Whats A Microcontroller v3

Electronic Building Blocks · Page 293

Here is a summary of each of the AD5220’s pins and functions:

1.CLK: The pin that receives clock pulses (low-high-low signals) to move the wiper terminal.

2.U/D: The pin that receives a high signal to make the wiper (W1) terminal move towards A1, and a low signal to make it move towards B1. This pin just sets the direction, the wiper terminal doesn’t actually move until a pulse (a low – high – low signal) is sent to the CLK pin.

3.A1: The potentiometer’s A terminal.

4.GND: The ground connection. The ground on the Board of Education and BASIC Stamp HomeWork Board is the Vss terminal.

5.W1: The potentiometer’s wiper (W) terminal.

6.B1: The potentiometer’s B terminal.

7.CS: The chip select pin. Apply a high signal to this pin, and the chip ignores all control signals sent to CLK and U/D. Apply a low signal to this pin, and it acts on any control signals it receives.

8.Vdd: Connect to +5 V, which is Vdd on the Board of Education and BASIC Stamp HomeWork Board.

The AD5220 Part Datasheet: To see the part datasheet for the AD5220: Go to www.analog.com. Enter “AD5220” into the Search field on Analog Devices’ home page, and click the Search button. Click the Data Sheets link. Click the link that reads “AD5220: Increment/Decrement Digital Potentiometer Datasheet”.

Digital Pot Controlled Transistor Parts

(1)Transistor – 2N3904

(2)Resistors – 100 kΩ (brown-black-yellow)

(1)LED – any color

(1)Digital potentiometer – AD5220

(10)Jumper wires

Building the Digital Potentiometer Circuit

Figure 9-6 shows a circuit schematic with the digital potentiometer used in place of a manual potentiometer, and Figure 9-7 shows a wiring diagram for the circuit. The BASIC Stamp can control the digital potentiometer by issuing control signals to P5 and P6.

9 Build the circuit shown in Figure 9-6 and Figure 9-7.

Electronic Building Blocks · Page 295

Imagine next that you turn your manual potentiometer 3 steps counterclockwise. The LED will get a little bit dimmer. This would be the same as sending a low signal to the U/D pin on the AD5220 and sending three pulses to the CLK pin.

LOW 5

FOR counter = 1 TO 3

PULSOUT 6, 1

PAUSE 1

NEXT

Imagine next that you turn the potentiometer all the way clockwise. That’s the same as sending a high signal to the AD5220’s U/D pin and sending 65 pulses to the CLK pin. Now the LED should be shining brightly.

HIGH 5

FOR counter = 1 TO 65

PULSOUT 6, 1

PAUSE 1

NEXT

Finally, imagine that you turn your manual potentiometer all the way counterclockwise. The LED should emit no light. That’s the same as sending a low signal to the U/D pin, and applying 128 pulses to the CLK pin

LOW 5

FOR counter = 0 TO 127

PULSOUT 6, 1

PAUSE 1

NEXT

Example Program: DigitalPotUpDown.bs2

This example program adjusts the potentiometer up and down, from one end of its range to the other, causing the LED to get gradually brighter, then gradually dimmer.

9 Enter and run DigitalPotUpDown.bs2.

Page 296 · What’s a Microcontroller?

'What's a Microcontroller - DigitalPotUpDown.bs2

'Sweep digital pot through values.

'{$STAMP BS2}

'{$PBASIC 2.5}

DEBUG "Program Running!"

DO

LOW 5

FOR counter = 0 TO 127

PULSOUT 6, 1

PAUSE 10

NEXT

HIGH 5

FOR counter = 0 TO 127

PULSOUT 6, 1

PAUSE 10

NEXT

LOOP

Your Turn – Changing the Rate and Condensing the Code

You can increase or decrease the rate at which the LED gets brighter and dimmer by changing the PAUSE command’s Duration argument.

9Modify and re-run the program using PAUSE 20 and note the difference in the rate that the LED gets brighter and dimmer.

9Repeat for PAUSE 5.

You can also use a command called TOGGLE to make this program simpler. TOGGLE changes the state of a BASIC Stamp I/O pin. If the I/O pin was sending a high signal, TOGGLE makes it send a low signal. If the I/O pin was sending a low signal, TOGGLE makes it send a high signal.

