What is a Microcontroller (Paralax, v2.2, student guide, 2004)

Page 82 · What’s a Microcontroller?

DEBUG ? IN3

IF (IN3 = 1) THEN

HIGH 14

PAUSE 50

LOW 14

PAUSE 50

ELSE

PAUSE 100

ENDIF

LOOP

How PushbuttonControlledLed.bs2 Works

This program is a modified version of ReadPushbuttonState.bs2 from the previous activity. The DO…LOOP and DEBUG ? IN3 commands are the same. The PAUSE 250 was replaced with an IF…THEN…ELSE statement. When the condition after the IF is true (IN3 = 1), the commands that come after the THEN statement are executed. They will be executed until the ELSE statement is reached, at which point the program skips to the ENDIF and moves on. When the condition after the IF is not true (IN3 = 0), the commands after the ELSE statement are executed until the ENDIF is reached.

You can make a detailed list of what a program should do, to either help you plan the program or to describe what it does. This kind of list is called pseudo code, and the example below uses pseudo code to describe how PushbuttonControlledLed.bs2 works.

• Do the commands between here and the Loop statement over and over again o Display the value of IN3 in the Debug Terminal

oIf the value of IN3 is 1, Then

Turn the LED on

Wait for 1/20 of a second

Turn the LED off

Wait for 1/20 of a second

oElse, (if the value of IN3 is 0)

do nothing, but wait for the same amount of time it would have taken to briefly flash the LED (1/10 of a second).

•Loop

Chapter #3: Digital Input - Pushbuttons · Page 83

Your Turn – Faster/Slower

√Save the example program under a different name.

√Modify the program so that the LED flashes twice as fast when you press and hold the pushbutton.

√Modify the program so that the LED flashes half as fast when you press and hold the pushbutton.

ACTIVITY #4: TWO PUSHBUTTONS CONTROLLING TWO LED CIRCUITS

Let’s add a second pushbutton into the project and see how it works. To make things a little more interesting, let’s also add a second LED circuit and use the second pushbutton to control it.

Pushbutton and LED Circuit Parts

(2) Pushbuttons – normally open

(2) Resistors - 10 kΩ (brown-black-orange)

(2) Resistors – 470 Ω (yellow-violet-brown)

(2) Resistors – 220 Ω (red-red-brown)

(2) LEDs – any color

Adding a Pushbutton and LED Circuit

Figure 3-12 shows a second LED and pushbutton circuit added to the circuit you tested in the previous activity.

√Build the circuit shown in Figure 3-12. If you need help building the circuit shown in the schematic, use the wiring diagram in Figure 3-13 as a guide.

√Modify ReadPushbuttonState.bs2 so that it reads IN4 instead of IN3, and use it to test your second pushbutton circuit.

Chapter #3: Digital Input - Pushbuttons · Page 85

Programming Pushbutton Control

In the previous activity, you experimented with making decisions using an IF…THEN…ELSE statement. There is also such a thing as an IF…ELSEIF…ELSE statement. It works great for deciding which LED to flash on and off. The next example program shows how it works.

Example Program: PushbuttonControlOfTwoLeds.bs2

√Enter PushbuttonControlOfTwoLeds.bs2 into the BASIC Stamp Editor and run it.

√Verify that the LED in the circuit connected to P14 flashes on and off while the pushbutton in the circuit connected to P3 is held down.

√Also check to make sure the LED in the circuit connected to P15 flashes while the pushbutton in the circuit connected to P4 is held down

'What's a Microcontroller - PushbuttonControlOfTwoLeds.bs2

'Blink P14 LED if P3 pushbutton is pressed, and blink P15 LED if

'P4 pushbutton is pressed.

'{$STAMP BS2}

'{$PBASIC 2.5}

DO

Page 86 · What’s a Microcontroller?

DEBUG HOME

DEBUG ? IN4

DEBUG ? IN3

IF (IN3 = 1) THEN

HIGH 14

PAUSE 50

ELSEIF (IN4 = 1) THEN

HIGH 15

PAUSE 50

ELSE

PAUSE 50

ENDIF

LOW 14

LOW 15

PAUSE 50

LOOP

How PushbuttonControlOfTwoLeds.bs2 Works

If the display of IN3 and IN4 scrolled down the Debug Terminal as they did in the previous example, it would be difficult to read. One way to fix this is to always send the cursor to the top-left position in the Debug Terminal using the HOME formatter:

DEBUG HOME

By sending the cursor to the home position each time through the DO…LOOP, the commands:

DEBUG ? IN4

DEBUG ? IN3

display the values of IN4 and IN3 in the same part of the Debug Terminal each time.

