MicroController Based Moving Message Display

Mini Project Report

MICROCONTROLLER BASED MOVING MESSAGE DISPLAY

1. INTRODUCTION LED-based moving-message displays are becoming popular for transmitting information to large groups of people quickly. These can be used indoors or outdoors. We can find such displays in areas like railway platforms, banks, public offices, hotels, training institutes, nightclubs and shops. Compared to LEDs, liquid-crystal displays (LCDs) are easy to interface with a microcontroller for displaying information as these have many built-in functions. But these can’t be observed from a distance and large size LCDs are very costly. We preferred to use 16 single digit alphanumeric displays over the LED dot-matrix type since the former is much cost effective and has less programming burden compared to other. We have used Atmel’s AT89C51 microcontroller as the heart of the circuit along with IC 74LS138 which is a 3to8 decoder, BC558 transistors, LED displays and power supply unit in the circuit. We have programmed to move the message from the rightmost display to the left and the message stayed stationary for a few seconds when the first character reaches the leftmost display, then it continues to move. A 4 pin dip switch connected to the microcontroller through a port is used to select the desired message stored in the memory of the microcontroller. The microcontroller provides the data signal to the 16 display units through other two ports. Another port is used to provide the address of the displays to the 3to8 decoders which are actually controlling the turning on and off of the displays.

Rajagiri School of Engineering and Technology

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Mini Project Report


The report provides a brief idea about the project through the block diagram explanation given in the first part. It is followed by a circuit diagram and circuit description given in a nice manner. Then come the software part of the project, it includes a software description, algorithm and finally the software code. The PCB designs are provided in the final part of the report. Some random screenshots of the software used for the program debugging and circuit simulation in also provided.


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2. SYSTEM INFORMATION 2.1 BLOCK DIAGRAM

4 PIN DIP Switch

LED Display AT89C51 Microcontroller Decoder

2.2 BLOCK DIAGRAM EXPLANATION 

An at89c51 microcontroller is the heart of the circuit. It has two timers to control the display timing and it also controls the entire data transfer in the circuit.



A 4 pin dip switch is connected to microcontroller which gives the input information to the microcontroller. Each pin in the dip switch has two states 0 and 1, by using 4 pins we can actually give 16 different input signals to the microcontroller.



The output data is displayed through a set of 16 single segment alphanumeric LED displays connected to the microcontroller.

 A decoder is connected to microcontroller which receives the address information to control the turning on and off of the LED displays.


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3. HARDWARE IMPLEMENTATION 3.1 CIRCUIT DIAGRAM


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3.2 CIRCUIT DESCRIPTION The diagram above shows the circuit of the microcontroller based moving message display. It comprises microcontroller AT89C51, three-to-eight decoder 74LS138, common anode alphanumeric displays, regulator 7805 and a few discrete components. At the heart of the moving-message display is Atmel AT89C51 microcontroller (IC1). It is a low-power, high-performance, 8-bit microcontroller with 4 kB of flash programmable and erasable read-only memory (PEROM) used as on-chip program memory, 128 bytes of RAM used as internal data memory, 32 individually programmable input/output (I/O) lines divided into four 8-bit ports, two 16-bit programmable timers/counters, a five-vector two-level interrupt architecture, on-chip oscillator and clock circuitry. Ports P0 and P2 of the microcontroller have been configured to act as a common data bus for all the 16 alphanumeric displays whose corresponding data pins have been tied together to make a common 16-bit data bus. Port-2 provides the higher byte of data, while port-0 provides the lower one to light up a character on the display. Port pins P1.2-P1.4 and P1.5-P1.7 of the microcontroller have been used as address inputs for decoder IC3 and IC4 (74LS138) to enable one of the fourteen alphanumeric displays (DIS3 through DIS16) at a time, respectively. However, displays DIS1 and DIS2 are enabled or disabled directly by port pins P1.0 and P1.1. Pins 4 and 5 are grounded and pin 6 is made high to enable decoder 74LS138. All the corresponding data pins Dis 1 through DIS16 of alphanumeric displays have been tied together, while the common anode of each display is separately powered via a BC558 transistor which switches ‘on’ or ‘off’ as required, through outputs of 74LS138 ICs and pins P1.0 and P1.1 of IC1. The higher nibble of port P3 (P3.4 through P3.7) is used as a selection bus to select one of the 16


