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RAJA GOPAL AKELLA

Remotely ProgrammableRTC-Interfaced Microcontrollerfor Multiple Device Control

This project based on AtmelAT89C52 and Dallas real-time-clock (RTC) chip DS12887 can

be used to control and remotely pro-gram the switching operation of 24electrically operated devices. The de-vices can be switched on/off at pre-cise times repeatedly every day, everymonth. The microcontroller can beprogrammed for device control usinga normal Philips TV remote control.

RC5 codingSince the circuit makes use of PhilipsTV remote for device-switching timeparameters, you need to know the fun-damentals of the coding format usedin these IR remotes.

The Philips IR format makes useof RC5 code, which is also known as‘bi-phase coding.’ In RC5-coded sig-nals (Fig. 2), each bit has a uniformduration. A transition in the middle ofthe time interval assigned to each bitencodes its logical value (‘0’ or ‘1’). Ahigh-to-low transition assigns the bita logic value of ‘0,’ and a low-to-hightransition assigns the bit a logic valueof ‘1.’ We need additional transitionsat the beginning of each bit if a streamof equal bits is sent. However, there isno need of additional transitions if thenext bit has a different logic value.

Table II shows how all the com-mands of an RC5 remote control areencoded.

The first two bits are ‘start’ bits,which are used to adjust andsynchronise the receiver. These bits areused to calculate and analyse the bitlength of the other bits.

The third bit is a ‘toggle’ bit, whichis toggled every time a button is

pressed at the remote control. This bitis used to identify whether the buttonis really pressed or whether an obstaclecame in between the IR path of theremote and the IR receiver.

The five bits (A4 through A0) im-mediately following the toggle bit areused to identify the device (see TableIII). So, a maximum of 32 devices canbe identified to and respond individu-

ally to the same type of coding with-out any disturbance, i.e., one amongthe 64 devices can be identifieduniquely. Addresses of some of theremotes are shown in Table III.

The six bits (C5 through C0) im-mediately following the five addressbits are the control/command bits.Therefore a maximum of 64 commandscan be equipped in an RC5-type re-mote. Some of the command codes(decimal equivalents), as used in thisproject, are shown in Table IV.

When any of the command/con-trol buttons on the remote is pressed,the coded signal is received by theIR receiver-demodulator TSOP1738.The output of the IR demodulatorcircuit is normally high, but whenany of the buttons in the remote ispressed, a stream of low-going de-modulated pulses will appear at itsoutput. These pulses are fed to theexternal active-low interrupt inputpin (INT/0) of 89C52. On receipt ofthe first low-going pulse, the moni-tor program of 89C52 will get inter-rupted and jump to the location‘0003H,’ where the execution is redi-rected to ‘receive’ sub-routine of theprogram. The outputs from the sub-routine are:

1. Toggle bit, which toggles (either‘0’ or ‘1’) each time the button in aremote is pressed.

2. Address byte, whose value iszero for a normal Philips-type TV re-mote control (see Table III).

3. Control byte, which has a uniquevalue for each button in the remotecontrol (see Table IV).

The hardwareMicrocontroller AT89C52 is interfacedto DS12887 (RTC), a 16x2 LCD mod-

PARTS LISTSemiconductors:IC1 - AT89C52 microcontrollerIC2 - 74LS573 octal D-type

latchIC3 - DS12887 real-time clockIC4 - 74LS138 decoderIC5 - 7400 NAND gateIC6 - 82C55 programmable

peripheral interfaceIC7- IC9 - ULN2803 high current

octal Darlington arrayIC10 - 7805 5V regulatorIRX1 - TSOP1738 IR receiver

moduleBR1 - 1A bridge rectifierT1 - BC547 npn transistorResistors (all ¼-watt, ±5% carbon):R1, R6-R29 - 4.7-kilo-ohmR2, R3 - 10-kilo-ohmR4 - 100-ohmR5 - 1-kilo-ohmVR1 - 10-kilo-ohm presetVR2 - 1-kilo-ohm presetCapacitors:C1, C2 - 33pF ceramic diskC3 - 10μF, 16V electrolyticC4-C8, C11 - 100nF ceramic diskC9 - 1μF, 16V electrolytic

capacitorC10 - 1000μF, 35V electrolytic

capacitorMiscellaneous:X1 - 230V AC primary to 15V,

500mA secondarytransformer

RL1-RL24 - 12V, 200-ohm, 1C/Orelay

S1 - Push-to-on switch- 16×2 LCD module

XTAL - 11.09 MHz crystal

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ule and an 8255 programmable periph-eral interface (PPI). The address-decod-ing circuitry comprises NAND gates74LS00 and 3-to-8 line decoder 74LS138

as shown in Fig. 1. The interfacing cir-cuitry for the external electrical appli-ances comprises Darlington array ICULN2803. The addressing range of

various peripheral devices is shown inTable I.

