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A liquid crystal display driven from a PC printer port Last month we published a demo board which let you manu­ally feed text into a one-line alphanumeric display by manipulat­ing switches. This month we take the same one-line display and hook it to the parallel port of a PC to produce moving messag­es. By RICK WALTERS 66  Silicon Chip Fig.1: horribly complicated, innit? The display requires 8 bits of data, a register select line, a strobe line and a 5V supply. Fig.2: the component overlay for the PC board. Parts List 1 PC board, code 04104981, 81 x 58mm 1 Liquid Crystal Display with HD44780 controller 1 25-way “D” male PC mount connector 1 20kΩ PC trimpot 1 100µF 25VW PC electrolytic capacitor 1 0.1µF MKT polyester capacitor 1 15-pin “D” male connector (for +5V from games port) Fig.3: actual size artwork for the PC board. D ID YOU DECIDE to have a go at the demo board last month or did you feel that all those switches were just not worth the has­ sle? Well, some of us at SILICON CHIP are quite lazy too and they (no names, no pack drill) thought that this sort of drudgery is the sort of task that com­ puters are ideal for. So the boss told me, “Some readers won’t be bothered with all that. You’d better do a version which can be driven from the parallel port of a PC”. Naturally I saluted and said “It will be done”. And here it is. So if you want all the hard work to be done for you by a computer so that you can display moving messages or create, save and display your own symbols, get stuck into this extremely simple project. It uses the same one-line 16-char­ acter Liquid Crystal Dis­play (LCD) as previously with an HD44780 control­ ler. This is mounted on a small PC board with a 25-way male D connector to mate with a standard 25-way cable from the parallel port. A trimpot and a couple of capacitors complete the assembly. The trimpot is used to ad­ just the optimum contrast for different LCD operating param­ eters. The 5V for the display can be provided by a power supply or taken from the games port of the computer. You could even use the 9V battery and regulator setup as in the previous article. To get the display working requires eight bits of data, a reg­ister select line, a strobe line and a 5V supply. The circuit arrangement is shown Fig.1. The 8-bit data we use for the display is the same eight bits that a printer uses and we also use two of the printer control lines for the other functions we need; ie, register select (RS) and strobe. When the register select line is low (ground) it allows the computer to send internal commands to the LCD and when it is high (5V) we can write to the LCD screen. The printer port line we use to control the RS line is port C bit 3. This is the only line from this port that does not have inverted logic. When logic highs are output from the computer to port C, only bit 3 will be high; the other three bits (1, 2 and 4) will be low (inverted). From this you can see that when we require the March 1998  67 Listing 1: LCD Driver Program 10 REM Driver program for single line LC Display 11 ‘driven from the parallel port of a PC 12 ‘R.W. 23/11/97 V1.0 20 GOSUB 1000 ‘Initialise software routines 30 GOSUB 2000 ‘Initialise display 40 GOSUB 3000 ‘Write data to display 50 GOSUB 6000 ‘Scroll message back & forth 60 GOSUB 7000 ‘Scroll message to left & wrap 70 GOSUB 8000 ‘Create your own character 999 END’ CLS: SYSTEM 1000 ‘*********************** 1010 ‘Initialisation routine. 1020 ‘*********************** 1030 KEY OFF: CLS: DEFINT A,B,S: DEFSTR D,K 1035 ‘A,B,S integers, D,K are strings, rest single precision 1040 DEF FNCEOL$ = STRING$(79 - POS(Q),” “) 1050 DEF FNCENTRE$(M$) = SPACE$((78-LEN(M$))/2) + M$ 1400 PORTA = &H378 ‘This is LPT1, use &H278 for LPT2 1410 PORTC = PORTA + 2 ‘Define Port C. Port B is not used 1420 COMMAND = 0: CHR = 4: STH = 0: STL = 1 ‘Define function & STrobe 1999 RETURN 2000 ‘******************* 2010 ‘Initialise display. 