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previously stored messages using the 4-bit binary value present on these pins. This value can be changed through a 4-pin DIP switch (S0 through S3). Selection pins P3.4 through P3.7 are pulled high via resistors R36 through R33, respectively. When the switch connected to a given pin is open the value is high (1), and when it is closed the pin is held low and the value becomes ‘0.’ In this way, by using a 4-bit number you can select any of the 16 messages stored in ROM Capacitor C5 and resistor R37 form the power-‘on’ reset circuit, while a push-toconnect switch has been used for manual reset. An 11.0592MHz crystal generates the basic clock frequency for the microcontroller. To change the message being displayed while the circuit is working, first change the number present at the selection bus, then press ‘reset’ key. The 220V AC mains is stepped down by transformer X1 to deliver the secondary output of 9V, 500 mA. The output of the transformer is rectified by a full-wave bridge rectifier comprising diodes D1 through D4, filtered by capacitor C3 and then regulated by IC 7805 (IC4). Capacitor C4 bypasses any ripple present in the regulated power supply. LED1 acts as the power-‘on’ indicator.


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3.3 CIRCUIT SIMULATION We used software named Proteus 7 for simulating the working of our project, especially the timer part. Some of the screenshots of the program is included here.


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4. SOFTWARE IMPLEMENTATION

4.1 SOFTWARE DESCRIPTION The software is written in assembly language and its main concept is as follows. Timer 1 has been used to generate a delay of around 1 ms for the switching gap between two consecutive displays. Thus, each display is enabled for 1 ms while displaying a message. The length of this cycle depends upon the length of the message string. The cycle repeats after a ‘0’ is encountered at the end of each message stored in the look-up table at the end of the program. Each time, to display a character at a given display, first two bytes (16 bits) of data are sent to Port-2 and Port-0, then the desired display is enabled by sending its address to Port-1. Thereafter, a delay of 1 ms (slightly more than that) is generated by timer 1. Upon timer overflow, the entire display panel is refreshed by passing ‘FFFFH’ to the data bus. Then the next character at the next display is passed in the similar manner. The cycle frequency is variable (depending upon the length of the message) but always high enough so that the message appears continuous to the human eye. Timer 0, with its interrupt enabled, is used to change the starting address of the message in cyclic manner so that the characters scroll from left to right with a proper gap between each shift. Meanwhile, the interrupt service sub-routine also checks for the starting address of DIS16 (right-most display). As soon as the first character reaches DIS16, the message stays for a longer time so that the entire message (message length not longer than 16 characters) can be easily read. Thereafter, characters again start scrolling rightwards, so the entire message goes out and disappears after a while to reappear from left side. All the messages are stored in the form of a look-up table in the program memory (ROM) itself. When the circuit is switched ‘on’ (or reset), the monitoring program


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first checks for the binary number present at the selection bus and according to that, the ROM address of the starting character of the selected message is loaded into the data-pointer. Thereafter, on-chip ROM reading is used to read the entire message over there. Each character is represented in the look-up table of the source code by two bytes. For example, ‘S’ is represented by ‘Sh’ and ‘Sl’ separated by a comma. In addition to the alphabets, Arabic numerals and a few special characters have been defined in the program. For instance, a blank space is represented by ‘bsh, bsl.’ Thus, it is very easy to modify the program.


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4.2 SOFTWARE DEVELOPMENT We used software named Keil µVision 3 for building the target software and debugging it. We could analyze each and every data bit in the ROM and RAM throughout the program execution along with the states of all the 4 ports of the microcontroller. We could also analyze the working of the timers according to the written program. Some screen shots of the program is included here.


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4.3 ALGORITHM Algorithm for at89c51 programming 

Port 1 is used as address bus



Port 2 is used as higher order data bus



Port 0 is used as lower order data bus

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Select inputs are given to the higher nibble of Port 3



Since a 16 segment display is used, each character is represented by two 8 bit values as DBH at port 2 and DBL at port 0

// Program execution starts from here. 

