Statement: Design a microprocessor system to control traffic lights. The traffic light arrangement is as shown in Fig. The traffic should be controlled in the following manner.
1) Allow traffic from W to E and E to W transition for 20 seconds. 2) Give transition period of 5 seconds (Yellow bulbs ON) 3) Allow traffic from N to 5 and 5 to N for 20 seconds 4) Give transition period of 5 seconds (Yellow bulbs ON) 5) Repeat the process.

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HARDWARE FOR TRAFFIC LIGHT CONTROL
Source program:

       MVI A, 80H                        : Initialize 8255, port A and port B
       OUT 83H (CR)                : in output mode
START: MVI A, 09H
       OUT 80H (PA)                : Send data on PA to glow R1 and R2
       MVI A, 24H
       OUT 81H (PB)                : Send data on PB to glow G3 and G4
       MVI C, 28H                        : Load multiplier count (40ıο) for delay
       CALL DELAY                        : Call delay subroutine
       MVI A, 12H
       OUT (81H) PA                : Send data on Port A to glow Y1 and Y2
       OUT (81H) PB                : Send data on port B to glow Y3 and Y4
       MVI C, 0AH                        : Load multiplier count (10ıο) for delay
CALL: DELAY                                : Call delay subroutine
       MVI A, 24H
       OUT (80H) PA                : Send data on port A to glow G1 and G2
       MVI A, 09H
       OUT (81H) PB                : Send data on port B to glow R3 and R4
       MVI C, 28H                        : Load multiplier count (40ıο) for delay
       CALL DELAY                        : Call delay subroutine
       MVI A, 12H
       OUT PA                        : Send data on port A to glow Y1 and Y2
       OUT PB                        : Send data on port B to glow Y3 and Y4
       MVI C, 0AH                        : Load multiplier count (10ıο) for delay
       CALL DELAY                        : Call delay subroutine
       JMP START


Delay Subroutine:

DELAY: LXI D, Count                : Load count to give 0.5 sec delay
BACK: DCX D                                : Decrement counter
       MOV A, D
       ORA E                                : Check whether count is 0
       JNZ BACK                        : If not zero, repeat
       DCR C                                : Check if multiplier zero, otherwise repeat
       JNZ DELAY
       RET                                : Return to main program

INTERFACING DIAGRAM




Fig. shows the interfacing diagram to control 12 electric bulbs. Port A is used to control lights on N-S road and Port B is used to control lights on W-E road. Actual pin connections are listed in Table 1 below.
The electric bulbs are controlled by relays. The 8255 pins are used to control relay on-off action with the help of relay driver circuits. The driver circuit includes 12 transistors to drive 12 relays. Fig. also shows the interfacing of 8255 to the system.
SOFTWARE FOR TRAFFIC LIGHT CONTROL