code. The controlpragram calcuJates the corresponding ad-dress, saves the three bytes of opeode fOW1dthere, and replaces them with an instructionthat calls up the breakpoint handler. After anumber of manipulations, the initial condi-tions are restered. and we can progress to thenext breakpoint.

Insingle-step mode, the control program,while executtng the breakpoint routine,automatically places a dynamic breakpointafter the next instruction. Ln single-stepmode you can see exactly what happens tothe register contents as the program evolves.A powerfuJ debugging aid!

HardwareAt the heart of the emulator circuit (Fig. 2) isthe Type SC80C451 rnicrocontroller frornStgnencs, the North-American branch ofPhilips Components. Its internal structure isgiven in Fig. 4. By taking a close look at theblock diagram, it becomes clear that the8OC451 is an extended version of the BOC51,with three additional 1/0ports (which givesa total of six), and four additional 1/0 con-trollines.

The 80C451 has no internal ROM, and fel-ches its instructions from an external ROMer EPROM. The microcontroller offers128 bytes of RAM, and the DIP version usedhere has no fewer than six 8-bit ports and one4-bit port. The 80C451 is capable of address-

COMPUTERS AND MICROPROCESSORS

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ELEKTOR ELECTRONICS MARCH 1992

COMPONENTS LIST

8751 EMULATOR

Resistors:1 1500 Al3 10kQ A2;A4;R5 '~-I: ", ,.- -1 lOOkQ A3

_e, .-...... "'7

Capacitors:9 100nF Cl-C97 10llF 16V radial Cl0;C13-C182 27pF Cll;C12

Semiconductors:1 lN4148 011 74HCT86 ICl1 SCBOC451 CCN64

(Signetics) IC21 74HC373 IC31 27C64 (ESS605l) 1C41 62256 IC51 74HCTOa IC61 74HCT32 IC71 MAX232 (Maxim) tca1 4013 IC9

Miscellaneous:1 9-way female sub-D

connectar of pes mounting K1push-button n.o. 818MHzquartzcrystal X164-pin strip 10 make le socket40-way rc socket withturned pins

2 40-way IDG style DIP header1 length of 40-way flatcable1 enclosure; approx. dimensions:

92x146x28mm.printed circuit boardcontral program on disk

Ha

920019ESS6051

ing 64 Kbytes of ROM, and an equal amountof RAM. The eurrent consumption of the de-vice is only about 24 mA at a supply voltageof 5 V and a dock frequency of 12 MHz,3 mA in stand-by mode, end about 50 ~A inthe power-down ('sleep') mode.

The mask-programmable ROM versinnof the SOC451, the SCS3C451, has 4 KBytes ofROM. Both the 80C451 and the SC83C451have two 16-bit tirners/ counters. Their inter-rupt strueture allows two priority levels tobe implemented.

The presence of a serial 1/0 port in the80C451 allows a UART (universal asyn-ehronous receiver /transrrutter) with trueduplex operation to be realized quite easily.Alternahvely, the serial 1/0 port may beused to extend the functionsof the 1/0 Iines,or to set up an inter-processor ccmmunica-tion systcm.

The srand-by and the power-downmodes can be entered via software. In stand-by mode, the CPU proper is haI ted, while theRAM, the rirners, the serial port and the in-terrupt system continue to function. In thepower-down mode. the clock oscillator isdfsabled. which causes all functions to beswitched off, but the RAM contents to be re-tained.

ELEKTOR ELECTRONICS MARCH 1992

ConstructionThe availabiIity of a ready-made. through-plated printed circuit board (Fig. 5) makesthe construction of the 8751 emulator rela-tively simple. A number of passive compo-nents are fitted upright. Due attentionshould be paid to the orientation of the ICson the board, since they are not a11mountedwith the same dlrection. Naturally, the samegoes for the orientation of the electrolytic ca-pacitors and the diode.

The push-buttcn, SI, may be fitted on tothe enclosure, and is connected to the appro-priate pins on the board.

The SC80C451 is best litted into a socketmade from two 32-pin strips. The RAM andthe EPROM are also fitted in TCsockets.

SOCKET! is best made from a goodquality 40-pin JC socket. The connection tothe 8751 socket in the target system is thenreadily made via a short length of flatcablefitted with a 40-way lDC-style DIP header ateither end. Unfortunately, the pins of theseDIP headers are pretty fragile, so take carenot to break orte, or the whole header is use-less.

The completed printed circuit board isbuilt into a suitable enclosure. A slot is cut inone of the sides to allow the 40-way flatcableto pass.

Practical useIrutially. the application (target) circuit andthe PC are switched off. Note that the emula-tor is normally powered by the target circuit.Connect COM1: or COM2: of the PC to theemulator via an RS232 cable (the serial portselection is made in the file CONFIG.EMU asdiscussed below).

Do not connect the application circuit asyet. Connect a 5- V supply to the emulator.

The + goes to pin 40 (+), and the - to pins 9and pin 20 of the DIL socket on the emulatorboard. Next, run the program DEV.EXE onthe PC, and check that the error message"Emulator not connected -- PRESS ANYKEY TO CONTlNUE" does not appear.