Main



Set global interrupt bit



Enable timer 0 interrupt



Timer 0 is configured in mode 1



Set initial count of timer 0 to 00H



Initialize the RAM address locations starting from 30H to 60H as FFH

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Set the address for displays from 1st to 16th in memory locations starting from 41H to 50H ( data given to decoder to enable each display )

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Read the data from port 3 to accumulator ( select inputs )

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Mask lower nibble of accumulator



According to data in accumulator load the data pointer with the base address of the corresponding message stored in the look up table in the RAM

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Store the higher and lower bits of data pointer in register R3 and R2



Initialize register R1 with 41H ( address of left most display )



Start the timer 0

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Step A 

Copy values in R1 to R0


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


Move the data pointed by data pointer to port 2 and port 0 through accumulator



Move the data ( address of displays ) pointed by R0 to port 1

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Start timer 1 in mode 1 for generating a delay of 1ms ( delay between each display )

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

When timer 1 overflows ( interrupts ) turn the display off and continue

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Decrement R0



Increment data pointer, so that next character of message is loaded



Go back to Step A and repeat these steps until the accumulator = 0

If accumulator = 0, reload the data pointer with the base address of selected message using registers R3 and R2 and go back to Step A

// Interrupt Service Routine for Timer 0 ( executed when timer 0 overflows ) 

Initialize R6 with 10 in Main at the beginning of program execution



Initialize R7 with 46 in Main at the beginning of program execution so that timer 0 need to overflow 46 times before R1 increments ( by one ) to changing the starting display



Start timer 0 again and return back

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Continue above steps till R1 reaches 50H i.e. the first letter in selected message reaches the rightmost display

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When R1 = 50H, decrement R6 and restart timer 0 and return back, so that display remains stationary till R6 =0



When R6 = 0, increment R1 and restart timer 0 and return back.



When R1 = 60H, reload R1 with 41H i.e. the address of the leftmost display and R6 with 10


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4.4 SOFTWARE CODE

$mod51 DBH equ p2

; Higher byte of Data Bus

DBL equ p0

; Lower byte of Data Bus

ADB equ p1

; Address Bus

input equ p3

; message select input

;** codes for decimal digits are given below: ; ('h' refers to higher byte, 'l' to lower one) zeroh equ 85h zerol equ 0d0h oneh equ 0d7h onel equ 0ffh twoh equ 0b2h twol equ 8dh threeh equ 92h threel equ 0cdh fourh equ 0c2h fourl equ 0ffh fiveh equ 8ah fivel equ 0cdh


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sixh equ 8ah sixl equ 8dh sevenh equ 0d7h sevenl equ 0ddh eighth equ 82h eightl equ 8dh nineh equ 82h ninel equ 0cdh ;** codes for alphabets are given below: Ah equ 0c2h Al equ 9dh Bh equ 82h Bl equ 8dh Ch equ 0afh Cl equ 8dh Dh equ 87h Dl equ 8dh Eh equ 0aah El equ 8dh Fh equ 0eah Fl equ 9dh GH equ 8dh Rajagiri School of Engineering and Technology

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Gl equ 8dh Hh equ 0c2h Hl equ 0bfh Ih equ 0bfh Il equ 0c4h Jh equ 0bfh Jl equ 0b4h Kh equ 6ch Kl equ 0bfh Lh equ 0afh Ll equ 0afh Mh equ 0c5h Ml equ 0bbh Nh equ 47h Nl equ 0bbh Oh equ 87h Ol equ 8dh Ph equ 0e2h Pl equ 9dh Qh equ 27h Ql equ 9dh Rh equ 62h Rajagiri School of Engineering and Technology

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Rlw equ 9dh Sh equ 8ah Sl equ 0cdh Th equ 0ffh Tl equ 0d4h Uh equ 87h Ul equ 0afh Vh equ 57h Vl equ 0fbh Wh equ 87h Wl equ 0aeh Xh equ 7dh Xl equ 7bh Yh equ 0fbh Yl equ 0fah Zh equ 0bdh Zl equ 4dh ;** codes for few special characters: strh equ 78h

;for star sign (asterisk)

strl equ 72h plsh equ 0fah

;for '+' sign

plsl equ 0f6h Rajagiri School of Engineering and Technology

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mnsh equ 0fah


;minus sign

mnsl equ 0ffh _h

equ 0bfh

_l

equ 0efh

bsh

equ 0ffh

bsl

equ 0ffh

pieh equ 0d7h

; underscore sign

;blank space

;for pie

piel equ 0f4h mueh equ 0dfh

;for micro (mu)

muel equ 0eeh org 0000h sjmp main org 000bh clr tr0

;timer0 interrupt vector address ;clear timer0 run bit

mov tl0,#00h mov th0,#00h

;reload timer0 with initial count

djnz r7,a1 mov r7,#46 cjne r1,#60h,a5

;check to again start entering from left-side

sjmp a4 a5: cjne r1,#50h,a2

;check for display to stay on reaching display-16

sjmp a3 a2: inc r1 sjmp a1 a3: djnz r6,a1 inc r1 Rajagiri School of Engineering and Technology

