9.12 Interfacing an 8086-Based Microcomputer to a Hexadecimal Keyboard and Seven-Segment Displays
This section describes the characteristics of the 8086-based microcomputer used with a hexadecimal keyboard and a seven-segment display.
9.12.1 Basics of Keyboard and Display Interface to a Microcomputer
A common method of entering programs into a microcomputer is via a keyboard. A popular way of displaying results by the microcomputer is by using seven-segment displays. The main functions to be performed for interfacing a keyboard are:
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Sense a key actuation.
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Debounce the key.
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Decode the key.
Let us now elaborate on keyboard interfacing concepts. A keyboard is arranged in rows and columns. Figure 9.28 shows a 2 x 2 keyboard interfaced to a typical microcomputer. In Figure 9.28, the columns are normally at a HIGH level. A key actuation is sensed by sending a LOW (closing the diode switch) to each row one at a time via PAO and PAl of port A. The two columns can then be input via PB2 and PB3 of port B to see whether any of the normally HIGH columns are pulled LOW by a key actuation. If so, the rows can be
checked individually to determine the row in which the key is down. The row and column code for the pressed key can thus be found.
The next step is to debounce the key. Key bounce occurs when a key is pressed or released-it bounces for a short time before making the contact. When this bounce occurs, it may appear to the microcomputer that the same key has been actuated several times instead of just once. This problem can be eliminated by reading the keyboard after about 20 ms and then verifying to see if it is still down. If it is, then the key actuation is valid. The next step is to translate the row and column code into a more popular code such as hexadecimal or ASCII. This can easily be accomplished by a program. Certain characteristics associated with keyboard actuations must be considered while interfacing to a microcomputer. Typically, these are two-key lockout and N-key rollover. The two-key lockout ensures that only one key is pressed. An additional key depressed and released does not generate any codes. The system is simple to implement and most often used. However, it might slow down the typing because each key must be fully released before the next one is pressed down. On the other hand, the N-key rollover will ignore all keys pressed until only one remains down.
Now let us elaborate on the interfacing characteristics of typical displays. The following functions are typically performed for displays:
I. Output the appropriate display code.
2. Output the code via right entry or left entry into the displays if there are more than one displays.
These functions can easily be realized by a microcomputer program. If there are more than one display, the displays are typically arranged in rows. A row of four displays is shown in Figure 9.29. In the figure, one has the option of outputting the display code via right entry or left entry. If the code is entered via right entry, the code for the least significant digit of the four-digit display should be output first, then the next digit code, and so on. The program outputs to the displays are so fast that visually all four digits will appear on the display simultaneously. If the displays are entered via left entry, then the most significant digit must be output first and the rest of the sequence is similar to the right entry.
Two techniques are typically used to interface a hexadecimal display to the microcomputer: nonmultiplexed and multiplexed. In nonmultiplexed methods, each hexadecimal display digit is interfaced to the microcomputer via an I/O port. Figure
9.30 illustrates this method. BCD to seven-segment conversion is done in software.
The microcomputer can be programmed to output to the two display digits in sequence. However, the microcomputer executes the display instruction sequence so fast that the displays appear to the human eye at the same time. Figure 9.31 illustrates the multiplexing method of interfacing the two hexadecimal displays to the microcomputer. In the multiplexing scheme, appropriate seven-segment code is sent to the desired displays on
seven lines common to all displays. However, the display to be illuminated is grounded. Some displays such as Texas Instrument’s TIL 311 have on-chip decoder. In this case, the microcomputer is required to output four bits (decimal) to a display.
The keyboard and display interfacing concepts described here can be realized by either software or hardware. To relieve the microprocessor of these functions, microprocessor manufacturers have developed a number of keyboard/display controller chips. These chips are typically initialized by the microprocessor. The keyboard/display functions are then performed by the chip independent ofthe microprocessor. The amount of keyboard/display functions performed by the controller chip varies from one manufacturer to another. However, these functions are usually shared between the controller chip and the microprocessor.
9.13.2 Hex Keyboard Interface to an 8086-Based Microcomputer
In this section, an 8086-based microcomputer is designed to display a hexadecimal digit entered via a keypad (16 keys). Figure 9.32 shows the hardware schematic.
I. Port A is configured as an input port to receive the row-column code.
2. Port B is configured as an output port to display the key(s) pressed.
3. Port Cis configured as an output port to output zeros to the rows to detect a key actuation.
The system is designed to run at 2 MHz. Debouncing is provided to avoid unwanted oscillation caused by the opening and closing of the key contacts. To ensure stability for the input signal, a delay of 20 ms is used for debouncing the input.
The program begins by performing all necessary initializations. Next, it makes sure that all the keys are opened (not pressed). A delay loop of 20 ms is included for debouncing, and the following instruction sequence is used (Section 9.8):
The next three lines detect a key closure. If a key closure is detected, it is debounced. It is necessary to determine exactly which key is pressed. To do this, a sequence of row-control codes (OFH, OEH, ODH, OBH, 07H) are output via port C. The row-column code is input via port A to determine if the column code changes corresponding to each different row code. If the column code is not OFH (changed), the input key is identified. The program then indexes through a look-up table to determine the row-column code saved in DL. If the code is found, the corresponding index value, which equals the input
key’s value (a single hexadecimal digit) is displayed. The program is written such that it will continuously scan for input key and update the display for each new input. Note that lowercase letters are used to represent the 8086 registers in the program. For example, al, ah, and ax in the program represent the 8086 AL, AH, and AX registers, respectively.
The memory and I/O maps are arbitrarily chosen. A listing of the 8086 assembly language program is given in the following:
In the program, the "Key-open" loop ensures that no keys are closed. On the other hand, the "Key-close" waits in the loop for a key actuation. Note that in this program, the table for the codes for the hexadecimal numbers 0 through F are obtained by inspecting Figure 9.32.
For example, consider key F. When key F is pressed and if a LOW is output by the program to bit 0 of port C, the top row and the rightmost column of the keyboard will be LOW. This will make the content of port A as:
Thus, a code of 7716 is obtained at Port A when the key F is pressed. Diodes are connected at the four bits (Bits 0-3) of Port C. This is done to make sure that when a 0 is output by the program to one of these bits (row of the keyboard), the diode switch will close and will generate a LOW on that row.
Now, if a key is pressed on a particular row which is LOW, the column connected to this key will also be LOW. This will enable the programmer to obtain the appropriate key code for each key.