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Switches and Keypads

Listing 7-3. Test program for Figure 7-9’s circuits.

10XBY(1F00H)=49 : REM 1

11XBY(1F01H)=50 : REM 2

12XBY(1F02H)=51 : REM 3

13XBY(1F03H)=67 : REM C

14XBY(1F04H)=52 : REM 4

15XBY(1F05H)=53 : REM 5

16XBY(1F06H)=54 : REM 6

17XBY(1F07H)=68 : REM D

18XBY(1F08H)=55 : REM 7

19XBY(1F09H)=56 : REM 8

20XBY(1F0AH)=57 : REM 9

21XBY(1F0BH)=69 : REM E

22XBY(1F0CH)=65 : REM A

23XBY(1F0DH)=48 : REM 0

24XBY(1F0EH)=66 : REM B

25XBY(1F0FH)=70 : REM F

requested function. Chapter 8 describes how to add a small display to a system, so you don’t have to use the host computer’s display for the menu.

For some projects, you may want to use the numeric values of the keys directly, rather than interpreting them as ASCII codes. In this case, you’ll need to revise the lookup table, or create a second table that matches the numeric values of the key legends with their data outputs. For example, a data output of 0 would correspond to 1, instead of 49 (the ASCII code for 1). Again, you usually can’t use the keypad encoder’s data outputs directly because they don’t correspond to the values printed on the keys.

If an application requires that users enter multi-digit numbers on the keypad, your program will have to translate the individual digits into a single value. Listing 7-4 is a program that waits for the user to enter a 4-digit hex address, then displays the data stored at that address in external RAM.

Chapter 7

Listing 7-4. Reads the data stored a a 4-digit address entered by the user on a keypad.

1 REM lookup table stores the numeric value of each key

10XBY(1F00H)=1 : REM 1

11XBY(1F01H)=2 : REM 2

12XBY(1F02H)=3 : REM 3

13XBY(1F03H)=0CH :REM C

14XBY(1F04H)=4 : REM 4

15XBY(1F05H)=5 : REM 5

16XBY(1F06H)=6 : REM 6

17XBY(1F07H)=0DH :REM D

18XBY(1F08H)=7 : REM 7

19XBY(1F09H)=8 : REM 8

20XBY(1F0AH)=9 : REM 9

21XBY(1F0BH)=0EH :REM E

22XBY(1F0CH)=0AH :REM A

23XBY(1F0DH)=0 : REM 0

24XBY(1F0EH)=0BH :REM B

25XBY(1F0FH)=0FH :REM F

Displays

8

Displays

In addition to switches and keypads for user input, most projects also include a display to let users know what’s going on inside. The type of display depends on the kinds of information you want to show. For simple status indicators, discrete LEDs will do the job. For numbers, you can use 7-segment displays. And if you need to display text or other symbols in addition to numbers, character-based LCD modules are a good solution. This chapter will show how to use each of these in an 8052-BASIC system.

Using LEDs

Discrete, or individual LEDs (light-emitting diodes) are an easy way to indicate status, such as On, Ready, Mode selected, and so on. They are colorful, eye-catching, and easy to interface to 5-volt logic. Available colors now include blue as well as red, green, and yellow. Some individual LED packages can emit red, green, or amber light, depending on the voltages applied.

Like other diodes, current passes through an LED in one direction only. When a positive voltage is applied to the anode, current flows and electrons migrate across an energy gap in the LED, causing it to emit light. The size of the energy gap determines the voltage drop across the LED, as well as the color of light emitted. A tinted case can also vary the color.

Displays

Figure 8-1. LED interfaces to output ports. Use an HCMOS output if you want a high output to turn on the LED. Use either HCMOS or LSTTL if you want a low output to turn on the LED.

Chapter 8

outputs of a 74LS374 addressed at E000h. The LEDs are connected as in Figure 8-1B , with logic-low outputs turning on the LEDs. Listing 8-1 tests the circuit by allowing you to turn individual LEDs on and off.

Bi-color LEDs

Bi-color LEDs have both a red and a green LED inside a single package. By turning on one, both, or neither, you can use a single indicator to show as many as four states. Some bicolor LEDs have two leads, while others have three. Figure 8-2 illustrates.