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sjmp a1 a4: mov r6,#10 mov r1,#41h a1: setb tr0 reti

; set timer0 run bit ;return from timer0 ISR and clear tf0

main: mov ie,#00h setb ea

;set global interrupt bit

setb et0

;enable timer0 interrupt

mov tmod,#01h

;timer0 configured in mode 1

mov tcon,#00h mov tl0,#00h mov th0,#00h

;set initial count to 0000H

mov r7,#46

;provides gap between each shift

mov r6,#10 mov r0,#60h blank: mov @r0,#0ffh

;initialize the pointed location by null address

dec r0 cjne r0,#2fh,blank mov r1,#41h

;load address-pointer with initial address

mov 50h,#0dfh

;address for 16th Display (rightmost)

mov 4fh,#0bfh

;address for 15th Display

mov 4eh,#9fh


mov 4dh,#7fh


mov 4ch,#5fh


mov 4bh,#3fh


mov 4ah,#1fh


mov 49h,#0fbh


mov 48h,#0f7h


mov 47h,#0f3h



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mov 46h,#0efh


mov 45h,#0ebh


mov 44h,#0e7h


mov 43h,#0e3h

;address for 3rd Display

mov 42h,#0fdh

;address for 2nd Display

mov 41h,#0feh

;address for 1st Display (leftmost)

chk: mov a,input

;load accumulator with value at P3

orl a,#0fh

;mask lower nibble to get selection bus value

cjne a,#0ffh,chk0 mov dptr,#default

;load dptr with starting address of default message

sjmp read

; now start reading

chk0: cjne a,#0fh,chk1 mov dptr,#msg0

;load dptr with starting address of msg0

sjmp read

; now start reading

chk1: cjne a,#1fh,chk2 mov dptr,#msg1 sjmp read chk2: cjne a,#2fh,chk3 mov dptr,#msg2 sjmp read chk3: cjne a,#3fh,chk4 mov dptr,#msg3 sjmp read chk4: cjne a,#4fh,chk5 mov dptr,#msg4 sjmp read Rajagiri School of Engineering and Technology

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chk5: cjne a,#5fh,chk6 mov dptr,#msg5 sjmp read chk6: cjne a,#6fh,chk7 mov dptr,#msg6 sjmp read chk7: cjne a,#7fh,chk8 mov dptr,#msg7 sjmp read chk8: cjne a,#8fh,chk9 mov dptr,#msg8 sjmp read chk9: cjne a,#9fh,chk10 mov dptr,#msg9 sjmp read chk10: cjne a,#0afh,chk11 mov dptr,#msg10 sjmp read chk11: cjne a,#0bfh,chk12 mov dptr,#msg11 sjmp read chk12: cjne a,#0cfh,chk13 mov dptr,#msg12 sjmp read


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chk13: cjne a,#0dfh,chk14 mov dptr,#msg13 sjmp read chk14: mov dptr,#msg14 sjmp read read: mov r3,dph mov r2,dpl setb tr0 rd1: mov r0,01h rd2: clr a movc a,@a+dptr jz down mov DBH,a clr a inc dptr movc a,@a+dptr mov DBL,a mov ADB,@r0 acall timer dec r0 inc dptr sjmp rd2 down: mov dph,r3

;reload dph

mov dpl,r2

;reload dpl

sjmp rd1 timer: mov tmod,#10h mov th1,#0fch

;set mode 1 for timer1 ;FC66H will generate a delay of 1ms with 11.0592MHz

Xtal mov tl1,#66h setb tr1 Rajagiri School of Engineering and Technology

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jnb tf1,$


;wait until timer1 overflows

clr tr1 clr tf1 mov DBH,#0ffh mov DBL,#0ffh ret ;** look-up table starts from here: msg0:

db

Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Bh,Bl,Ih,Il,Rh,Rlw,Th,Tl,Hh,Hl,bsh,bsl,Dh,D l,Ah,Al,Yh,Yl,0 msg1:

db

Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Nh,Nl,Eh,El,Wh,Wl,bsh,bsl,Yh,Yl,Eh,El,Ah, Al,Rh,Rlw,0 msg2:

db

strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Dh,Dl,Ih,Il,Wh,Wl,Ah,Al,Lh, Ll,Ih,Il,bsh,bsl,strh,strl,0 msg3:

