The Microcontroller Idea Book (Jan Axelson, 1994)

Switches and Keypads

Figure 7-2. Use a rotary switch to allow users to choose from among several options.

where address is the location of the byte to be read in memory and bit is the bit number of the switch.

Detecting a Switch Press

Momentary switches are useful when you want to get the computer’s attention. For example, you might have a program that normally displays the current temperature and time, but switches to a setup routine when you press a switch. On a normally open pushbutton (momentary) switch, the contacts close when you press the switch, then open as you release it. In a normally closed momentary switch, the contacts open when you press, and close on release.

Using Interrupts

An easy way to detect a switch press is with an external interrupt. The 8052-BASIC automatically detects the switch press and branches to an interrupt-handling subroutine.

Chapter 7

Listing 7-1. Displays a message when external interrupt 1 is detected.

10 ONEX1 100

20 DO

30 WHILE 1=1

40 END

100 PRINT “Interrupt detected”

110 RETI

For a pushbutton-triggered interrupt, connect a switch like Figure 7-1’s to the 8052-BASIC’s INT1 input at pin 13. Pin 13 will then be normally high. When you press and release the switch, pin 13 will briefly go low.

Listing 7-1 is a simple program that waits for interrupts and jumps to an interrupt-handling routine when it detects one, signified by pin 13 going low. Line 10 enables external interrupt 1 and specifies line 100 as the location to branch to when the 8052-BASIC detects an interrupt request at pin 13. Lines 20-30 are an endless loop that waits for an interrupt. Lines 100 and 110 are the interrupt-service routine. In this example the routine doesn’t do very much; it just displays an on-screen message that the interrupt was detected, then returns to the main program loop.

Edge-detecting Interrupts

BASIC-52’s TCON operator allows you to write to the 8052’s special-function register TCON, which enables you to set up interrupt 1 as edge-detecting or level-detecting. The default after bootup or reset is edge-detecting, where interrupts are triggered by a falling edge at pin 13. If you want a rising edge to trigger an interrupt, you’ll have to add an inverter at pin 13.

Edge-triggering is handy for detecting switch presses, because the interrupt routine executes only once, when the switch is first pressed, no matter how long you hold down the switch.

Switch debouncing. Even with edge-triggered interrupts, however, switch bounce can cause multiple interrupts to occur with a single switch press. Switch bounce occurs because manual presses of mechanical switches tend to be sloppy. When you press a switch, the contacts normally bounce open and closed several times before they close positively, and bounce again as you lift your finger and the contacts open.

The computer has to be able to tell the difference between a bounce and a genuine switch press. Otherwise, each time you press a switch, and again when you release it, the computer will detect several rapid switch presses. One way to handle switch bounce is to ignore keypresses that are less than a certain length, usually around 10-20 milliseconds, with the

Switches and Keypads

exact value depending on the switch characteristics. Ignoring switch bounce is called switch debouncing.

You can debounce a switch in hardware or software. Because BASIC-52 is slow, it has some debouncing built-in. When the 8052-BASIC detects an interrupt, it will ignore all interrupts that occur before it exits the interrupt-handling routine. So, if an interrupt-handling routine takes 20 milliseconds to execute, you probably don’t need to add any debouncing circuits or delays. To add additional software debouncing, you can just add a delay loop like this to the interrupt routine:

105 FOR I=1 TO 100:NEXT I

Adjust the total count to the minimum value that prevents extra interrupts due to switch bounce.

Figure 7-3 shows a hardware debouncing circuit that uses a 74HC14 inverter with Schmitttrigger input. The circuit generates a clean pulse when S1 is pressed, in spite of switch bounce that may occur.

Point B, the inverter’s input, is normally low, and point C, the inverter’s output, is normally high. When S1 is pressed, C1 discharges slowly through R2 and the switch contacts. If switch bounce occurs, the voltage at the inverter’s input can’t change rapidly enough to affect the logic state of the input. Only when the switch remains closed for around 50 milliseconds does point B go high enough to cause point C to switch low.

In a similar way, when the switch contacts open, C1 charges slowly though R1 and R2, and pin C goes high again only when the contacts have remained closed for around 50 milliseconds. The Schmitt-trigger input ensures that the output pulse is clean even if the input changes slowly.

As with software debouncing, you can experiment to find the minimum values that prevent unwanted interrupts due to switch bounce. The debounce time increases as you increase the values of C1 and R2.

Level-detecting interrupts

If you use a level-detecting interrupt instead of an edge-detecting one, the interrupt routine executes whenever there is a low logic level at pin 13. So, for example, if you press and hold the switch in the above example, the interrupt routine will execute again and again, until you release the switch.

Level-triggered interrupts can be useful if you have multiple interrupt sources. If each source generates an interrupt request by turning on an open-collector output, you can tie all of the

Switches and Keypads

Figure 7-4. Two pushbuttons connected to port pins on the 8052-BASIC.

isn’t required. When you use polling, you have to be sure to check the switch often enough so that you won’t miss a switch press. For example, if you read the switch once per second, and you press the switch for just 100 milliseconds, you may not detect the switch press.