2020 ‘******************* 2030 FUNCT = COMMAND: OUT PORTA,0 ‘Define RS & set LCD inputs low 2040 GOSUB 4040 ‘clock RS line low 2050 OUT PORTA,48: GOSUB 4000 2060 T = TIMER: WHILE T + .03 > TIMER: WEND ‘wait 30ms 2070 GOSUB 4000 2080 T = TIMER: WHILE T + .01 > TIMER: WEND ‘wait 10ms 2090 GOSUB 4000 2100 OUT PORTA,60: GOSUB 4000 ’16 character display 2110 OUT PORTA,14: GOSUB 4000 ‘display & cursor ON, UNDERLINE cursor 2120 OUT PORTA,1: GOSUB 4000 ‘clear home 2130 T = TIMER: WHILE T + .01 > TIMER: WEND ‘clear home can take 5ms 2999 RETURN 3000 ‘******************* 3010 ‘Write data to Port. 3020 ‘******************* 3030 FUNCT = CHR ‘Set register select line high 3040 GOSUB 4040 ‘Clock RS line high 3050 DAT = “Silicon Chip is a great magazine.” 3060 FOR A = 1 TO LEN(DAT): TOWRITE = ASC(MID$(DAT,A,1)) 3070 OUT PORTA,TOWRITE 3080 GOSUB 4000 ‘Clock full message into LCD from left hand edge 3090 NEXT 3100 GOSUB 5000 ‘Locate cursor at 64 decimal 3110 FOR A = 9 TO LEN(DAT): TOWRITE = ASC(MID$(DAT,A,1)) ‘Write nessage 3120 OUT PORTA,TOWRITE ‘starting at position 9 i.e. “C” for correct scroll 3130 GOSUB 4000 ‘Clock data into LCD 3140 NEXT 3999 RETURN 4000 ‘********** 4010 ‘Clock LCD. 4020 ‘********** 4030 OUT PORTC,FUNCT OR STL ‘Set R/S line & take strobe low 4040 OUT PORTC,FUNCT OR STH ‘Set R/S line, take strobe high. 4999 RETURN 5000 ‘*************************** 5010 ‘Shift cursor to Position 9. 5020 ‘*************************** 5030 FUNCT = COMMAND: OUT PORTC,FUNCT OR STH ‘Set command mode 5040 OUT PORTA,64: GOSUB 4000 ‘Set CG to 9 . . . continued next page 68  Silicon Chip strobe to be high we actually have to output a low and vice versa. If you look at Listing 1, on line 1420 you will see that STH (strobe high) is defined as 0 (low) and STL (strobe low) is defined as 1 (high). This allows us to forget about the logic inversion and just select the strobe logic level we want. The strobe line is normally held high and is taken low to transfer information from the data lines to the display. The LCD commands for the HD44780 controller are listed in Ta­ ble 1 and the character set is listed in Table 2, as published in last month’s issue. After initialising the LCD, the software (Listing 1), firstly writes a string of text, which is longer than the 16 character window, to the display. This message then scrolls backwards and forwards until a key is pressed, whereupon it will scroll in a contin­ uous loop, disappearing on the left side of the window, then scrolling in from the right. Another keypress will clear the display screen then write a custom symbol to the first eight positions. The Listing should make it clear how you code the symbol you want. With a little ingenuity you can create and place two or three symbols side by side to make a shape. With each of these functions writ­ ten as a complete subrou­ tine, you should have no trouble customising the Listing to suit your particular needs. Board assembly & testing We will not insult you by telling you how to assemble the PC board. It should be quite apparent from the overlay dia­gram of Fig.2 and should not take more than a few minutes. Be sure to double check the orientation of the electrolytic capaci­tor for correct polarity. The display will only draw 4-5mA from its power supply so it does not impose any significant load. If you intend to power the display from the PC, connect a wire from the battery positive terminal to pin 1 on the 15way “D” connector, then plug the PC board into the end of the cable, the cable into the printer port and the 15 way connector into the games port. Turn the computer on, load GW-Ba­ sic or Q-Basic and type in listing 1. If this gives you the horrors, you can get the com­plete listing from SILICON CHIP on a floppy disc at $7 plus $3 postage and packing. Make sure that you enter the printer port you intend to use on line 1400. When you run the program the results should be as previously described. To change the message just enter a different string for DAT in line 3050. Don’t forget to enclose it in quota­ tion marks and remember it is case sensitive. Understanding the display The single line display we have used has a few quirks. Instead of the addresses of the 16 visible characters being continuous from 0-15, the first eight characters reside in