LABORATORY 10: DISPLAYING A 7-SEGMENT LED AND FLASHING AN LED ARRAY NAME: STUDENT ID#: Objectives Learn how to: Program the 8255 to show digits on a 7-segment LED. Program both the 8255 and 8279 in order to accept keypad inputs from 0 to F and display it on a 7-segment LED. Program the 8255 to flash an LED array. Part 1: Displaying a 7-Semgent LED from keypad inputs The 7-segment LED display can be found in many displays such as microwave or fancy toaster ovens and occasionally in non-cooking devices. It is composed of 7 LEDs that are fabricated in one case to make a convenient device for displaying numbers and some letters. Figure L10.1 shows a typical 7-segment LED display with its pin layout. The 7-segment LED shown in Figure L10.1 is a common anode LED display. This means that the positive leg of each LED is connected to a common point which is pin 3 in this case. Each LED has a negative leg that is connected to one of the pins of the device. To make it work, you need to connect pin 3 to +5 volts. Then to make each segment light up, connect the ground pin to ground. A resistor is required to limit the current. Rather than using a resistor from each LED to ground, you can just use one resistor from Vcc to pin 3 to limit the current. Figure L10.1 A 7-segment LED display and its pin layout. LAB10-1
Table L10.1 provides the information that is needed to display digits from 0 to 9 and the letters A, b, C, d, E, and F. '0' means that pin is connected to ground and '1' means that pin is connected to Vcc. Table L10.1 Logic states of 7-segment LED to display different digits or letters. a (Pin 1) b (Pin 10) c (Pin 8) d (Pin 6) e (Pin 5) f (Pin 2) g (Pin 9) 0 0 0 0 0 0 0 1 1 1 0 0 1 1 1 1 2 0 0 1 0 0 1 0 3 0 0 0 0 1 1 0 4 1 0 0 1 1 0 0 5 0 1 0 0 1 0 0 6 0 1 0 0 0 0 0 7 0 0 0 1 1 1 1 8 0 0 0 0 0 0 0 9 0 0 0 1 1 0 0 A 0 0 0 1 0 0 0 b 1 1 0 0 0 0 0 C 0 1 1 0 0 0 1 d 1 0 0 0 0 1 0 E 0 1 1 0 0 0 0 F 0 1 1 1 0 0 0 Figure L10.2 shows the 7-segment LED decoder circuit. With this information we can design assembly instructions to display different digits or letters on the 7-segment LED. For example, the instruction to display 1 on the 7-segment LED is: OUT 3FF0, F9 To avoid showing the consecutive digits from 0 to F too quickly, we need to insert time delay before outputting the next digit on the 7-segment LED. This can be accomplished by writing a time delay loop as follows: MOV CX, 0A000H LOOP $ LAB10-2
Figure L10.2 The 7-segment LED decoder circuit Here we will begin our practice of displaying digits or letters on a 7-segment LED with a simple program. We will analyze, run, and modify the program. Check off each step as it is completed. Check Step Procedure 1. Examine the data segment and constant definitions in the program L10P1.ASM. a. How many digits does the program generate? b. What is the name of the repeat count that is used to set the duration between digits? Find the value assigned to this constant in the program. 2. Examine the part of the program in L10P1.ASM. What is the address of FND? a. What does the instruction sequence that follows do? J2: MOV AL, [SI] MOV CX, 0A000H LOOP $ INC SI DEC BL JNZ J2 b. Is this FND a common cathode or anode LED? 3. List the 16-bit words corresponding to the display of each digit from 0 to F. LAB10-3
4. Do the following steps to generate a downloadable hex format file and download it through RS-232. C:\> MASM ( ) Enter L10P1 C:\> LINK ( ) Enter L10P1 C:\> EXE2BIN L10P1 ( ) C:\> BIN2HEX ( ) Enter L10P1.BIN 5. Use debug program to trace L10P1.EXE and describe what you see. 6. Modify the program to show only even digits. Reassemble, relink, and generate L10P1A.EXE. Download the new program to MTS-86C and verify the program that it works correctly. Describe the difference between what was seen in step 4 and what you now see. Hand in your program listing with this lab report. Part 2: Scan a key from the keypad and display it on a 7-segment LED In this part of the lab, we will learn to read inputs from a keypad and display the corresponding key that is pressed on a 7-segment LED display. You will need to write an assembly program that will determine which key has been pressed on the key pad. The program will then display the corresponding character by configuring the output port correctly (the 7-segment displays). A schematic diagram is provided below to show connections needed to implement this lab. To do this, we will briefly introduce the fundamentals of a keypad and we will also show how an 8279 is used to interface a keypad. The circuit of 8279 is shown in Figure L10.3. In order to scan the keypad, the 8279 outputs a nibble to force one (only one) of the columns low and then reads the rows to see if any key in that column have been pressed. The rows are pulled up by the internal weak pull-ups in the 8279 ports. Consequently, as long as no key is pressed, the 8279 detects a logic high on each of the pins attached to the keypad rows. The nibble driven onto the columns always contains only a single 0. The only way that the 8279 can find a 0 on any row pin is when the keypad button to be pressed. In that case, a logic 0 will be read on the corresponding column LAB10-4
line. The 8279 identifies which column is at a 0-level and which row reads 0, allowing it to determine which key is pressed. For the keypad, the pins from left to right are: R1, R2, R3, R4, C1, C2, C3, C4. Figure L10.3 Keypad interface with 8279 Check Step Procedure 1. View the file L10P2.ASM with an editor. What are the contents of the data table in the program? 2. What is the address of 8279? 3. What does the instruction sequence that follows do? _ CNTR79 EQU 0FFEAH MOV DX, CNTR79 MOV AL, 0 MOV AL, 39H 4. What happens if skipping the below instructions in LP10P2.ASM? _ LOOP1: IN AL, DX TEST AL, 7 JZ LOOP1 5. Write a new program, L10P2A.ASM, modified from the program L10P2.ASM, to scan the keypad and display digits only from 0 to F on the 7-segment LED. Hand in your program listing with this lab report. LAB10-5
Part 3: A Simple Program to flash an LED array The objective of this part of the lab is to know how to flash an LED array through 8255 interface. Figure L10.4 shows the interface circuit for the LED array. Figure L10.4 Interface circuit for an LED array Check Step Procedure 1. View the file L10P3.ASM with an editor. What are the addresses of the control register, port A and port B? 2. What is the operation mode of 8255? 3. What does the instruction sequence that follows do? CNT3 EQU 3FD6H APORT3 EQU 3FD0H BPORT3 EQU 3FD2H MOV AL, 90H MOV DX, CNT3 J1: MOV DX, APORT3 IN AL, DX NOT AL MOV DX, BPORT3 JMP J1 4. Why is the purpose of the instruction 'NOT AL? 5. Write a new program, L10P3A.ASM, to light the LED array with different flashing pattern. The assembly program will flash the LED array continuously at approximately 1Hz. You need to implement a simple loop in the program that delays for 500 ms (half a second) with the LED on, and then delays for another 500 ms with the LED off. Thus, the LED flashes at 1 Hz! Hand in your program listing with this lab report. LAB10-6