In 89C52 (IC1), port P0 is used foroutputing multiplexed address (lower

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8-bit) and data. The address is latchedinto 74LS573 (IC2) octal latch and RTC

DS12887 (IC3)with the help ofALE (addresslatch-enable) out-put from pin 30 ofIC1.

Only two ad-dress lines fromIC2 (A0 and A1)have been usedfor addressing thefour registers of8255 PPI (IC6) in

conjunction with the chip-select signal at pin 6 (fromIC4) and read/write signalsfrom IC1. Higher-addressbits from port P2 of IC1 (A8,A9 and A10 from outputpins P2.0, P2.1 and P2.2) areused for generating the chip-select signals from 74LS138(IC4) covering address

ranges 000H-0FFH, 100H-1FF and 200-2FF for RTC, LCD module and PPIchip, respectively.

Quad NAND gate 7400 (IC5) inconjunction with read and write sig-nals from IC1 and chip-select signalfrom pin 14 of IC4 is used for select-ing the LCD module both during readand write cycles of IC1.

PPI chip 8255 is configured withport A, port B and port C as outputports for controlling up to 24 electricalappliances via relays RL1 throughRL24. Relays are energised throughhigh-current octal Darlington arrays

inside ULN2803 (IC7 through IC9) inaccordance with programmed datastored in the non-volatile RAM (NVRAM) of RTC chip DS12887. There isno need of connecting external free-wheeling diodes across relays asinbuilt diodes are provided inULN2803 ICs.

All the 24 devices/electrical appli-ances are considered as 24 bits (three

TABLE II/O Address Range ofPeripheral Devices

Address range Used address range Device name

0000 00FF 0000 to 007F DS12887 (RTC chip)0100 01FF 0100 LCD Module (16×2)0200 02FF 0200 to 0203 8255 (PPI)

Note. Please refer device datasheets for more details.

TABLE IIRC5 Coding Format

S S T A4 A3 A2 A1 A0 C5 C4 C3 C2 C1 C0

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bytes at locations 200H, 201H and202H) of the three ports (ports A, Band C) of 8255. The LCD is used fordisplaying real time (year, month, date,day and time in 24-hour mode) ob-tained from RTC DS12887 as also someother information during time setting,device programming, searching (de-vice-switching programmed data),password entry, etc, as described later.

RTC DS12887 is clock-cum-calen-dar chip with 128 NV RAM (14 bytesused for its control registers and 114bytes as general-purpose RAM). It hasan inbuilt lithium battery and can re-tain stored data for over ten years inthe absence of external power.

Memory map of DS12887 is shownin Table V. Data stored in location07FH (decimal 127) indicates the ad-dress of the last RAM location used.

TABLE IIIRemote Address Codes

Address Device/Equipment

0 TV11 TV22 Videotext3 Expansion for TV1 and TV24 Laser video player5 VCR16 VCR27 Reserved8 Sat1

27-31 Reserved

TABLE IVRemote Command Codes

Button Command Function (as used)

0 – 9 0 – 9 Number keys‘--’ 10 10+‘sfx’ 36 20+Mute 13 Delete taskAC 34 Clear prog memoryPWR 12 Change passwordTimer 38 Change timeSearch 30 Chk existing tasksCH+ 32 See next taskCH- 33 See before taskRCL 15 Turn on/off LCD

back lightPP 14 Enter new taskStore 41 Enable/disable

child lock

The relay-switchingdata that is outputfrom ports A, B and Cof the PPI is stored asconsecutive bits at00EH, 00FH and 010Hlocations of the RAM.The relay/device pro-gramming timing datais stored at five con-secutive locations foreach device. This dataincludes month, date,hour and minute infirst four bytes, whilethe fifth byte contains5-bit address of the de-vice with MSB indicat-ing ‘on’/‘off’ status ofthe device. Bits 6 and 7of this byte are ‘don’tcare’ bits. Address lo-cations 123 through 126are used for storing the4-byte long password.Thus only 106 locationsare available for stor-ing the 5-byte long de-vice data and as suchthe program for amaximum of only 21devices out of 24 de-vices can be stored.The remaining threedevices can beswitched on/offthrough remote keyoperation as explainedbelow.

