EXERCISE 3: Calculations and Input Conditioning

Transcription

1 EXERCISE 3: Calculations and Input Conditioning EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe switch bounce, explain switch debouncing, and use Flowcode to program your FACET board to perform binary addition. EXERCISE DISCUSSION Switch Bounce Switches contain two metal contacts, made of brass. When switched off, these contacts are held apart, separated by an insulator - air. To switch on, these contacts are slapped together by some lever mechanism, aided by a strong spring. Now, metals are springy. When the contacts hit each other, they bounce apart, hit each other again and may continue to do this a number of times before coming to rest in contact with each other. All of this happens very quickly. We normally do not notice it, and it causes no problem. When the switch is part of a switch unit, this bouncing causes a series of changes in output, as shown in the diagram. We can show these changes on a voltage/time graph: 150 FACET by Lab-Volt

2 Loops, Messages, and Calculations Switch debouncing Switch bounce is a problem when the switch is part of a counting system. Each time the switch is pressed, instead of the count increasing by one, it can increase by two, three or four, or more! The solution is to debounce the switch. This does not stop the mechanical bounces of the switch, but cuts out the repeated electrical pulses that the bounce otherwise causes. We add a debouncing circuit to the output of the switch unit. There are a number of techniques that can be used in the debouncing circuit. We will use a time delay. This relies on making the first of the electrical pulses trigger a long pulse that lasts longer than the train of pulses caused by the bouncing. To do this, add a time delay circuit between the switch unit and the counting system. The effect of this is illustrated in the following graphs. The counting system receives just a single pulse each time the switch is pressed. Switch bounce is very fast - a simple way of avoiding switch bounce problems is to simply make your program wait for a millisecond after a switch input is detected. EXERCISE QUESTIONS One way to eliminate the errors in counting that can be caused by switch bouncing is to add a a. loop. b. string. c. time delay circuit FACET by Lab-Volt 151

3 EXERCISE PROCEDURE Set up the equipment The following is the equipment you will need for this exercise: Multiprogrammer Hardware: Setting: PICmicro Device Toggle Switch (Fast/Slow) Toggle Switch (RC/XTAL) XTAL PORTA PORTB PORTC PORTD PORTE 16F877A N/A XTAL 19,660,800 Hz N/A Switch Block Switch Block LED Block N/A A binary adder In this section you can see how the PICmicro brain works by making a system that can add two numbers. The simplest way to input a binary number is to use a set of switches attached to the input port. We want to input two binary numbers, so we will need two sets of switches, and two input ports. We want to see the result of the calculation. For now, we will use a LED array, connected to the output port, to show the result. We need a PICmicro chip with three ports! 152 FACET by Lab-Volt

4 Loops, Messages, and Calculations Set up the flowchart 1. Launch Flowcode. Start a new flowchart. 2. Select 16F877A from the Choose a Target Dialog box scroll down menu, then click OK to close the Dialog box. 3. Click and drag a loop icon between the Begin and End boxes. 4. Click and drag an input icon and drop it in between the ends of the loop. 5. Click and drag a second input icon and drop it in between the ends of the loop. 6. Click and drag an output icon and drop it just below the input boxes. 7. Click and drag a Calculation icon, and place it in between the second input icon and the output icon. 8. Your flowchart should look like the one shown. FACET by Lab-Volt 153

5 Create the variables 1. Click on Edit, on the menu bar, and then on 'Variables...'. This opens up the Variable Manager dialogue box. 2. We are going to create three variables, called 'input1', 'input2' and 'sum'. 3. The first two will store the numbers that we feed in from the switches. The variable SUM will store the result of adding together the two numbers stored in INPUT1 and INPUT2. 4. Click on the Add New Variable button in the dialogue box. You now see another dialogue box. 5. Type in the word 'input1', and then click on OK. Leave 'Variable type:' as Byte. This dialogue box closes and the name of the new variable appears in the Variable Manager dialogue box. 6. Create the other two variables, 'input2' and 'sum', in the same way. 7. Click on the Close button to shut down the Variable Manager. 154 FACET by Lab-Volt

6 Loops, Messages, and Calculations Set up the inputs 1. Right click on the top input icon, and select Properties from the menu. The following dialogue box appears. 2. Double click on the word 'Input' in the Display name window to highlight it. Type "Input the first number" to replace it. This will appear alongside the input icon on the flowchart. In a complex flowchart, adding labels like this helps users to understand what is happening. 3. Click on the 'Variables...' button. This will open the Variable Manager. It lists the three variables that you just created. 4. Click on the word 'input1'. Next click on the 'Use Variable' button. FACET by Lab-Volt 155

7 5. Back to the Input Properties box again. It is set to Port A at the moment. Click on the down arrow at the end of the Port window, and select PORT B. Click on OK to close the Input Properties box. 6. Double-click on the second input icon. (This is a quick way to open the Properties box, instead of right clicking on the icon and selecting from the menu.) Set up this input to: display the label 'Input the second number' use the variable input2 and use PORT C 7. Close the Input Properties box. Set up the calculation Double click on the Calculation icon to open the Properties dialogue box. 1. Change the 'Display name:' text box to "Add the two numbers together". 2. Tell the Calculations icon what it is supposed to calculate by adding the following to the 'Calculations:' box: 156 FACET by Lab-Volt

8 Loops, Messages, and Calculations sum = input1 + input2 You can either type this in directly, or use the 'Browse for variable...' button to select each variable in turn, and then add the '=' and '+' signs in the right place! 3. Then click on the 'OK' button, to close the dialogue box. Set up the output 1. Double click on the output icon, to open the Output Properties dialogue box. 2. Click on the 'Variables...' button. 3. Click on the word 'sum' and then on the 'Use Variable' button. 4. You are now back on the Output Properties box. Change the port used to PORT D. 5. Click on 'OK' to close the Output Properties box. 6. The flowchart should now look like the one shown. FACET by Lab-Volt 157

