Lab2: 8255 Interfacing

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AL-Hussein Bin Talal University College of Engineering Department of Computer Engineering Computer Interfacing and Peripherals Lab Student Name: Student Number: Dr. Fadi Abu-Amara Eng.

2. Introduction The 82C55 is a popular interfacing component, that can interface any TTL-compatible I/O device to the MP.

1 AL-Hussein Bin Talal University College of Engineering Department of Computer Engineering Computer Interfacing and Peripherals Lab Student Name: Student Number: Dr. Fadi Abu-Amara Eng. Huda Saqallah Lab2: 8255 Interfacing 1. Objectives The objective of this lab is to show how the 8255 chip can be used as an interface between the 8086 microprocessor, the LEDs, and the TACT switches. 2. Introduction The 82C55 is a popular interfacing component, that can interface any TTL-compatible I/O device to the MP. It has 24 pins for I/O that are programmable in groups of 12 pins and has three distinct modes of operation. The following figure shows the pin-out diagram of the 82C55. Figure 1: Pin-out diagram of the 8255 interfacing chip. The 82C55 is programmed through the two internal command registers as shown in figure 2. - Group B operation: o Mode 0: the basic input/output mode that allows the pins of group B to be programmed as a buffered input or latched output connections. o Mode 1: data are transferred through port B and handshaking signals are provided by port C. - Group A operation: o Mode 2: port A becomes bi-directional allowing data to be transmitted and received over the same eight wires. o When the command byte bit-7 is reset, any bit of port C can be set or reset; if the 82C55 is operated in either mode 1 or mode 2. o The bit set/reset: often used to control a system via setting/resetting control pins. 3. Procedure In this section different exercises will be performed such as: Controlling the LEDs directly from the 8086 microprocessor and controlling the LEDs through the TACT switches.

2 Figure 2: The 8255 programming diagram Displaying two Hex-Digits on the LEDs The 8086 microprocessor sends the information to be displayed to the 8255 and continues its work (no need to wait!). The 8255 chip sends the information to the 74LS240 buffer and from which to the LEDs as shown in the following figure. Figure 3: Functional circuit description of the 8086 interfacing with LEDs via The following code is used to program the 8255 to transfer the data from the accumulator register to the LEDs through the buffer. For this experiment, answer the following: - What the 3FD6H and 3FD2H represent? - Find the control word that must be written to the 8255 (the seventh line in the following code). - Modify the code to display 77H, waits for 2s, display 55H, waits for 2s, and exit. START: SEGMENT MOV SP, 4000H Page 2 of 5

Figure 4: Results of the LEDs blinking exercise-1.

3 MOV AL, 95H MOV DX, HLT 3.2. Blinking the LEDs Upon executing the following code, the LEDs will display the following pattern repeatedly. Figure 4: Results of the LEDs blinking exercise-1. START: SEGMENT MOV SP, 4000H OUT DX, AL MOV AL,0FH MOV DX, J1: MOV CX,0A000H LOOP $ JMP J1 On the other hand, when the following code is executed, the LEDs will show the following pattern repeatedly. Figure 5: Results of the LEDs blinking exercise-2. Page 3 of 5

SEGMENT START: MOV SP, 4000H J0: MOV AH,6 MOV AL,0FH MAIN: CALL OUTPUT JNZ MAIN MOV AH,8 MOV AL,1 J1: CALL OUTPUT ROL AL,1 JNZ J1 MOV AH,8 MOV AL,1 J2: CALL OUTPUT ROL AL,1 ADD AL,1 JNZ J2 JMP J0

4 SEGMENT START: MOV SP, 4000H J0: MOV AH,6 MOV AL,0FH MAIN: CALL OUTPUT JNZ MAIN MOV AH,8 MOV AL,1 J1: CALL OUTPUT ROL AL,1 JNZ J1 MOV AH,8 MOV AL,1 J2: CALL OUTPUT ROL AL,1 ADD AL,1 JNZ J2 JMP J0 OUTPUT: MOV DX, MOV CX,9000H LOOP $ RET 3.3. Controlling the LEDs through the TACT Switches The information is sent from the TACT switches to the 8086 through the 8255 chip (port A). Then, the same information is sent back from the 8086 microprocessor to the LEDs through the 8255 chip (port B) as shown in the following figure. Figure 6: Functional circuit description of the TACT switches interfacing with LEDs. Page 4 of 5

5 When the following code is executed, the information input by the TACT switches will be displayed on the LEDs. APORT3 EQU 3FD0H SEGMENT START: MOV SP, 4000H J1: MOV DX,APORT3 IN AL,DX MOV DX, JMP J1 Exercises 1. Referring to the circuit of Figure 3, why the information is not sent directly from the 8086 to the LEDs. Why the 8255 and the 74LS240 chips are used? 2. If the circuit of Figure 6 is to be programmed such port A is connected to the LEDS while port B is connected to the TACT switches, find the new control word. 3. Write and execute a program that displays a bouncing ball on the LEDs. Use a TACT switch to start and stop the program execution. 4. Write and execute a program that make the LEDs show the following pattern. Page 5 of 5

CHAPTER: 3 PROGRAMMABLE PERIPHERAL INTERFACE & ELECTROMECHANICAL DEVICES INTERFACING

CHAPTER: 3 PROGRAMMABLE PERIPHERAL INTERFACE & ELECTROMECHANICAL DEVICES INTERFACING CHAPTER: 3 1 PROGRAMMABLE PERIPHERAL INTERFACE & ELECTROMECHANICAL DEVICES INTERFACING Introduction to 8255 PPI 2 The Intel 8255A is a high-performance, general purpose programmable I/O device is designed