At this stage, it should be possible toemulate the example program (see the syn-tax requirernents mentioned in READ.ME),without the need of inserting the DIP pluginto the 8751 socket in the target system. Ifthis works, remove the temporary 5-V sup-ply connections, and plug the 40-way DIPheader into the socket on the emulatorboard. Insert the DIP header at the other endof the cable into the 8751 socket on your tar-get system board. Power up the applicationbefore switching on the PC

Programs may be edited and assembledeven when the emulator is not connected orpowered. The coniiguration file, CON-FIG.EMU, contains only two characters. Thefirst is an M (for the monochrome Herculesvideo adapter) or a C (for the coIour videoadapters CGA, EGA and VGA). The secondcharacter selects the serial port, and is eithera l forCOM1:, or a 2 forCOM2:.lf necessary,edit the configuration file ustng any ASCIIcompatible word processor. On the disk sup-plied through the Readers Services, the con-figuration is set to eolour and COM1:.

The main program, DEV.EXE, is menu-driven, and uses the arrow keys to make se-lections. If you have the emulator poweredup and connected to the target system at thisstage, press switch Si before runningDEV.EXE.

Pinally, READ.ME contains more infcr-mation on the operation of the serial datalink between the pe and the emulator. •

I COMPUTERS AND MICROPROCESSORS

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ELEKTOR ELECTRONICS MARCH 1992

Mains (power line) voltages are not listedin the articles. It is assumed that our readersknow what voltage is standard in their partof the world.

Readers in countries that use 60 Hz sup-plies, should note that our circuits are usu-ally designed for 50 Hz. This will not nor-mally cause problems, although if the mainsfrequency is used for synchronization, samemodificat.ion may be required.

The international letter symbol' U' is usedfor voltage instead of the ambiguous 'V'.The letter V is reserved for 'volts'.

CORRECTIONSPlant warmer (June 1992)

Resistor Re was omitted from Fig. 2. Thecorreet diagram is shown below.

Inductance-capacitance meter (March 1992)The value of Rt6 and R'7 should be 39 0,not 30 0 as shown in the parts list.

8751 Emulator (March 1992)The features list in the first column on page53 should read:- download, modify, and upload 8751 pro-

grams without having to erase and pro-gram an 8751.

- put breakpoints in programs.- display register and memory contents.

ete.

FM tuner - Part 3 (May 1992)In the PSU parts list on page 54, R301shouldbe 1500,1%, not 150 kO, 1%.

Video enhaneer (July 1992)Preset P2 is best adjusted for a signal levelof2 Vpp at the collector of Tj. Output tran-sistor T3 may run fairly hot: this is normal.

The third paragraph of the text on page73 should read: The frequeney characteris-tie of the signal at the base of T 3 is shapedby P" R6 and C8, and is, therefore, to a cer-tain extent under the contral of the user (withP,).

Mark 2 QTC 80/40 loop antenna (July 1992)The frequeney '3800 kHz' mentioned twieeunder 2. 40-metre band (page 90) shouldhave read '7300 kHz'.

Audible fluid level indicator (July 1992)Owing lO a printing error, the diagram in thisarticle is incorrect. The right diagram is shownbelow.

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Write the advertisement, which MUSTrelate to electronics, in the coupon on thispage; n MUST INCLUDE a private telephonenumber or name and address; post officebcxes are NOT acceptable.

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Elektor Electronics (PubHshing) reservethe right to refuse advertisernents withoutgiving reasons or without returning them.

WANTED: Signeties linear LSI data & appli-eations manual 1985., or article p.9-198.Phone Frank Cosgrove on (0202) 432 973.

FOR SALE. Tektronix 465 100 MHz portablescope, calibrated, manuals and probes. Verygood.t295. Phone (0344)27869.

FOR SALE. Atari 520 STFM computer, 1 Mdrive, mouse, joystick, manual. t200. Phone092684279.

WANTED. Helpwith laserPSU, 150 mW, argonion; heater OK; tube current and voltage con-trol. All expenses paid. Write to Mr W. Owen,

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HELP. Electrohome ECM1311 monitor. Anyonewho hassuecessfully replaced the CRT, pleasephone (0543) 491 867.

WANTED. Sindair ZX81 ULA logic chip orworking ZX81. Phone Jon on (0602) 307 140belween 6 and 7 p.m.

WANTED. Cireuit diagram for TEAC FD-55disk drive fitted with Apple 2 adapter. Pleasewrite to Stephen Shaw, 21 KanferSt, GreenhilIs1760, South Afriea.

WANTED. Ouantity of 18-pin RAMs typenumbers5514 HM4334 M58981 6514. PhoneMark on 081 6938200.

LOAN. Schematic for Ferguson hi-fi system15 model ML40001 C. Write to R. Barnell, 26Roseholme Road, Northampton NN1 4TO.

FOR SALE. Intel8-bit embedded controllershandbook 1991 ed. Contains 8051 data. t15ono. Phone John on 0920 462 414.