db

Mh,Ml,Eh,El,Rh,Rlw,Rh,Rlw,Yh,Yl,bsh,bsl,Ch,Cl,Hh,Hl,Rh,Rlw,Ih,Il,Sh,Sl,Th,Tl,M h,Ml,Ah,Al,Sh,Sl,0 msg4:

db

strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Hh,Hl,Oh,Ol,Lh,Ll,Ih,Il,bsh,b sl,strh,strl,0 msg5:

db

strh,strl,bsh,bsl,Eh,El,Ih,Il,Dh,Dl,bsh,bsl,Mh,Ml,Uh,Ul,Bh,Bl,Ah,Al,Rh,Rlw,Ah,Al,K h,Kl,bsh,bsl,strh,strl,0 msg6:

db

Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Dh,Dl,Ah,Al,Sh,Sl,Hh,Hl,Eh,El,Hh,Hl,Rh,Rl w,Ah,Al,0 msg7:

db

Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Wh,Wl,Eh,El,Dh,Dl,Dh,Dl,Ih,Il,Nh,Nl,Gh,Gl, 0 Rajagiri School of Engineering and Technology

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msg8:


db

Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Jh,Jl,Ah,Al,Nh,Nl,Mh,Ml,Ah,Al,Sh,Sl,Hh,Hl, Th,Tl,Mh,Ml,Ih,Il,0 msg9:

db

strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Rh,Rlw,Ah,Al,Kh,Kl,Hh,Hl,I h,Il,bsh,bsl,strh,strl,0 msg10: db strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Ph,Pl,Oh,Ol,Nh,Nl,Gh,Gl,Ah, Al,Lh,Ll,bsh,bsl,strh,strl,0 msg11: db Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Mh,Ml,Oh,Ol,Th,Tl,Hh,Hl,Eh,El,Rh,Rlw,Sh, Sl,Dh,Dl,Ah,Al,Yh,Yl,0 msg12: db strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Rh,Rlw,Ah,Al,Mh,Ml,Jh,Jl,A h,Al,Nh,Nl,bsh,bsl,strh,strl,0 msg13: db strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Lh,Ll,Oh,Ol,Hh,Hl,Rh,Rlw,Ih ,Il,bsh,bsl,strh,strl,0 msg14: db strh,strl,bsh,bsl,Hh,Hl,Ah,Al,Ph,Pl,Ph,Pl,Yh,Yl,bsh,bsl,Eh,El,Ah,Al,Sh,Sl,Th,Tl,Eh,E l,Rh,Rlw,bsh,bsl,strh,strl,0 default: db Wh,Wl,Eh,El,Lh,Ll,Ch,Cl,Oh,Ol,Mh,Ml,Eh,El,bsh,bsl,Th,Tl,Oh,Ol,bsh,bsl,Ah,Al,Lh, Ll,Lh,Ll,0 end


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5. PCB DESIGN

Acual-size, single side PCB for Microcontroller Based Moving Message Display except LED Display part.

Single side PCB for LED Display part of Microcontroller Based Moving Message Display. Not Actual Size.


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6. RESULT The breadboard prototype of the circuit was made and tested for a single display. It was verified by the guide. Then the circuit including the all display units was mounted on PCB. The selected messages moved from the leftmost display to the right and it stayed stationary for a few seconds, when the first character reached the leftmost display. Thus the desired result was obtained.


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7. CONCLUSION It may be concluded that the mini project has helped us to develop a deep practical knowledge of the at89c51 microcontroller. We have dealt with the timer programming and the interrupt programming of the microcontroller. The LED displays proved to very cost effective and simple to program compared to others. We could also use the software like Proteus 7 and Keil µVision 3 that are very indispensible in embedded software development.


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8. FUTURE SCOPE Many more messages would be possible if complete Port-3 is used for message selection. Pins RxD, TxD, INT0 and INT1 have been kept free, so that these can be used for interfacing with the serial port of the PC. Also, interrupt pins can be used to display some message and sound an alarm in the case of an emergency. For example, a fire sensor can be connected to ‘INT0’ and a vibration detector to ‘INT1.’ These pins can also be used to send signals to synchronise a similar system that displays another related message at the same time, so a 16-character, twoline display is made possible. A PC keyboard can be interfaced with microcontroller so that messages can display on the fly.


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9. REFERENCES The 8051 Microcontroller and Embedded Systems by Muhammad Ali Mazidi Websites  www.atmel.com  www.alldatasheets.com


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APPENDIX


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MicroController Based Moving Message Display

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