9 Adding the LED array 1. Click once on the LED icon. Place the LED array where you want it on the work area by moving the cursor over the title bar of the LED's box and then clicking and dragging the box into position. 2. Click once on the button. Then click once on the 'Components Connections...' option on the menu to open the 'Specify Component Pin Connections' dialogue box. 3. Click on the down arrow at the end of the Port window, and click once on the PORT D option to connect the LED's to the pins on Port D. Click on the 'Done' button to finish the process. Adding the switches You will use two sets of switches, one for each binary number. The output port has only eight bits, and so the biggest number that it can output is This is equal to 255 in decimal. We are going to limit ourselves to inputting only seven bit numbers. This means that the biggest number we can input is , which equals 127 in decimal. If we used bigger numbers, then we would overflow the capacity of the output. 1. Click once on the switch icon, and drag it into a suitable spot on the work area. 2. Click once on the button, and select the 'Properties' option. Using the arrow in the 'Number of Switches' box, select 7 switches. In the same way, select 'Toggle' in the 'Switch Type' box. Then click on the 'OK' button. 3. Click once again on the button, and select the 'Component Connections' option. Connect the switches to Port B, using the down arrow at the end of the Port box. 4. Now set up the second set of switches by following the same process. This time, connect the switches to Port C. Slow simulation Flowcode allows you to run through the flowchart one step (icon) at a time. You can watch the effect of each step on the variables and on the output. 1. There are three ways to run the program step-by-step: a. on the menu click on the RUN... STEP INTO b. press the F8 function key on the keyboard c. click on the 'Step Into' icon on the simulation bar. Do one of these! 2. Several things happen. a. A red rectangle appears around the BEGIN icon, showing that this is the current step. b. Two more boxes appear - Variables and Call Stack. 158 FACET by Lab-Volt

10 Loops, Messages, and Calculations c. The Variables box lists the three variables that you defined for this program, and shows their current values - all zero at the moment. Ignore the Call Stack box for the moment. 3. Now set up two numbers on the switch components. Move the cursor over the switch box connected to Port B. Click on switches B0, B1, and B3, to close these switches. The switch box now looks like the one shown. You have set up the binary number (which is the number eleven in decimal). B6 gives the most significant bit of the number, and B0 the least significant bit. Set up the number (fifteen) on the switch box connected to Port C. 4. Now step on to the next icon in the program by, for example, pressing F8 once more. 5. The red rectangle moves on to the next icon, the Loop icon, but little else happens. 6. Press F8 once again. The red rectangle moves on to the first INPUT icon. 7. Press F8 again and the Variable box shows that the INPUT1 variable now contains 11. This is the result of the INPUT instruction just carried out. 8. Press F8 again and the Variable box shows that INPUT2 now contains Press F8 again and the calculation is carried out. The SUM variable stores the result. 10. Press F8 again. The value stored in SUM is transferred to the LED array. FACET by Lab-Volt 159

11 Reading from Most significant bit (D7) to Least significant bit (D0), the LED array shows the number In decimal, this is the number 26. No surprises there then! 11.Repeat step 3 using different numbers, and step through the program to check what the sum of the numbers is. The program is finished, and working. As usual, you need to let Flowcode translate it into machine code, and burn it into a PICmicro chip. 12. Set up the Clock for XTAL in Flowcode and on the FACET Board using the Toggle switch. 13. Save the program as Ex 3: Calculations and Input Conditioning with your initials in your folder and run the program on the FACET Board. Note that the Switches on the FACET Board are momentary switches rather than toggle switches. You must, therefore, hold down the switches you want the program to add. 160 FACET by Lab-Volt

12 Loops, Messages, and Calculations EXERCISE QUESTIONS 1. In this exercise you input two binary numbers by using a. two sets of switches and one input port. b. one set of switches and two input ports. c. two sets of switches and two input ports. 2. The results were shown using a(n) a. LCD array b. LED array c. Key pad EXERCISE CONCLUSION When switch contacts hit each other, they bounce apart. They may repeat this process a number of times before coming to rest. Switch bounce creates a problem when the switch is part of a counting system. A debounce circuit cuts out the repeated electrical pulses caused by bouncing. A debounce circuit is created by programming a time delay between a switch unit and a counting system. REVIEW QUESTIONS 1. When a switch is turned on, two metal contacts a. always make contact immediately and remain that way until the switch is turned off. b. always separate from each other and remain that way until the switch is turned off. c. often make and break contact before they come to rest with an insulator between them. d. often make and break contact several times before coming to rest in contact with each other. 2. Using a time delay to debounce a switch a. does not stop the mechanical bounces of the switch. b. stops the mechanical bounces of the switch. c. cuts out the repeated electrical pulses caused by bouncing. d. Both a and c are correct. 3. The following sequence represents an attempt to debounce a switch: a. Time delay - Switch unit - Counting system b. Switch unit - Time delay - Counting system c. Switch unit - Counting system - Time delay d. Counting system - Time delay - Switch unit 4. When a switch is part of a switch unit, bouncing causes: a. a series of changes in output. b. a series of changes in input. c. a series of changes in both output and input. d. a short in the circuit. FACET by Lab-Volt 161

13 5. The decimal number 10 is represented by in the binary system. a. 10 b. 101 c d CMS AVAILABLE FAULTS AVAILABLE 162 FACET by Lab-Volt