You will need to write a C program that will determine which key has been pressed on the keypad. The program will then display the corresponding character on the character LCD module. A schematic diagram is provided below to show the connections needed to implement this lab. To do this, we will briefly introduce the fundamentals of a keypad.
Matrix Keypad
Keypads are used in all types of devices, including cell phones, fax machines, microwaves, ovens, door locks, etc. There are practically everywhere. Tone of electronic devices use them for user input.
In this section we will discuss logic and interface of a matrix keypad with microcontroller to reduce the number of port pins required to read a certain number of digital input. The same logic applies to any matric keypad.
Why Matrix Keypad?
Typically, one digital input is connected to one port pin. When there are a lot of digital inputs that have to be read by microcontroller, it requires same number of port pins to read each input signal separately. It would not be feasible to allocate one pin for each of them, because these will occupy a lot of I/O pins. the main reason is that microcontrollers grow with the number of pins, and growth means more power, capabilities and most of all higher price. So in the end you can either get a cheap chip with little capabilities (that is what you need) but with few I/O pins, or a more powerful chip, which is mush more than you need.
Therefore, a new interface technique will be needed to reduce number of required pins in this kind of situation. A easiest way to do that is to arrange inputs in matrix form, which divides I/O pins into two sections: the rows and the columns. For example, a 64-key keyboard would require 64 digital input port pins. With a matrix circuit, 16 I/O pins arranged in 8 rows and 8 columns can connect 64 keys — 8 output pins to drive rows and 8 input pins to read columns.
What are the Key Matrices?
Matrix Keypad is made by arranging push button switches in rows and columns. If you want to interface a 4 by 4 (16 keys) matrix keypad with a microcontroller. In the straight forward way, you will need 16 I/O pins of a microcontroller for that, but by using a simple technique we can reduce it to 8 I/O pins. In the matrix keypad, switches are connected in a special manner a shown in the figure below.
The blue lines are the columns and the red lines the rows. There are 16 knots that the rows and columns intersect, and each knot connects one switch button. There will be no connections between columns and rows. When any of the switches are pressed, the corresponding columns and rows are connected (short circuited), which can be detected by microcontroller to identify which keys have been pressed.
How does Key Matrix Works?
We make the columns as input pins and we drive the rows making them output pins. In order for the microcontroller to determine which button is pressed, it first needs to pull each of the four rows (R1 ~ R4) either logic low or high one at a time, and then poll the states of the four columns (C1 ~ C4). Depending on the states of the columns, the microcontroller can detect which button is pressed.
Lets assume that a logic high signal is given to Row 2 (R2). If any of the key belongs to Row 2 is pressed, the high signal from Row 2 will pass to the corresponding column as high. Watch the above animation, the button '5' is pressed, then the column 2 will also have high as long as the button '5' is pressed. What this means it that. if we know which row has currently logic high signal, and we watch the columns, then we can understand which button was pressed, if we detect power on a column. For example, our program pulls all four rows low and then pulls the second row (R2) high. It then reads the input states of each column, and reads column 2 high. This means that a connection has been made between column 2 and row 2, so button '5' has been pressed.
4x4 Matrix Keypad Pinout
Matrix keypads use a combination of four rows and four columns to provide button states to the host device, typically a microcontroller. Underneath each key is a pushbutton, with one end connected to one row, and the other end connected to one column. These connections are shown in below Figure
Connection Diagram
Keypad Connection for Open Drain Output Pins
Keypad Connection with Resistors for Regular Output Pins
Required Components List
4x4 Matrix Pad
× 1
Character LCD module
× 1
10 KΩ Potentiometer
× 1
Breadboard
× 1
Breadboard Power Adapter
× 1
Circuit / Schematic Diagram
Connect the character LCD and matrix keypad to the Tiva LaunchPad board, as shown below. The pin directional need to be configured as follows:
All the pins connected to the character LCD module are output direction.
The pins connected to the matrix keypad are divided into two parts: rows and columns. The pins connected to row wires are output direction, and connected to column wires are input direction.
Create a new folder under the EE3450 folder and name it Lab05_Keypad. Then double-click the folder you just created to jump into it.
Launch the Keil μVisio, and create a new project, save the project as Lab05_Keypad.
Add MyDefines.h to the Source Group, and then add the Common folder to the include paths under the "Options for Target" setting.