Latching a Switch Press

Another solution to switch detecting is to latch the switch press, as Figure 7-5 shows. In this circuit, a switch press causes the Q output of a 74HC74 flip-flop to go low and remain low until an external signal clears the flip-flop.

The circuit uses two port bits: one input (P1.0) and one output (P1.1). Both should be set high to begin. The flip-flop’s Q output connects to P1.0, which the program can read at its leisure. If the bit is low, someone has pressed the switch. To reset the bit, the program brings P1.1, low, then high to clear the flip-flop and bring Q high again, ready to detect another switch press. You can use any free port bits in place of P1.0 and P1.1.

Listing 7-2. Monitors two switches and displays a message when one is pressed.

Chapter 7

Figure 7-5. You can use a 74HC74 D-type flip-flop to latch a switch press. A second port pin clears the flip-flop.

Adding a Keypad

Keypads offer more options than individual switches or pushbuttons, but at lower cost and smaller size than a full keyboard. Examples of products that may use keypads include electronic locks, EPROM programmers, and many test instruments. On some devices, the keys have custom legends that describe the specific functions of the keys, but generic numbered keypads are also useful.

Keypad Types

Some keypads have attached cables that terminate in a connector. Others have headers to which you can solder wires or connect your own cable.

Different keypads follow different decoding schemes for detecting which key is pressed. Some have a dedicated connector pin for each key and a single common pin to which the pins connect when a key is pressed (Figure 7-6). You can wire and access these like a series of individual switches, with a pull-up or pull-down resistor at each switch.

Other keypads use matrix encoding, where the switch connections are arranged in a rectangular array. Figure 7-7 illustrates a typical hex keypad that uses matrix encoding. There are four rows (Y) and four columns (X) to which the switches connect. Each key corresponds to a hexadecimal digit.

Switches and Keypads

Figure 7-6. A line-per-key keypad is a series of momentary switches, with each switch having a common terminal.

In this keypad, each key acts as a normally open pushbutton whose contacts connect one row and one column when the key is pressed. In Figure 7-7, pressing key #1 connects Y1 and X1, pressing key #2 connects Y1 and X2, and so on down to key #F at Y4, X4. By determining which row and column are connected, you can detect which key has been pressed.

Matrix encoding saves on hardware, since each key doesn’t require a dedicated signal line. With Figure 7-7’s keypad, you can detect any of 16 key presses with 8 signal lines. Sixteen keys is a popular size for keypads, but larger and smaller sizes are also available. Telephone-

Switches and Keypads

Custom Keypads

It’s also possible to create a keypad with legends that match your application. One source for these in single or small quantities is Sil-Walker, which offers a variety of membrane keypads in kit form.

A kit consists of the basic keypad, optional colored pads, a faceplate, and a bezel. To create a custom keypad, you apply an optional colored pad and your own legend to each key. You can apply a legend using silk-screen printing, transfer-lettering, or a felt marker. You then press the faceplate over the keypad, and press the bezel onto the faceplate and its mounting surface to secure the keypad in its final location.

Using a Matrix-encoded Keypad

In a matrix-encoded keypad, the usual way to detect a keypress is with scanning circuits. In Figure 7-7’s keypad, rows Y1-Y4 could be tied high through pull-up resistors. Columns X1-X4 could then be scanned, or brought low in sequence. As each column goes low, the logic level at each row is checked, repeating until a row goes low, indicating that a key is pressed.

The column and row that are low identify the key that is pressed. For example, if X3 and Y2 are low at the same time, key #6 has been pressed.

A Keypad Encoder

An easy way to interface a matrix-encoded keypad to the 8052-BASIC is to use National Semiconductor’s 74C922 16-key encoder chip (or the 74C923, which handles up to 20 keys). Figure 7-8 shows the pinout, truth table, and waveforms of the 74C922. The chip is a member of the 74C family, which uses CMOS technology but TTL-type part designations (like the HCMOS family, but not high-speed). The chip is available from many parts sources.

The 74C922 has several useful features:

•It automatically translates each keypress into a 4-bit number (0000 to 1111). The chip has four inputs (Y1-Y4) and four outputs (X1-X4) that connect to the X and Y lines on a keypad, and 4 data outputs (A, B, C, D) that identify the key that was pressed. The 74C922 contains its own scanning circuits, including internal row pullups. All you need to add is a capacitor at the OSC input to set the scanning frequency, or you can use an external clock to control the scanning.

•Keypresses are signaled automatically by a Data Available (DA) output, which goes high when a key is pressed. You can tie DA to an interrupt or port pin on the 8052-BASIC.

Chapter 7

Figure 7-8. Pinout, truth table, and waveforms for the 74C922 16-key encoder for matrix-encoded keypads.

•Latches store the last keypress. When you lift your finger from a key, the keypad’s X and Y lines no longer connect, and there is no way to know that a key was pressed. If the computer is busy doing something else while the key is pressed, it may not see the keypress at all. The 74C922 takes care of this by latching the data that corresponds to the last key pressed.