Bit P1.1 output ofIC1 is fed to transistorBC547 (T1) through R5.Transistor T1 acts likea switch for LCD back-light. So you canswitch the backlight ofLCD ‘on’/‘off’ just bysetting/resetting theP1.1 bit of 89C52.

Power supply (Fig.3). While most of thecircuit requires regu-lated 5V for its opera-tion, the relays andDarlington drivers IC7through IC9(ULN2803) are oper-

TABLE VMemory Map of DS12887

Decimal Memory Location Data description Pointer

0 0000 Seconds Memory usedby Clock

1 0001 Seconds Alarm2 0002 Minutes3 0003 Minutes Alarm4 0004 Hours5 0005 Hours Alarm6 0006 Day of the Week

(Sun=1)7 0007 Date of the month8 0008 Month9 0009 Year10 000A Register A11 000B Register B12 000C Register C13 000D Register D14 000E Data on Port A 24 bits treated

as 24 devices15 000F Data on Port B16 0010 Data on Port C17 0011 Month Prog 1 data18 0012 Date19 0013 Hour20 0014 Minute21 0015 Device # (MSB

indicates ON/OFF)22 0016 Month Prog 2 data23 0017 Date24 0018 Hour25 0019 Minute26 001A Device # (MSB

indicates ON/OFF)….. ….. ….. …..….. ….. ….. …..….. ….. ….. …..….. ….. ….. …..….. ….. ….. …..112 0070 Month Prog 20 data113 0071 Date114 0072 Hour115 0073 Minute116 0074 Device # (MSB

indicates ON/OFF)117 0075 Month Prog 21 data118 0076 Date119 0077 Hour120 0078 Minute121 0079 Device # (MSB

indicates ON/OFF)122 007A Mem. Location

not used Nil123 007B * Used to store

the Password124 007C *125 007D *126 007E *127 007F Pointer value

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directly fed to all the relays andULN2803 ICs. The same output is usedfor regulation by 7805 (IC10). Theripple in the regulator output is fil-tered by capacitor C11.

An actual-size, single-side PCB forthe remotely-programmable RTC-inter-faced microcontroller for multiple de-vice control and power supply circuits

(Figs 1 and 3) is shown in Fig. 4 and itscomponent layout in Fig. 5. The con-nections for relays are to be extendedfrom the 16-pin FRC connectors on thePCB. Each connector is meant for ex-tending connections to eight relays.

The author’s prototype is shownin Fig. 6.

Remote key operationsRefer Table IV for details ofremote buttons/keys. The functions ofthese keys follow:

Keys ‘0’ through ‘9’, ‘--’ and ‘sfx’.Used to switch on/off the 24 devicesmanually. Each time you press any ofthese buttons, the status of the corre-sponding device will toggle, i.e., itsoutput will be the complement of theprevious state.

Button ‘--’ is used as ‘10+’ button.When it is pressed, the system waitsfor around three seconds before thenext button (which should be between‘0’ and ‘9’ to determine the device be-tween ‘10’ and ‘19’) is pressed. Simi-larly, ‘sfx’ is used as ‘20+’ key. Thebutton following the ‘sfx’ buttonshould be between ‘0’ and ‘3’ (as theproject supports only 24 electrical ap-pliances numbering ‘0’ through ‘23’).

ated with unregulated 12V DC sup-ply. A step-down transformer rated at15V AC secondary voltage at 500 mAis used to supply 12V unregulated and5V regulated power to the circuit. Thesecondary output is rectified by 1Arated bridge rectifier BR1 andsmoothed by 1000μF, 35V capacitorC10. The output from the capacitor is

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RCL. Turns the LCD backlight‘on’/‘off.’

PWR. Used to change the 4-digitpassword (initial value ‘0000’). Whenyou press this button, the system willask for the existing password. If thecorrect password is entered, it will askfor the new password. If a wrong pass-word is entered, ‘invalid’ message willflash on the LCD.

Note that the password can be any4-digit value, which need not be thenumbers from ‘0000’ to ‘9999.’ Otherbuttons representing various codes arealso accepted.

Timer. Used to change the realtime. As the circuit operations dependon the real (set) time, changing thesame is password-protected. A valid4-digit password will let youchange/set the time. When youpress ‘timer’ button, the top rowon the LCD defines the format‘Hr:Mn:ScWkDyMnYr.’ You need toenter the valid data as follows:

Hr: 00 to 23 (24-hour mode)Mn: 00 to 59 minutesSc: 00 to 59 secondsWk: 01 to 07 (01 is Sunday)Dy: 01 to 31 datesMn: 01 to 12 (01 is January)Yr: 00 to 99 (year)Any value out of the range will not

be accepted and message ‘invalidvalue’ will be displayed on the LCD.