Add ezTiva LIB into your project, increase the stack and heap size under the "startup_TM4cXXX.s (Startup)" setting, and then add the ezTivaLIB folder to the include paths under the "Options for Target" setting. (Lesson 09: Add ezTiva Library into Your Project)
Next, we need to configure all the GPIO ports and pins that are used in the designd.
According to the pin connections, complete the following GPIO configurations for each port. Fills the pin field by the value below:
0: Clean the bit
1: Set the bit
x: Do not change the bit
d: Do not care
For both TM4C123GXL and TM4C1294XL LaunchPads, the Port C [3:0] are used for JTAG/SWD. Therefore, when you configure the Port C, you have to use bitwise operators to make sure your new configuration settings do not affect the JTAG/SWD function (PC3 ~ PC0).
Most of GPIO pins are configured as GPIOs and tri-stated by default (GPIOPCTL = 0, CPIOAFSEL = 0, GPIODIR = 0, GPIOPUR = 0, GPIOPDR = 0, GPIOODR = 0)
Enable Clock to the GPIO Modules (RCGCGPIO register) TM4C123G: SYSCTL->RCGCGPIO|= binary = hex
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port F
Port E
Port D
Port C
Port B
Port A
port
0
0
-
TM4C1294: SYSCTL->RCGCGPIO |= binary = hex
8
4
2
1
8
4
2
1
8
4
2
1
8
4
2
1
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
bit
Port Q
Port P
Port N
Port M
Port L
Port K
Port J
Port H
Port G
Port F
Port E
Port D
Port C
Port B
Port A
port
0
-
-
-
After enable clock signal, check the PRGPIO register until the correspoding bit set to 1.
Unlock Port TM4C123G: PD7 and PF0 are locked after reset. TM4C1294: PD7 and PE7 are locked after reset If those pins are used in the design, it must be unlocked first. To unlock the port, 0x4C4F434B must be written into GPIOLOCK register and uncommit it by setting the GPIOCR register.
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port
Pin 7
Pin 6
Pin 5
Pin 4
-
Pin 3
Pin 2
Pin 1
Pin 0
pin
Value in Hex
Register
Value to Register
-
-
=
➤
GPIO->LOCK
=
0x4C4F434B
-
-
=
➤
GPIO->CR
-
-
=
➤
GPIO->LOCK
=
0x4C4F434B
-
-
=
➤
GPIO->CR
Convert above configuration into registers
GPIO Analog Mode Select If any pin is used as an Analog signal (check Signal Type field on the table 1), the appropriate bit in AMSEL must be set.
0: Digital signal
1: Analog signal
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port
Pin 7
Pin 6
Pin 5
Pin 4
-
Pin 3
Pin 2
Pin 1
Pin 0
pin
Value in Hex
Register
Value to Register
-
-
=
➤
GPIO->AMSEL
-
-
=
➤
GPIO->AMSEL
-
-
=
➤
GPIO->AMSEL
-
-
=
➤
GPIO->AMSEL
-
-
=
➤
GPIO->AMSEL
-
-
=
➤
GPIO->AMSEL
-
-
=
➤
GPIO->AMSEL
GPIO Port Control (PCTL) The PCTL register is used to select the specific periheral signal for each GPIO pin when using the alternate function mode.
0: GPIO
1~0xF: Check the GPIO Pins and Alternate Function table
GPIO Alternate Function Select (AFSEL) Setting a bit in AFSEL register configures the corresponding GPIO pin to be controlled by PCTL peripheral function.
0: General I/O
1: Pin connected to the digital function that defined in PCTL register
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port
Pin 7
Pin 6
Pin 5
Pin 4
-
Pin 3
Pin 2
Pin 1
Pin 0
pin
Value in Hex
Register
Value to Register
-
-
=
➤
GPIO->AFSEL
-
-
=
➤
GPIO->AFSEL
-
-
=
➤
GPIO->AFSEL
-
-
=
➤
GPIO->AFSEL
-
-
=
➤
GPIO->AFSEL
-
-
=
➤
GPIO->AFSEL
-
-
=
➤
GPIO->AFSEL
GPIO Pin Direction (DIR) Set pin direction
0: Input pin
1: Output pin
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port
Pin 7
Pin 6
Pin 5
Pin 4
-
Pin 3
Pin 2
Pin 1
Pin 0
pin
Value in Hex
Register
Value to Register
-
-
=
➤
GPIO->DIR
-
-
=
➤
GPIO->DIR
-
-
=
➤
GPIO->DIR
-
-
=
➤
GPIO->DIR
-
-
=
➤
GPIO->DIR
-
-
=
➤
GPIO->DIR
-
-
=
➤
GPIO->DIR
Internal Pull-Up Resistor (PUR), Pull-Down Resistor (PDR), and Open-Drain (ODR) PUR: The pull-up control register PDR: The pull-down control register ODR: The open-drain control register
0: Disable
1: Enable
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port
Pin 7
Pin 6
Pin 5
Pin 4
-
Pin 3
Pin 2
Pin 1
Pin 0
pin
Value in Hex
Register
Value to Register
-
-
=
➤
GPIO->
-
-
=
➤
GPIO->
-
-
=
➤
GPIO->
-
-
=
➤
GPIO->
-
-
=
➤
GPIO->
-
-
=
➤
GPIO->
-
-
=
➤
GPIO->
GPIO Digital Enable Enables all the pins that are used in the design, including GPIO pins and alternate function pins.
