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Embedded Systems Development and Labs; The English Edition

5.1.8 Exercises

Refer to the sample program; display the 4 x 4 keyboard values on the LCD panel.

5.2 The 4 x 4 Keyboard Control Lab

5.2.1 Purpose

● Understand the design method of keyboard interrupt control program. ● Understand the design of the keyboard interrupt test program.

● Understand the interrupt service routine programming using the ARM core processor.

5.2.2 Lab Equipment

● Hardware: Embest S3CEV40 hardware platform, Embest Standard/Power Emulator, PC. ● Software: Embest IDE 2003, Windows 98/2000/NT/XP operation system.

5.2.3 Content of the Lab

Develop a project that accepts the keys of the keyboard pad through interrupt service routine and display the values on the 8-SEG LED.

5.2.4 Principles of the Lab

For the matrix keyboard interface, there are normally three ways of getting the keyboard values: through interrupts, through scanning, and through inversion.

●Interrupts: When a key is pressed, CPU will receive an interrupt signal. The interrupt service routine will

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Embedded Systems Development and Labs; The English Edition

read the keyboard status on the data bus through different addresses and determine which key is pressed.

●Scanning: Send low voltage to one horizontal line and high level to the other horizontal lines. If any vertical line is low, the key that sits at the intersection of the selected row and column is pressed.

●Inversion: Send low voltage to the horizontal lines and read the vertical lines. If any vertical line is low, it indicates one key is pressed on that column. Then send low voltage to the vertical lines and read the horizontal lines. If any horizontal line is low, it indicates one key is pressed on that row. The intersection of the identified row and column will give the position of the key.

5.2.5 Lab Design

1. Keyboard Hardware Circuit Design

1) 4 x 4 Keyboard

The 4 x 4 keyboard has 4 rows and 4 columns. The circuit is shown in Figure 5-12. Any pressed key will generate a pass route.

Figure 5-12 4 x 4 Keyboard Circuit

2) CPU Recognition Circuit

The keyboard recognition circuit is shown bellow:

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Embedded Systems Development and Labs; The English Edition

Figure 5-13 4 x 4 Keyboard Recognition Circuit

3) Circuit Functionality

As shown in Figure5-13, the keyboard connection electric circuit, a 4×4 matrix keyboard port is expanded on the board. This keyboard supports the interrupt mode and the scanning mode. 4 data wires represent the rows and 4 address wires represent the columns. Row wires are connected with pull-up resistors to maintain high level. These row signals are used to generate the EXINT1 MCU’s interrupt signal through a 74HC08 AND gate. The column wires are connected with pull-down resistors to maintain low level. When some key is pressed down, the row wires are pulled to low level, which causes EXINT1 input to become low and activate the MCU interrupt system. After the interrupt is recognized, the pressed key can be found by scanning the rows and columns of the keyboard then the corresponding key is processed. Chip 74HC541 is selected through the chip select signal nGCS3. This guarantees that MCU reads the row wire’s information only when the keyboard is used. For example, if the key that connects pin1 and pin5 of J7 is pressed, the interrupt routine will read data using the following addresses (x means 0 or 1):

•Xxx11101, A1 is logic low. Analyze whether the button on L0 line is pressed. Because the fourth pin on J7 is in the off status, and high logic on A4 causes that the first pin is disconnected with the fifth pin of J7, output of data bus from U10 is still 0xF

•Xxx11011, A2 is low logic. Analyze whether the buttons on L1 line are pressed. Because the third pin of J7 is in the off status, and high logic on A4 causes that the first pin is disconnected with the fifth pin of J7, output of data bus from U10 is still 0xF.

•Xxx10111, A3 is low logic. Analyze whether the buttons on L2 line are pressed. Because the second

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Embedded Systems Development and Labs; The English Edition

pin of J7 is in the off status, and high logic on A4 causes that the first pin is disconnected with the fifth pin of J7, output of data bus from U10 is still 0xF.

•Xxx01111, A4 is low logic. Analyze whether the buttons on L3 line are pressed. Because the first pin is connected with the fifth pin of J7, and low logic on A4 causes that input of data bus pass through the loop from U11 to U10, the output of data bus D0 is pulled down by U10 and becomes 0xE. The interrupt service routine (ISR) can analyze whether the button SB16 is pressed according to the rules.

The addresses and the data for the 16 keys are shown in Table 5-7.

Table 5-7. Key value decisions

4) Key Display Control

When a key is pressed, the corresponding key value will be displayed on the 8-SEG LED. The circuit of 8-SEG LED is shown in Figure 5-14. (Refer to Section 4.6 “8-SEG LED Display Lab”)

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