Store. Enables/disables the childlock function. You can lock the remotekeypad by enabling the child lock.When you press this button, the sys-tem will prompt the message ‘Lock?’or ‘UnLock?’ depending on the presentstatus of the child lock. If ‘1’ is pressed,the child lock feature is enabled/dis-abled. Any button other than ‘1’ willbe treated as zero.

PP. Takes you to programming ofa task.

If the NV RAM is full in DS12887,the message ‘prog memory full’ will

flash on the LCD.If the memory is not full, a new

device program is accepted by display-ing a message in the first line of theLCD as ‘Mn Dt Hr:Mn Dv S’ and ablinking cursor will appear on the sec-ond line to take the data. ‘Mn’ indi-cates ‘month’ (‘01’ to ‘12’), ‘Dt’ indi-cates ‘date’ (‘01’ to ‘31’), ‘Hr’ indicates‘hours’ (‘00’ to ‘23’), ‘Mn’ indicates‘minutes’ (‘00’ to ‘59’), ‘Dv’ indicates‘device number’ (‘00’ to ‘23’) and ‘S’stands for ‘programmed status’ (‘1’ for‘on’ or ‘0’ for ‘off’). Enter the desireddata in this format, which will getstored in the NV RAM of the RTC. Ifmonth (Mn) is entered as ‘00,’ the sametask will repeat every month on thesame date and time. If date (Dt) is en-tered as ‘00,’ the same task will repeatevery day on the same time.

Search. Shows the existing deviceprograms that are stored in the memorystarting from location 011H onwardsone by one. Each time, you need topress CH+/CH- button to move for-ward/backward. In this mode, you maydelete the displayed device programdata entry simply by pressing ‘Mute’button. Then the program that is resid-ing next to this task moves to the loca-tion of the deleted task and the wholememory is refreshed.

See the example shown above forclarity. The pointer value in memorylocation 007FH of DS12887 changesaccordingly.

AC. Deletes the entire programmeddata in one stroke. So use this key verycautiously.

RTC initialisationWhen DS12887 is shipped from the fac-tory, the oscillator is in disabled state.The following program will makeDS12887 work and also reset the pass-word to ‘0000’ and make the programpointer to point to the location 0011H,i.e., it clears the existing tasks by mak-

ing the program memory empty:$MOD52 ORG 00H JMP MAIN ORG 20HMAIN: MOV DPTR, #000AH

MOVX A, @DPTRANL A, #0A0HMOVX @DPTR, AMOV DPTR, #007FHMOV A, #0011HMOVX @DPTR, AMOV A, #00HMOV DPTR, #007BHMOVX @DPTR, AINC DPTRMOVX @DPTR, AINC DPTRMOVX @DPTR, AINC DPTRMOVX @DPTR, AJMP $

END

Before getting started, you need tomake this program run for the firsttime after all the components and ICsare inserted into the circuit. That is, tomake DS12887 work, burn the pro-gram shown above in microcontroller89C52, put the programmedmicrocontroller in the circuit, switchon the circuit for five seconds and thenturn it off. By doing so, the internaloscillator of DS12887 starts oscillatingand IC DS12887 is now ready for use.Now, remove 89C52 from the circuitand load into it the main program tomake the circuit work with all the fea-tures. (For more details on DS12887RTC, refer to the datasheets.)

The monitor program in 89C52 getsthe relevant time data (time, date, day,year, etc) from DS12887 RTC and dis-plays it on the LCD. The data is alsocompared against the user-entereddata (programmed timing data formultiple devices), which had beenstored in the NV RAM of DS12887.When the timing data that was storedin the NV RAM equals the real-timedata fetched from the DS12887, it sets/resets the MSB of the fifth byte of thestored program for that device.

The main software program‘proj.asm’ for the project is given inthe EFY-CD of this month. Beforeburning the code for main program‘proj.asm’ into AT89C52, erase theinitialisation program ‘rtcint.asm’ thatis programmed initially into it.

EFY note. The software and datafiles required for the project are in-cluded in this month’s EFY-CD.

Program memory in DS12887

Task 1 Task 2 Task 3 … … Task ‘n–1’ Task ‘n’

When Task 2 is deleted, the whole memory is refreshed as:

Task 1 Task 3 … … Task ‘n–1’ replaces Task ‘n’ replaces Emptytask ‘n–2’ task ‘n–1’

Example