Traffic light controller - Interfacing 8255 with 8085 | 8085 Lab Program

 Section: Hardware Interfacing Experiments

Aim: To design a traffic light controller using the 8085 microprocessor through the Programmable Peripheral Interface 8255.

Apparatus Required: 8085 µp kit, 8255 Interface board, DC regulated power supply, VXT parallel bus, Traffic light controller interface board.

Algorithm:

1. Start.
2. Write the control word to initialize the 8255 IC. Obtain the signal data for each direction and store it in memory.
3. Initialize a counter to indicate the number of directions.
4. Initialize the HL pair to the starting address of the data.
5. Send the control/status data to the ports.
6. Call a delay subroutine between signal switching.
7. Decrement the counter and repeat the process for all directions.

Program:

Address	Label	Mnemonics	Operand	Comment
4100		LXI	H, Data	Load the data starting address in HL register pair
4103		MVI	C, 04	Move 04 to C register for 4 directions
4105		MOV	A, M	Move control word to Accumulator
4106		OUT	CNT	Send control word to control register
4108		INX	H	Increment HL pair
4109	LOOP1	MOV	A, M	Move Port C signal data to Accumulator
410A		OUT	CPRT	Send status word to Port C
410C		INX	H	Increment HL pair
410D		MOV	A, M	Move Port B signal data to Accumulator
410E		OUT	BPRT	Send status word to Port B
4110		INX	H	Increment HL pair
4111		MOV	A, M	Move Port A signal data to Accumulator
4112		OUT	APRT	Send status word to Port A
4114		CALL	DELAY	Call delay subroutine
4117		INX	H	Increment HL pair
4118		DCR	C	Decrement direction counter C
4119		JNZ	LOOP1	Jump to LOOP1 if counter is non-zero
411C		JMP	START	Repeat traffic cycle
411F	DELAY	PUSH	B	Push BC register pair onto stack
4120		MVI	C, 0D	Move 0D to C register
4122	LOOP3	LXI	D, FFFFH	Load DE pair with FFFFH
4125	LOOP2	DCX	D	Decrement DE pair
4126		MOV	A, D	Copy D to Accumulator
4127		ORA	E	OR Accumulator with E
4128		JNZ	LOOP2	Loop until DE becomes 0
412C		JNZ	LOOP3	Loop until C becomes 0
412F		POP	B	Pop BC pair from stack
4130		RET		Return to main program

Observation:

• Mode 0 (Simple I/O) control word is sent to initialize Ports A, B, and C as output ports.
• Output signals are sent sequentially to Port A, Port B, and Port C to illuminate Red, Yellow, and Green LEDs for four traffic directions with designated delay intervals.

Result: Thus the design of traffic light controller using 8085 microprocessor through programmable peripheral interface 8255 is done and the output is verified.

Flowchart:

               +-------------------+
               |       START       |
               +---------+---------+
                         |
                         v
               +-------------------+
               | Initialize 8255   |
               | Write Control Word|
               +---------+---------+
                         |
                         v
               +-------------------+
               | Load Data Pointer |
               |   [HL] <- Data    |
               +---------+---------+
                         |
                         v
               +-------------------+
               |    [C] <- 04H     |
               | (4 Directions)    |
               +---------+---------+
                         |
                         v <-------------------+
               +-------------------+           |
               | Output Signals to |           |
               | Ports C, B, and A |           |
               +---------+---------+           |
                         |                     |
                         v                     |
               +-------------------+           |
               |   Call Delay      |           |
               |   Subroutine      |           |
               +---------+---------+           |
                         |                     |
                         v                     |
               +-------------------+           |
               |   [C] <- [C] - 1  |           |
               +---------+---------+           |
                         |                     |
                         v                     |
                       /   \                   |
                     /  Is   \                 |
                   /  [C] = 0? \               |
                  <             >              |
                   \ Is zero?  / --- NO -------+
                     \       /
                       \   /
                        | YES
                        v
               +-------------------+
               | Jump to START     |
               | (Continuous Loop) |
               +-------------------+

Viva Questions:

1. When the 8255 is reset, its I/O ports are all initialized as what?
2. If the programmable counter timer 8254 is set in mode 1 and is to be used to count six events, the output will remain at logic 0 for how many number of counts?
3. The devices that provide the means for a computer to communicate with the user or other computers are referred to as what?
4. What is the maximum number of I/O devices which can be interfaced in the memory-mapped I/O technique?
5. Interaction between a CPU and a peripheral device that takes place during an input/output operation is known as what?
6. What is the other name for Programmable Peripheral Input-Output port?
7. All the functions of the ports of 8255 are achieved by programming the bits of an internal register called what?
8. What is the port that is used for the generation of handshake lines in Mode 1 or Mode 2?
9. What is the pin that clears the control word register of 8255 when enabled?
10. In 8255, if A1 = 0 and A0 = 1, then from where is the input read cycle performed?

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