0: Pin undriven
1: Enable pin
8
4
2
1
8
4
2
1
7
6
5
4
3
2
1
0
bit
Port
Pin 7
Pin 6
Pin 5
Pin 4
-
Pin 3
Pin 2
Pin 1
Pin 0
pin
Value in Hex
Register
Value to Register
-
-
=
➤
GPIO->DEN
-
-
=
➤
GPIO->DEN
-
-
=
➤
GPIO->DEN
-
-
=
➤
GPIO->DEN
-
-
=
➤
GPIO->DEN
-
-
=
➤
GPIO->DEN
-
-
=
➤
GPIO->DEN
Example Source Code
Copy the following example code to your main.c file.
Note: The example code is not yet complete, you must implement all functions following the lab instructions.
Inside the ReadKeypad() function, if no keys are pressed, it will return 0. If a key is pressed, it will return the ASCII value from the Keypad[] array that corresponds to that key. For example, if you press a key on the 3rd row, second column, the ReadKeypad() function will return the character '8' ( or ASCII code for '8').
Implement the following algorithm to "scan" the keypad in the ReadKeypad() function.
Scan Algorithm
Drive row wire R1 low, and other rows high
Read and test the states of all column wires (check the value of C1, C2, C3, and C4)
If column Cn is low, it is shorted to row R1
if column Cn is high, Cn is not shorted to any row wire & remains pulled high
Repeat steps 1 & 2, but with different row wire driven low (and others high)
A keypress is detected if a column line is detected in the low state
The key position is the intersection of that column and the row being driven low
From the lookup table to get the ASCII code of the pressed key
If no key is pressed, then the function should return null (zero value)
There is a short time delay from the time a pattern is written to an output port to the appearance of that pattern on the external pins. After writing a pattern to an output port (to drive row lines), insert a short program delay before reading the input port and testing the keypad column lines.
while-loop (row < 4) set R1 output to 1 set R2 output to 1 set R3 output to 1 set R4 output to 1
if row equal to 0 set R1 output to 0 else if row equal to 1 set R2 output to 0 else if row equal to 2 set R3 output to 0 else if row equal to 3 set R4 output to 0 endif
Delay for 10us
if C1 equal to 0 set key from KeyPad array[row, 0] else if C2 equal to 0 set key from KeyPad array[row, 1] else if C3 equal to 0 set key from KeyPad array[row, 2] else if C4 equal to 0 set key from KeyPad array[row, 3] endif
if key not equal to 0 jump out while-loop endif
increase row by 1 endwhile
return key value }
Lab Questions
Can you briefly describe your GPIO configuration for the matrix keypad? Explain what happens if the row pins are configured as general output pins without open-drain function?
What is the status of the column input if no key is pressed?
In a 4 x 4 matrix keypad used in this lab, can we press two keys at the same time? Explain.
In Exp #1, if you keep holding down a key, what happens to the system?
Now, modify your ReadKeypad() function to support key detection; when the user presses a key, the ASCII code is returned to the main code. If the same key is held, the function will return 0x00, unless the user releases the key or presses another key.
Hint:
Create a static variable preKey in the ReadKeypad()function to store the previous key value. Then compare it with the current key value; if the current key is the same as the previous, then return 0x00 back to the main code.
(Don't forget to update the current key value to the preKey before you return the code.)
The pseudocode code as follow:
ReadKeypad(){ create static variable: preKey = 0
put the original code for scanning and reading the keypad here
if key is not equal to preKey set preKey equal to key else clear key to zero endif