addresses 0-7, while the second visible group of eight reside at addresses 64-71. All addresses are given in decimal as this is how both versions of Basic communicate with the printer port. Note that we are talking about vis­ ible addresses but there are actually 80 addresses available, 0-40 and 64104. This is the reason for subroutine 5000. After writing the full string of 33 characters starting from address 0 (lines 3060-3090), only the first eight will display. We then call subroutine 5000 to move the cursor to display position 9 or location 64. We then write from the 9th character to the 33rd again but only the next eight (“Chip is”) will show. By changing or adding lines be­ tween 40 and 999, changing the mes­ sage in subroutine 3000 and creating your own characters, you should soon become an expert with Liquid Crystal Displays. Acknowledgement Our thanks to Branco Justic of Oat­ ley Electronics for assistance in the SC development of this project. How To Get The Software The software shown in Listing 1 is available on floppy disc for $7 plus $3 postage and packing. Send your remittance or credit card authorisation (Bankcard, Visa or Mastercard) to Silicon Chip Publications, PO Box 139, Collaroy Beach, NSW 2097. Don’t forget to nominate whether you want a 3.5-inch or 5.25-inch floppy disc. Listing 1: LCD Driver Program – continued 5050 OUT PORTA,192: GOSUB 4000 ‘Set DD to 9 5060 FUNCT = CHR: OUT PORTC,FUNCT OR STH ‘Restore character function 5999 RETURN 6000 ‘************************ 6010 ‘Scroll message to & fro. 6020 ‘************************ 6030 FUNCT = COMMAND: OUT PORTC,FUNCT OR STH ‘Set command mode 6040 LOCATE 25,1: PRINT FNCENTRE$(“Press a key to end this demon­stration.”); 6050 OUT PORTA,24 ‘Shift data left 6060 FOR A = 1 TO LEN(DAT) - 1: GOSUB 4000 6070 K = INKEY$: IF K > “” THEN 6990 6080 T = TIMER: WHILE T + .5 > TIMER: WEND ‘Wait 0.5 seconds 6090 NEXT 6100 OUT PORTA,28 ‘Shift data right 6110 FOR A = 1 TO LEN(DAT) - 1: GOSUB 4000 6120 K = INKEY$: IF K > “” THEN 6990 6130 T = TIMER: WHILE T + .5 > TIMER: WEND 6140 NEXT 6150 GOTO 6050 6990 LOCATE 25,1: PRINT FNCEOL$;: LOCATE 1,1 6999 RETURN 7000 ‘*********************************** 7010 ‘Scroll message to left continuously. 7020 ‘************************************ 7030 FUNCT = COMMAND: OUT PORTC,FUNCT OR STH ‘Set command mode 7040 LOCATE 25,1: PRINT FNCENTRE$(“Press a key to end this demon­stration.”); 7050 OUT PORTA,24 ‘Shift data left 7060 FOR A = 1 TO LEN(DAT) * 32: GOSUB 4000 7070 K = INKEY$: IF K > “” THEN 7990 7080 T = TIMER: WHILE T + .5 > TIMER: WEND 7090 NEXT 7990 LOCATE 25,1: PRINT FNCEOL$;: LOCATE 1,1 7999 RETURN 8000 ‘************************** 8010 ‘Create your own character. 8020 ‘************************** 8030 ‘000 01110 decimal 14 The 1’s represent pixels that will 8040 ‘000 11011 decimal 27 be written (black) 8050 ‘000 00000 decimal 0 The 0’s will appear as background 8060 ‘000 00100 decimal 4 so the character will have the shape 8070 ‘000 10101 decimal 21 of the 1’s i.e. a crude smiley face 8080 ‘000 01010 decimal 10 Only the right 5 rows are significant 8090 ‘000 00100 decimal 4 The left 3 rows are ignored 8100 ‘000 00000 decimal 0 8110 FUNCT = COMMAND: OUT PORTC,FUNCT OR STH ‘set command mode 8120 OUT PORTA,1: GOSUB 4000 ‘clear home 8130 T = TIMER: WHILE T + .01 > TIMER: WEND ‘clear home can take 5ms 8140 OUT PORTA,64: GOSUB 4000 ‘Set RAM address 01000000 8150 FUNCT = CHR: GOSUB 4040 ‘Set character mode 8160 OUT PORTA,14: GOSUB 4000 ‘Start sending your character 8170 OUT PORTA,27: GOSUB 4000 8180 OUT PORTA,0: GOSUB 4000 8190 OUT PORTA,4: GOSUB 4000 8200 OUT PORTA,21: GOSUB 4000 8210 OUT PORTA,10: GOSUB 4000 8220 OUT PORTA,4: GOSUB 4000 8230 OUT PORTA,0: GOSUB 4000 ‘After the 8th bit 8240 FUNCT = COMMAND: GOSUB 4040 ‘change the RS line 8250 OUT PORTA,1: GOSUB 4000 ‘and clear/home the display 8260 T = TIMER: WHILE T + .01 > TIMER: WEND ‘Clear home can take 5ms 8270 FUNCT = CHR: GOSUB 4040 ‘Revert to character mode 8280 OUT PORTA,0: GOSUB 4000 ‘and load our character stored at location 0 8290 FOR B = 1 TO 7:GOSUB 4000: NEXT ‘Then write it to the next 7 locations 8999 RETURN March 1998  69