The DO keyword begins the loop in this program:

DO

These commands in the IF statement are the same as the ones in the example program from the previous activity:

IF (IN3 = 1) THEN

HIGH 14

Chapter #3: Digital Input - Pushbuttons · Page 87

PAUSE 50

This is where the ELSEIF keyword helps. If IN3 is not 1, but IN4 is 1, we want to turn the LED connected to P15 on instead of the one connected to P14.

ELSEIF (IN4 = 1) THEN

HIGH 15

PAUSE 50

If neither statement is true, we still want to pause for 50 ms without changing the state of any LED circuits.

ELSE

PAUSE 50

When you’re finished with all the decisions, don’t forget the ENDIF.

ENDIF

It’s time to turn the LEDs off and pause again. You could try to decide which LED you turned on and turn it back off. PBASIC commands execute pretty quickly, so why not just turn them both off and forget about more decision making?

LOW 14

LOW 15

PAUSE 50

The LOOP statement sends the program back up to the DO statement, and the process of checking the pushbuttons and changing the states of the LEDs starts all over again.

LOOP

Your Turn – What about Pressing Both Pushbuttons?

The example program has a flaw. Try pressing both pushbuttons at once, and you’ll see the flaw. You would expect both LEDs to flash on and off, but they don’t because only one code block in an IF...ELSEIF…ELSE statement gets executed before it skips to the

ENDIF.

Here is how you can fix this problem:

√Save PushbuttonControlOfTwoLeds.bs2 under a new name.

√Replace this IF statement and code block:

IF (IN3 = 1) THEN

Page 88 · What’s a Microcontroller?

HIGH 14

PAUSE 50

with this IF...ELSEIF statement:

IF (IN3 = 1) AND (IN4 = 1) THEN

HIGH 14

HIGH 15

PAUSE 50

ELSEIF (IN3 = 1) THEN

HIGH 14

PAUSE 50

A code block is a group of commands. The IF statement above has a code block with three commands (HIGH, HIGH, and PAUSE). The ELSEIF statement has a code block with two commands (HIGH, PAUSE).

√Run your modified program and see if it handles both pushbutton and LED circuits as you would expect.

The AND keyword can be used in an IF…THEN statement to check if more than one condition is true. All conditions with AND have to be true for the IF statement to be true.

The OR keyword can also be used to check if at least one of the conditions are true.

You can also modify the program so that the LED that’s flashing stays on for different amounts of time. For example, you can reduce the Duration of the PAUSE for both pushbuttons to 10, increase the PAUSE for the P14 LED to 100, and increase the PAUSE for the P15 LED to 200.

√Modify the PAUSE commands in the IF and the two ELSEIF statements as discussed.

√Run the modified program.

√Observe the difference in the behavior of each light.

ACTIVITY #5: REACTION TIMER TEST

You are the embedded systems engineer at a video game company. The marketing department recommends that a circuit to test the player’s reaction time be added to the

Chapter #3: Digital Input - Pushbuttons · Page 89

next hand held game controller. Your next task is to develop a proof of concept for the reaction timer test.

The solution you will build and test in this activity is an example of how to solve this problem, but it’s definitely not the only solution. Before continuing, take a moment to think about how you would design this reaction timer.

Reaction Timer Game Parts

(1) LED – bi-color

(1) Resistor – 470 Ω (yellow-violet-brown)

(1) Pushbutton – normally open

(1)Resistor – 10 kΩ (brown-black-orange)

(1)Resistor – 220 Ω (red-red-brown)

(2)Jumper wires

Building the Reaction Timer Circuit

Figure 3-14 shows a schematic and wiring diagram for a circuit that can be used with the BASIC Stamp to make a reaction timer game.

√Build the circuit shown in Figure 3-14.

√Run TestBiColorLED.bs2 from Chapter #2, Activity #5 to test the bi-color LED circuit and make sure your wiring is correct.

√If you re-built the pushbutton circuit for this activity, run ReadPushbuttonState.bs2 from Activity #2 in this chapter to make sure your pushbutton is working properly.