Файл: Basic Analog and Digital (Students guide, v13).pdf

Page 20 · Basic Analog and Digital

(misc.) Jumper wires

The Pushbutton

There's just one new part and circuit symbol to introduce for this experiment, the pushbutton in Figure 2-1. Note how each terminal on the circuit symbol corresponds to two pins on the part. If you want to connect to a particular terminal shown on the symbol, you can connect to either (or both) of the two corresponding pins on the part.

The open space in the circuit symbol indicates that the switch is normally open. When the two terminals of a switch are not connected, the switch is referred to as an open circuit. Under normal circumstances (when the pushbutton is not pressed), the circuit is open, thus the name normally open.

Figure 2-1

Pushbutton Circuit Symbol Compared to the Component

Building the Circuit

Figure 2-2 shows the schematic for this experiment. Remember to think of a schematic as a list of components and connections. For example, the anode of the right LED is connected to the P5 terminal on the Board of Education. The cathode is connected to one terminal of a 470 resistor. The other pin on that same resistor is connected to the Vss terminal on the Board of Education, and so on. Follow schematics faithfully when constructing circuits.

Chapter 2: Introduction to Bit Crunching · Page 23

'Basic Analog and Digital - PL2_1R0.bs2

'Program Listing 2.1 Revision 0.

'{$STAMP BS2}

'{$PBASIC 2.5}

INPUT 0

INPUT 1

OUTPUT 4

OUTPUT 5

DEBUG CLS

DO

a = IN0 b = IN1 OUT4 = b OUT5 = a

d = (2*b) + (1*a)

DEBUG HOME, "State of pin P0 is ", BIN a, CR DEBUG "State of pin P1 is ", BIN B, CR, CR DEBUG "2-bit binary number: ", CR

LOOP

The Output

Here's how the program should work. When no pushbuttons are pressed, the Debug Terminal output should match Figure 2-4, and both LEDs should be off. Try pressing the right button (in Figure 2-3). Did the right LED light up? Did the state of P0 in the Debug Terminal change to 1? Is the decimal equivalent 1? If so, it looks like your circuit and program are working well so far.

Page 24 · Basic Analog and Digital

Figure 2-4

Debug Terminal

Output for Program

Listing 2.1

So how do you count from decimal-0 to decimal-3 using the binary pushbuttons? The two-bit binary equivalent of decimal-0 is 00. When you don’t press either of the pushbuttons, the decimal output is 0 in the Debug Terminal. When you press the right button, you get 01, which has a decimal equivalent of 1. When you press the left button, you get 10, which has a decimal equivalent of 2. When you press both pushbuttons, you get 11, which has a decimal equivalent of 3.

About the Code

As with the program of the previous chapter, the first lines start with an apostrophe, so they are comments and compiler directives, and the BASIC Stamp ignores them.

'Basic Analog and Digital - PL2_1R0.bs2

'Program Listing 2.1 Revision 0.

'{$STAMP BS2}

'{$PBASIC 2.5}

Chapter 2: Introduction to Bit Crunching · Page 25

Next, three variables are defined. Variables can be used to store values while the program is running. The letters a and b are defined as variables that store 1-bit each. So, the variable a can store a single binary digit, likewise with the variable b. The letter d is defined to be a variable that stores a "nibble" of binary information.

Memory and Food: A bit of memory can store one binary digit, either a 0 or a 1.

A nibble of memory stores 4-bits.

A byte (pronounced bite) stores 8-bits.

A word stores 16 bits.

Since a bit, a nibble, and a byte, all sound like references to eating food, perhaps a better name for 16-bits might have been "dinner".

This segment of code uses commands introduced in the last chapter. First, two I/O pins are declared inputs and two more pins are declared outputs. Then the Debug Terminal is opened and cleared.

INPUT 0

INPUT 1

OUTPUT 4

OUTPUT 5

DEBUG CLS

We want the BASIC Stamp keep checking the inputs over and over again. We also want the BASIC Stamp to automatically update the LEDs and the Debug Terminal with the latest information on the pushbuttons. The way to accomplish this is to keep repeating the program inside a DO…LOOP loop. To define the start point of the loop we use DO and to send the program back to this point we’ll use the LOOP command.

DO

Next, we need to check the state of the pushbuttons by checking the input at pins P0 and P1. The first of these two commands sets the bit variable a equal to the state measured at pin P0. The second command sets the bit variable b equal to the state measured at pin P1.

a = IN0 b = IN1

Page 26 · Basic Analog and Digital

Next, we need to set the output at pin P4 equal to the input taken at pin P1. The left LED which is connected to P4 will light up when the left button, which is connected to P1, is pressed. Likewise, we need to set the output at pin P5 equal to the input measured at pin P0.

Since the input values were set to the variables, a and b, we can use a and b to dictate the output values at pins P4 and P5.

OUT4 = b

OUT5 = a

We could just as easily have used the commands OUT4=IN1 and OUT5=IN0; however, using variables to store the values in memory has advantages as the programs get more complicated. In the next experiment, it will be necessary to use variables to store values.

The reason we used variables in this program is because they can be manipulated arithmetically, and the next task is to convert from binary to decimal. To do this, multiply the variable b by 2 and the variable a by 1 and add them together. The nibble variable d is used to store this new value. This is the method for converting a 2-bit binary number to a decimal number. The next section shows how to do this for a binary number of any size.

d = (2*b) + (1*a)

BASIC Stamp Memory:

RAM: The BASIC Stamp has 26 bytes of RAM (random access memory) that can be used for storing variable values. Another 6 bytes of RAM is used to interface the BASIC Stamp with its I/O pins.

EEPROM: Short for electrically erasable programmable read only memory, EEPROM is used mainly to store the PBASIC programs. EEPROM can also be used to store data values that do not change frequently.

In the calculation we just did using PBASIC, the parentheses are necessary to maintain the normal algebraic order of operation. This is because the BASIC Stamp performs its math beginning at the left. Then, it performs each operation it encounters while checking the line from left to right.

Without the parentheses, d would be set equal to the value ((2 x b + 1) x a) because that's the order in which the operators (+, -,*, /, etc) are encountered. When parentheses are used, the BASIC Stamp completes operations within parentheses first, and then it does its sweep of operations from left to right.