9Save DigitalPotUpDown.bs2 as DigitalPotUpDownWithToggle.bs2.

9Modify the program so that it looks like the one by that name shown below.

9Run the program and verify that it functions the same way as the DigitalPotUpDown.bs2.

9Compare the number of lines of code it took to do the same job.

Electronic Building Blocks · Page 297

Running out of program memory is a problem some people encounter when their BASIC Stamp projects get large and complicated. Using TOGGLE instead of two FOR...NEXT loops is just one example of many techniques that can be used to do the same job with half the code.

'What's a Microcontroller - DigitalPotUpDownWithToggle.bs2

'Sweep digital pot through values.

'{$STAMP BS2}

'{$PBASIC 2.5}

DEBUG "Program Running!"

DO

FOR counter = 0 TO 127

PULSOUT 6,5

PAUSE 10

NEXT

TOGGLE 5

LOOP

Looking Inside the Digital Potentiometer

Figure 9-8 shows a diagram of the potentiometer inside the AD5220. The AD5220 has 128 resistive elements, each of which is 78.125 Ω (nominal value). All 128 of these add up to 10,000 Ω or 10 kΩ.

78 Ω pos. 1

78 Ω pos. 0 B1

6

Page 298 · What’s a Microcontroller?

A nominal value means a named value. Parts like resistors and capacitors typically have a nominal value and a tolerance. Each of the AD5220’s resistive elements has a nominal value of 78.125 Ω, with a tolerance of 30% (23.438 Ω) above or below the nominal value.

Between each of these resistive elements is a switch, called a tap. Each switch is actually a group of transistors that are switched on or off to let current pass or not pass. Only one of these switches can be closed at one time. If one of the upper switches is closed (like pos. 125, 126, or 127), it’s like having the manual potentiometer knob turned most or all the way clockwise. If pos. 0 or 1 is closed, it’s like having a manual potentiometer turned most or all the way counterclockwise.

Imagine that Pos. 126 is closed. If you want to set the tap to 125, (open pos. 126 and close pos. 125), set U/D low, then apply a pulse to CLK. If you want to set the tap to Pos 127, set U/D high, and apply 2 pulses. If you want to bring the tap down to 1, set U/D low, and apply 126 pulses.

This next example program uses the Debug Terminal to ask you which tap setting you want. Then it decides whether to set the U/D pin high or low, and applies the correct number of pulses to move the tap from its old setting to the new setting.

With the exception of EEPROM Data, the next example program also has all the sections you could normally expect to find in an application program:

•Title – comments that include the filename of a program, its description, and the Stamp and PBASIC directives

•EEPROM Data – DATA declarations that store predefined lists of values in portions of EEPROM memory that are not needed for program storage

•I/O Definitions – PIN directives that name I/O pins

•Constants – CON declarations that name values in the program

•Variables – VAR declarations that assign names to portions of BASIC Stamp’s RAM memory for storing values

•Initialization – a routine that gets the program started on the right foot. In this next program, the potentiometer’s tap needs to be brought down to zero

•Main – the routine that handles the primary jobs the program has to do

•Subroutines – the segments of code that do specific jobs, either for each other, or in this case, for the main routine

Electronic Building Blocks · Page 299

Example Program: TerminalControlledDigtialPot.bs2

You can use this example program and the Debug Terminal to set the digital pot’s tap. By changing the tap setting on the digital pot, you change the brightness of the LED connected to the transistor that the digital pot controls. Figure 9-9 shows an example of entering the value 120 into the Debug Terminal’s Transmit windowpane while the program is running. Since the old tap setting was 65, the LED becomes nearly twice as bright when it is adjusted to 120.

numbers for the new tap setting.

9Enter and run TerminalControlledDigtialPot.bs2.

9Make sure the Echo Off box is clear (no checkmark).

9Click the Debug Terminal’s Transmit windowpane to place the cursor there.

9Enter values between 0 and 127 into the Debug Terminal. Make sure to press the enter key after you type in the digits.

'-----[ Title ]-----------------------------------------------------------

'What's a Microcontroller - TerminalControlledDigitalPot.bs2

'Update digital pot's tap based on Debug Terminal user input.

'{$STAMP BS2}

'{$PBASIC 2.5}

'-----[ EEPROM Data ]-----------------------------------------------------

'-----[ I/O Definitions ]-------------------------------------------------