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

END

How ReadLightMeasurementsFromEeprom.bs2 Works

As with the WRITE command, the READ command uses byte-size addresses. Since words are being read from the EEPROM, the eepromAddress variable has to have 2 added to it each time through the FOR…NEXT loop.

FOR eepromAddress = 0 to 58 STEP 2

The READ command gets the word size value at eepromAddress. This value is loaded into the time variable.

READ eepromAddress, Word time

The value of the time and eepromAddress variables are displayed as columns in a table in the Debug Terminal.

NEXT

Your Turn – Plotting the Stored Data

√Modify ReadLightMeasurementsFromEeprom.bs2 so that it displays the data to Stamp Plot Lite. Remember, the DEBUG statement must only display the value and a carriage return.

ACTIVITY #4: SIMPLE LIGHT METER

Light sensor information can be communicated in a variety of ways. The light meter you will work with in this chapter changes the rate that the display flickers depending on the light intensity it detects.

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Light Meter Parts

(1) Photoresistor

(1) Resistor – 220 Ω (red-red-brown)

(1) Capacitor – 0.01 µF

(1) Capacitor – 0.1 µF

(1) 7-segment LED display

(8) Resistors – 1 kΩ (brown-black-red)

(6) Jumper wires

Building the Light Meter Circuit

Figure 7-9 shows the 7-segment LED display and photoresistor circuit schematics that will be used to make the light meter, and Figure 7-10 shows a wiring diagram of the circuit. The photoresistor circuit is the same one you have been using in the last two activities, and the 7-segment LED display circuit is the one that was controlled by the BASIC Stamp in Chapter #6.

Page 208 · What’s a Microcontroller?

PAUSE 2000

GOSUB First_Subroutine

DEBUG "Back in main.", CR

PAUSE 2000

GOSUB Second_Subroutine

DEBUG "Repeat main...", CR

PAUSE 2000

LOOP

First_Subroutine:

DEBUG " Executing first "

DEBUG "subroutine.", CR

PAUSE 3000

RETURN

Second_Subroutine:

DEBUG " Executing second "

DEBUG "subroutine.", CR

PAUSE 3000

RETURN

How SimpleSubroutines.bs2 Works

Figure 7-12 shows how the First_Subroutine call in the main routine (the DO…LOOP) works. The command GOSUB First_Subroutine sends the program to the First_Subroutine: label. The three commands inside that subroutine are executed. When the program gets to the RETURN statement, it jumps back to the command that comes right after the GOSUB command, which is DEBUG "Back in Main.", CR.

What’s a subroutine call? When you use the GOSUB command to make the program jump to a subroutine, it is called a subroutine call.

Page 210 · What’s a Microcontroller?

√Make any necessary adjustments to the DEBUG commands so that the display looks right with all three subroutines.

You can also call one subroutine from within another. This is called nesting subroutines.

√Try moving the GOSUB that calls Subroutine_Name into one of the other subroutines, and see how it works.

When nesting subroutines the rule is no more than four deep. See the BASIC Stamp Manual for more details. Look up GOSUB and RETURN.

Light Meter Using Subroutines

The segments on the display cycle in a circular pattern that gets faster when the light on the photoresistor gets brighter. When the light gets dimmer, the circular pattern displayed by the 7-segment LED display goes slower.

The program that runs the light meter will deal with three different operations:

1.Read the photoresistor.

2.Calculate how long to wait before updating the 7-segment LED display.

3.Update the 7-segment LED display.

Each operation is contained within its own subroutine, and the main DO…LOOP routine will cycle and call each one in sequence, over and over again.

Example Program: LightMeter.bs2

Controlled lighting conditions make a big difference. For best results, conduct this test in a room lit by fluorescent lights with no direct sunlight (close the blinds). For information on how to calibrate this meter to other lighting conditions, see the Your Turn section.

√Enter and run LightMeter.bs2.

√Verify that the circular pattern displayed by the 7-segment LED display is controlled by the lighting conditions the photoresistor is sensing. Do this by casting a shadow over it with your hand or a piece of paper.

Chapter #7: Measuring Light · Page 211

'What's a Microcontroller - LightMeter.bs2

'Indicate light level using 7-segment display.

'{$STAMP BS2}

'{$PBASIC 2.5}

DEBUG "Program Running!"

IF index = 6 THEN index = 0 ' BAFG.CDE LOOKUP index, [ %01000000,

%10000000,

%00000100,

%00000010,

%00000001, %00100000 ], OUTH

index = index + 1

RETURN

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How LightMeter.bs2 Works

The first two lines of the program declare variables. It doesn’t matter whether these variables are used in subroutines or the main routine, it’s always best to declare variables (and constants) at the beginning of your program. Since this is such a common practice, this section of code has a name, variable declarations. This name is shown in the comment to the right of the first variable declaration.

Many programs also have things that need to get done once at the beginning of the program. Setting all the 7-segment I/O pins low and then making them outputs is an example. This section of a PBASIC program also has a name, initialization.

This next segment of code is called the main routine. The main routine calls the Get_Rc_Time subroutine first. Then, it calls the Delay subroutine, and after that, it calls the Update_Display subroutine. Keep in mind that the program goes through the three subroutines as fast as it can, over and over again.

All subroutines are usually placed after the main routine. The first subroutine’s name is Get_Rc_Time:, and it takes the RC-time measurement on the photoresistor circuit. This subroutine has a PAUSE command that charges up the capacitor. The Duration of this command is small because it only needs to pause long enough to make sure the capacitor is charged. Note that the RCTIME command sets the value of the time variable. This variable will be used by the second subroutine.