In the 3-lead, or common-cathode type, the cathodes of both LEDs connect internally (A). To turn on an LED, you ground the cathodes through a current-limiting resistor and apply power to the anode of the desired LED. When both LEDs are powered, you get an amber light. Removing power from both turns the LED off, giving a total of four states that the device can display. Instead of the one current-limiting resistor shown, you can connect a resistor to each anode, to set the current through each LED individually.

In a 2-lead, or parallel-connected, bicolor LED, the anode of each LED connects internally to the other’s cathode (B). To turn on the red LED, you apply +5V to terminal A and ground terminal B. To turn on the green LED, you do the reverse: terminal A is ground, and terminal B is +5V. With this type, you can’t turn on both LEDs at once.

With either type, by adding an inverter, you can use a single output to control both LEDs (C, D).

Displays

Figure 8-2. Ways to connect bicolor LEDs .

7-segment Displays

If you want to display numbers, 7-segment displays will do the job. Each digit on the display contains seven segments. Numerals are displayed by turning on different combinations of segments, as Figure 8-3 shows. Decoder chips make it easy to operate one or more displays with a minimum of programming and added components. Seven-segment displays are available as LEDs, where each segment is a light-emitting diode, and as LCDs, where each segment is a liquid-crystal display. We’ll look at the LED type first.

7-segment LEDs

A 7-segment LED contains seven individual LEDs arranged in the pattern shown in Figure 8-3. Sometimes there is also a decimal point (or two, one on each side). There are also special leading-digit modules that display only a 1 and a plus-or-minus symbol.

Chapter 8

Figure 8-3. A 7-segment display can show numbers from 0 to 9, plus hex digits A-F. In a common-cathode LED display, all of the cathodes connect together, while in a common-anode display, all of the anodes connect.

The displays come in two types: common-anode or common-cathode. In a common-anode display, the anodes of each segment connect internally. To use the display, you connect the anodes to a voltage source and turn on individual segments by grounding them through a current-limiting resistor. A common-cathode display is the opposite: the cathodes connect internally, so you ground the cathodes and apply voltages through current-limiting resistors at the segments you want to light.

Deciphering pinouts

Unfortunately, there isn’t much standardization for pinouts of 7-segment displays. If you don’t know the pinout for a display, you can find it by experimenting. You’ll need a 330-ohm resistor and a 5-volt supply.

Sometimes you’ll find CC or CA stamped on the package to indicate common cathode or common anode. If even this information is lacking, begin by connecting one lead of the

Displays

Figure 8-4. Four output port pins can control a 7-segment LED.

resistor to ground on your power supply. Clip the resistor’s other end to one of the LED’s pins. Use a test lead to touch the power supply’s +5V output to each of the other pins in turn.

If only one or two connections cause a segment to light, you have a common-anode display, and the common anode is the pin or pins that connect to +5V when the segment lights. (There

Chapter 8

Listing 8-2. Causes a 7-segment LED to display each digit in sequence.

may be two common-anode pins.) To find the pin that controls each segment, leave the +5V lead on a common-anode pin, and connect the resistor to each pin in turn, noting the results.

For a common-cathode display, to find the common-cathode pin or pins, connect a pin to +5V, and touch the others to ground through the 330-ohm resistor. The pin or pins that cause the segment to light are the common-cathode connections. To find the pin that controls each segment, move the +5V lead to each pin in turn, and note the results.

Interfacing

For 7-segment decoder/drivers, you can choose from single-digit and multi-digit chips.

Single-digit driver. Figure 8-4 shows a 7-segment display controlled by a 4511B latch/decoder/driver. The display shows the value of the 4-bit number at the 4511’s data inputs A-D. The 4511 will drive common-cathode displays directly. Common-anode displays require inverters at the segment outputs. You can use any output port bits to control the display. An 8-bit port will control a 2-digit display.

Listing 8-2 tests Figure 8-4’s circuits by displaying each digit in sequence. The program assumes that a display connects to bits 0-3 of Port A on an 8255 addressed at FC00h. If your system has different addressing, change the program to match.

Multi-digit driver. If you want to display more than a couple of digits, there are specialized chips that will drive and control multiple-digit displays. One example from Intersil (now part of Harris Semiconductor) is the ICM7218D multiplexed display driver, which can control up to 8 common-cathode digits. Figure 8-5 illustrates. For common-anode displays, use the ICM7218C.

The segments of all eight displays connect to the 7218D’s segment-driver outputs (a-g, dp). Each display’s common cathode connects to one of eight DIGIT outputs. An internal oscillator