FOR SALE. PC-based disassemblerfor8031/32produces souree code with labels. t20. PhoneMark on (0626) 779 987.

FOR SALE. Electronic Components (book)by D.T. Horn (McGraw-HiIl1992)orexchangefor Elektor Electronics issues 5, 7-8, 10 and111991 and 11987. WritetoJarkko Laukkanen,Heleniuksenk 36A 17, SF-05860 Hyvinkaa,Finland.

Wantto EXPERIMENT with tubes? 20 valves

incl. triodes, pentodes, rectifiers, etc. t15.Phone (0254) 760 003.

Send this coupon toElektor Electrontcs (Publishing)

P.O. Box 1414Derchester DT2 SV"

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Block capitals please - one cnaracter 10 each box

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Name and address MUST be given

920022 - I - 20

Mini OlN63

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Fig. 25. Manual control: connections of thethree switches to the mini-OIN plug insertedinto the 12Ccontrol input.

with two to four projectors, where theslides are distributed over the projectors.Switch 56 serves to set the dissolve (fadeout/fade in) time. Pressing it once causes aLED bar to appear on all four lamp indica-tors. The LED bar indication is propor-tional to the dissolve time. The 'fcrward'and 'reverse' button then serve to increaseand decrease the dissolve time respec-tively, in steps of one second. The LED barwill change accordingly. Pressing S4 againtakes you back to the projector eontrolmode.

Provision has been made for the projec-tor illumination areas (cn the screen) to bematched. All four prcjectors light whenthe 'forward' or 'reverse' button is pressedwhen the unit is switched on. This allowsyou to position the projectors such thattheir light beams overlap exactly on thescreen. This mode Is left by pressing the'forward' or 'reverse' button aga in, where-upon the system can be used as describedabove.

DIGITAL AUmOlVlSUAL SYSTEM - 2

Fig. 26. The side panel of the Aetex enclosure has holes for the mains adaptor plug, theswitches and the potentiometer.

The case

If everything works to your satisfaction,the dissolve unit may be fitted into its en-closure. Self-adhesive foils are availablefor the front as weil as the rear panel of theRetex enclosure. The rear panel layout Isshown reduced in Fig. 21. These foils grvethe unit a professional and attractive fin-ish. There is one point to note about therear panel and the rear panel foil: in somecases, the hole for the mini-DIN plug mayhave to be made larger than indicated bythe drilling template. This is necessary be-cause the plug body has to touch thesocket to make proper contact.

The right-hand side panel of the Retexcase is driIIed and filed to allow 51, 52, S3and PI to be opera ted. Also note the holerequired to insert the mains adaptor plug(see Fig. 26). The voltage regulator isbolted on to the rear panel. lf you use an-other enclosure than the one we recorn-mend, remember that the metal part ofTC16 and the sides of the 12Cplugs are con-nected to ground. To prevent a short cir-cuit with the alternating voItage, themains adaptor input socket must be an in-sulated type if it is fitted on the same panelas the regulator. 0

8751 EmulatorMarch 1992, p. 53.(Corrections; component infcrmation)While in emulation mode, the regtster eon-tents are displayed with an offset of onevertical Iine from the associated registerdesignations. This error occurs on early re-leases of the system software, item ES51741, and is caused by one superfluous'spece' charaeter in the DEV.EXEprogram.This 'space' (20H) should be changed intoa 'line feed' (OAH). First, make a backupcopy of your original diskette. Next, use ahex editor to change the byte at addressoffset DEODH from 20H into OAH. Usingthe hex editor of PCTooIs V6, for instance,this byte is found in relative sector 111(decimall), at offset ODH.

Pins 52, 53 and 54 of the SC80C451must be connected to ground to giveproper access to (simulated) Port O.For noapparent reason, this is not indicated inthe Signetics datasheets. Port 0 is actually

ELEKTOR ELECTRONICS DECEMBER 1992

COf:lRECTIONSAND

UPDATES

simulated by Port 6 of the SC80C451. Forfurther inforrnation on this compatibilityproblern with generic 8051 assembler files,consult the SC80C451 (Signencs) or8xC451 (Intel) datasheets.

In addition to your local 5ignetics(Philips Semiconductors) distributors, twosuggested suppliers of the controller TypeSC80C45ICCN64 OIe:

(1) Macro Marketing Ltd., BurnhamLane, Slough SLl 6LN. Telephone (0628)604383.

(2) C-I Electronics, P.D. Box 22089, 6360AB Nuth, Holland. Fax: +31 45241877.

GAL programmerMay 1992, p. 55.(Update)The transistors Type BC369 in positions T6and T7 are apparently difficult to obtain,and may be replaced by BC640s.

The most recent version of the softwareis V. 6.53dt, June 1992. The README filecontains an update note on problems withthe programming of certain GAL makes,as weil as a suggestion to make GALs witha damaged electronic signature (type iden-tifier) useable again.

8051 Single boardcomputerOe tob er 1992. p. 40.(Update)Since the publieation of this arttele. wehave been advised that the telephonenumber of Suncoast Technologies is +1(904)596-7599.