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Intro to Physical Computing Syllabus code, circuits, & construction
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Lab Assignment |
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Variables and Analog Input |
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Minimum parts needed: (new parts in bold. see parts list for details)
Step 1: Connect a potentiometer up to pin RA0 of your PIC 18F452 or pin 13 of your BX-24 as shown.
PIC with pot on RA0. Note the decoupling capacitor to smooth out the results.
BX-24 with pot on pin 13 On the PIC, you'll also need to connect wires to pins RC6 and RC7 to a DB9 serial connector, and connect it to the computer's serial port. For more on this, see the debug notes. Program the PIC to read the varying voltage coming through as follows: ' PicBasic Pro program to display result of
' 10-bit A/D conversion through serial at 9600 baud
'
' Connect analog input to channel-0 (RA0)
' Define ADCIN parameters
DEFINE ADC_BITS 10 ' Set number of bits in result
DEFINE ADC_CLOCK 3 ' Set clock source (3=rc)
DEFINE ADC_SAMPLEUS 50 ' Set sampling time in uS
ADCvar VAR WORD ' Create variable to store result
TRISA = %11111111 ' Set PORTA to all input
ADCON1 = %10000010 ' Set PORTA analog and right justify result
Pause 500 ' Wait .5 second
main:
ADCIN 0, ADCvar ' Read channel 0 to adval
serout2 PORTC.6, 16468, [DEC ADCvar, 13, 10] ' print it to serial out,
' with linefeed and carriage return (10, 13)
GoTo main ' Do it forever
Program the BX-24 to read the varying voltage coming through the pot as follows: dim potVar as integer Sub main() call delay(0.5) ' start program with a half-second delay do potVar = getADC(13) debug.print "potVar = " ; cstr(potVar) loop end sub Notice the range of numbers the pot returns. See what happens when you put a resistor in series with the power end or the ground end. |
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| Step 2:
Connect another variable resistor RA0 or your PIC, or pin 13 of your BX-24 as shown:
PIC with flex sensor on RA0. Note the decoupling capacitor to smooth out the results.
BX-24 with flex sensor on pin 13 Program the PIC or BX-24 to read the varying voltage coming through the sensor as shown above. Try differing values for the fixed resistor in the circuit. Note how they change the value returned by getADC(). Try using a potentiometer instead of the fixed resistor (the center pin and either one of the side pins will do). Notice how the range changes when you vary the pot or the variable resistor. |
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| Step 3:
Use an analog sensor (a pot or other variable resistor) in an application. Perhaps the changing value of a sensor affects how many lights or motors are turned on or off; perhaps a seven-segment LED display is used to generate numbers. Or a practical joke with the BX-24. Make a sensor that lights a series of LEDs only after you affect it beyond a certain threshold, or a step pad that lights up only after the user steps away. Use variables to keep track of the changes of the switches and analog sensors as the user interacts with the project. Look for other digital output devices that will respond to the output of the BX-24: piezo buzzers are a start. See what else you can find to add some sensory stimulation to your project. Once you have exhausted your creativity on this lab assignment, explore the analog output lab assignment for more possibilities. |
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| Still not sure about variables?
The following steps are illustrations of how variables work. If you're still confused a bit about variables, these might help. |
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| Step 4:
Wire a switch to pin RB0 of your PIC, or pin 5 of your BX-24. Use the same digital input circuit from the previous lab. Wire an LED to pin RB7 of your PIC or pin 20 of your BX-24. Write a program that stores the state of pin 5 in a variable, and changes the LED based on that variable. PicBasic Pro: switchVar var byte
input portb.0
output portb.7
pause 500 ' start program with half-second delay
main:
switchVar = portb.0
portb.7 = switchVar
goto main
BX-24: dim switchVar as byte Sub main() call delay(0.5) ' start program with a half-second delay do switchVar = getPin(5) call putPin(20, switchVar) loop end sub |
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| Step 5:
Write a program that counts the number of times the switch has been switched on, and puts it in a variable: PicBasic Pro:
switchedOnVar var byte
input portb.0
pause 500 ' start program with a half-second delay
main:
if portb.0 = 1 then
switchedOnVar = switchedOnVar + 1
endif
serout2 portc.6, 16468, [DEC switchedOnVar, 13,10]
goto main
BX-Basic: dim switchedOnVar as byte Sub main() call delay(0.5) ' start program with a half-second delay do if getPin(5) = 1 then switchedOnVar = switchedOnVar + 1 end if debug.print cStr(switchedOnVar) loop end sub Why does the counter run so high? How can you make the variable increment only once per switch press? |
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| Step 6:
Connect 8 LED's to pins RB0 through RB7 of your PIC or pins 5-12 of your BX-24. Run the following code to see the binary contents of a memory register reflected in the LED's: x var byte
trisb = %00000000
pause 500 ' start program with a half-second delay
main:
for x = 0 to 255
portb = x
pause 1000
next
pause 3000
goto main
BX-24: Sub main() call delay(0.5) ' start program with a half-second delay dim counterVar as byte register.ddrc = bx1111_1111 register.portc = bx0000_0000 do for counterVar = 0 to 255 register.portc = counterVar call delay(1.0) next call delay(3.0) loop end sub The portb on the PIC, or register.portc on the BX-24, are special memory registers, connected directly to the output pins of the LEDs. When you increment the value in these memory registers, the change is seen in the pins. If you read all eight pins as an eight-bit number in base two, you will know what number is in the computer's memory at register.portc. See the BX-24 register example for more details. |
