EE 3501 Lab 5 Manual vSp2022 (1)

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Electrical Engineering

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Feb 20, 2024

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Department of Electrical and Computer Engineering EE 3501 Laboratory 5: GPIO, Scanning a Keypad Name: Date: Learning Objective: - Write a real-time polling routine to read inputs from a keypad. Introduction A Keypad as a Matrix of Switches A keypad consists of matrix of switches used to connect an input pin to an output pin (Fig. 1). When a button is pressed, a switch is closed, and a connection is made between an input pin arranged in columns and an output pin arranged in rows. Figure 1 - A 3x4 Keypad with Underlying Switch Representation The input pins of a keypad are typically connected to a high logic voltage via pull-up resistors. This means their default state is logic 1. The output pins can be set to either logic 1 or 0.

Mapping a Key Press to a Keypad Matrix Location Consider a situation where all the output pins are set to 0 and no keys are pressed (Fig. 2). In this situation there is no connection between inputs and outputs and all input pins are logic 1. Figure 2 – Input Pin Values with Output Pins R1, R2, R3, R4 = 0000, and no Key Pressed Now consider a situation where all the output pins are set to 0 and key “ 0 ” is pressed (Fig. 3 and Fig. 4). In this situation there is a connection between input pin C2 and output pin R4, which results in C2 being logic 0. Figure 3 – Switch Closure Between R4 and C2 when Key “0” is Pressed

Figure 4 – Input Pin Values with Output Pins R1, R2, R3, R4 = 0000 and Key “0” Pressed Fig. 4 illustrates that we can locate the input column of a key press when all the output rows are set to 0. To obtain the row location as well as column location of a key press, we must successively set each output row to logic 0, while keeping the other output rows high and read the input pins each time. As shown in Fig. 5 – 8, the input pins will only indicate that a key in a column is pressed when the row in which the key is pressed is set to 0. This polling procedure allows us to determine the row and column location of a key press in the keypad matrix. Figure 5 – Input Pin Values with Output Pins R1, R2, R3, R4 = 0111 and Key “0” Pressed

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Figure 6 – Input Pin Values with Output Pins R1, R2, R3, R4 = 1011 and Key “0” Pressed Figure 7 – Input Pin Values with Output Pins R1, R2, R3, R4 = 1101 and Key “0” Pressed

Figure 8 – Input Pin Values with Output Pins R1, R2, R3, R4 = 1110 and Key “0” Pressed In this scenario, the keypad matrix location of key “0” is obtained through the successive “scanning” of rows . The location is row 4 and column 2.

Laboratory Procedure: For this lab, you will create a C++ program that will use a polling procedure to read a key press from your keypad and output the corresponding character to the terminal. You will also modify this initial program to read a five-character sequence and turn LED1 on if the sequence matches a code. Polling Procedure to Read a Key Press 1. Your keypad will look like Fig. 9. Your output row pins and input column pins are arranged as shown in the figure. Figure 9 - Keypad Pinout Diagram 2. Connect the output pins and input pins of your keypad to GPIO pins of your choice on your microcontroller. 3. In your C++ program, create four global DigitalOut objects: ROW1, ROW2, ROW3, and ROW4, and initialize them to map to the GPIO pins that are connected to output pins of your keypad. 4. Also, create four global DigitalIn objects: COL1, COL2, COL3, and COL4, and initialize them to map to the GPIO pins that are connected to input pins of your keypad. Also initialize these objects to be pull-up inputs. 5. Within your C++ program, write a polling procedure to output a logic 0 on the GPIO pins connected to ROW1, ROW2, ROW3, and ROW4 one at a time while keeping the other pins at logic 1. An output row will be held at logic 0 for 5 ms before the input column values are read and the output row will remain at logic 0 for 5 ms after the columns are read. Refer to the timing diagram of Fig. 10.

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Figure 10 - Digital Output Timing Diagram for Keypad Rows 6. By using the polling procedure described in step 5, you can obtain the row and column location of a key press, which can be used to access elements of a char array that mimics the keypad layout. See an example char array structure below. Note that array index values start at 0, so key_map[0][0] is ‘1’ , key_map[1][2] is ‘ 6 ’ , and so on. char key_map [4][4] = { {'1', '2', '3', 'A'}, //1st row {'4', '5', '6', 'B'}, //2nd row {'7', '8', '9', 'C'}, //3rd row {'*', '0', '#', 'D'}, //4th row }; 7. Use the polling procedure of step 5 and char array of step 6 to detect and locate a key press and then access the correct character from the char array. Output this character to the terminal using a printf or cout statement. You can create your program in any way you see fit. However, I recommend that you use a structured hierarchical approach. One way to do this is create a function, that uses the polling procedure to return the character pressed by the keypad when called in the main() function. An example function declaration could be char keypad_scan(void); This function could also call a function that reads the state of the input columns to determine the column index of the key pressed. An example function declaration could be int col_scan(void); Read Five-Character Sequence and Turn LED1 on if Code Match 1. Modify the program you created in the previous procedure to read a five-character sequence. If the sequence matches the code 2753# then turn LED1 on. If not, LED1 remains off. 2. If LED1 is on, add to the program instructions that will turn LED1 off when * is pressed. You may find including the <string> library to be helpful. Specifically, a) You can add individual characters to a string using the += operator. b) You can determine the length of a string object using the string.size() function. c) You can also compare two strings to determine if they are equal using the == operator.

Discussion Questions 1. Refer to the webpage below and state the 4 input pin modes you can set for a DigitalIn object? https://os.mbed.com/docs/mbed-os/v6.11/mbed-os-api- doxy/classmbed_1_1_digital_in.html#a12ffae4af877bdcd41ad2fc6a0a444ad 2. Refer to the webpage below and describe the two methods for setting the value of a DigitalOut object. https://os.mbed.com/docs/mbed-os/v6.15/mbed-os-api- doxy/classmbed_1_1_digital_out.html Report Requirements Lab Results ▪ Copy of the code used to implement the Polling Procedure to Read a Key Press program. (20 points) ▪ Video or instructor inspection of a user pressing ALL the keys on the keypad and the corresponding keys being displayed in the terminal screen. (20 points) ▪ Copy of the code used to implement Read Five-Character Sequence program. (20 points) ▪ Video or instructor inspection of a user pressing the correct Five-Character sequence and LED1 turning on in response, followed by the user pressing ‘*’ and LED1 turning off in response. (20 points) Answers to Discussion Questions 1. 10 points 2. 10 points No formal lab report is needed for Lab 5.