EXERCISE 4: A Simple Hi-Fi

Transcription

1 EXERCISE 4: A Simple Hi-Fi EXERCISE OBJECTIVE When you have completed this exercise, you will be able to summarize the features of types of sensors that can be used with electronic control systems. You will use Flowcode and the Microcontroller System Development FACET board to program a simple Hi-Fi. EXERCISE DISCUSSION Adding Inputs - Sensors An electronic control system knows nothing about the outside world unless we give it sensors - its eyes, ears etc. The language it understands is that of electrical signals, voltages and currents. A sensor has to turn changes in the outside world, such as light level, temperature, sound level, humidity, pressure etc., into changing electrical signals. A different type of sensor is needed for each of these tasks. Here are some of them. Light sensor: The light dependent resistor (LDR) - Its resistance drops when a brighter light shines on it. It is relatively slow to react to changes in light level, though. This is the only type of sensor included on the Microcontroller System Development FACET board. The photodiode - A component that passes an electric current only when sufficient light shines on it. It reacts much faster than the LDR to changes in light level. Temperature sensor: The thermistor - There are two versions of thermistor. One type, called NTC (negative temperature coefficient) has a resistance that gets less when its temperature goes up. The other type, PTC, (positive temperature coefficient) has a resistance which gets bigger when the temperature rises. Sound sensor: The microphone - The common form of microphone generates a voltage when it picks up a sound. The louder the sound, the bigger the voltage! FACET by Lab-Volt 213

2 Microcontroller System Development Adding Inputs - Sensing units We have just seen that some sensors, like the thermistor and the LDR, produce a change in resistance when the conditions in the outside world change. We need to convert that change in resistance into a change in voltage or current in order for the electronic control system to understand it. The voltage divider is one way of doing that. It consists of two resistors connected in series (one after the other), so that they share the power supply voltage between them. The diagram shows a voltage divider where one resistor is twice as big as the other. They share the 6V power supply voltage between them in such a way that the 8k resistor takes twice as much of the voltage as the 4k resistor. The sum of the voltages must equal the power supply's 6V, of course. Suppose that the lower resistor is actually a thermistor. Hence the different symbol, the rectangle with the hockey stick through it, in the diagram. As luck would have it, the resistance of the thermistor is 8k when its temperature is 10 C! This means that the voltage across it will be 4V, as before. 214 FACET by Lab-Volt

3 Let's suppose that it is a NTC thermistor, and that its resistance drops to 2kΩ when the temperature has risen to 50 C. The voltage across the thermistor has now dropped to 2V. The thermistor resistance is now only half as big as the 4k resistor, and so the voltage across the thermistor is only half that across the 4k resistor. In other words, here we have a circuit that outputs a voltage that changes with temperature. Just what the control system needs to tell it what the weather is like in the outside world! Here are some more sensing units: Sensing unit Behavior A Temperature Output voltage drops as temperature rises B Temperature Output voltage rises as temperature rises C Light Output voltage drops as light level rises D Light Output voltage rises as light level rises Notice: the symbol for a LDR the fixed resistor in the earlier circuits has been replaced with a variable resistor (the symbol with an arrow through.) FACET by Lab-Volt 215

4 Microcontroller System Development EXERCISE QUESTIONS An electronic component that monitors changes in the environment, such as light, temperature, sound, and converts those changes into electrical signals is a. thermostat b. sound meter c. sensor Most sensors can interpret a combination of different conditions. a. True b. False The electrical signals passed by a sensor to a system are in the form of a. voltage changes b. current changes c. Both of the above are correct. 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 Sensor Block Switch Block LCD display N/A N/A 216 FACET by Lab-Volt

5 Port B interrupts There are two more common interrupts that you should understand: port B bit 0, and port B change. The port B bit 0 interrupt will trigger a given macro every time the bit 0 line on the PICmicro device goes to a logic 1 or 5V. The port B change interrupt triggers every time there is a change of state on the highest 4 bits of port B: bits 4, 5, 6, 7. The port B bit 0 interrupt is straight forward enough, but the port B change is a little more difficult to handle. Here we will set up part of a hi-fi system that has 4 preset radio channels - channels 1 to 4. Each one is selected by the use of one of 4 push-to-make switches connected to port B. The idea behind our system will be that every time a switch is pressed the port B change interrupt will trigger a macro that alters the channel number. Follow the instructions below to set up the program. 1. Start a new flowchart with a 16F877A device. 2. Add an interrupt icon to the program. 3. Add a loop to the program. Don't put anything in the loop yet. To start with the loop will just keep the program going. The real work will be done by the interrupt routine we will make. 4. Add 4 push to make switches connected to Port B: bits 4, 5, 6, 7, using the bit drop down menu. Label them 'Channel 1' to 'Channel 4'. 5. Add two new Byte variables: 'channels' and 'switchs' 6. Add a new macro called 'Chandet' FACET by Lab-Volt 217

6 Microcontroller System Development 7. Assemble this program in the 'Chandet" macro. 218 FACET by Lab-Volt

7 Here is an explanation of how the 'Chandet' macro works: The first icon assigns the value of the top 4 bits of port B to the variable switches. Use masking to get the state of only the top 4 bits. The difficulty (or advantage) of the port B change interrupt is that it triggers on a change of status in port B. That means that it triggers when the switch is pressed, and also when it is released. Because of this you need to filter out all switch releases. We want to detect only switch presses. The first decision box detects a value of 0 for the variable switches. If a 0 is detected the connection point diverts the program to the end of the interrupt macro. The next decision box detects a value of 0x10 (16 decimal). This corresponds to the switch on RB4 being pressed. If RB4 is pressed then we assign the variable 'channel' to have a value of 1. The next decision box detects a value of 0x20 (32 decimal). This corresponds to the switch on RB5 being pressed. If RB5 is pressed then we assign the variable 'channel' to have a value of 2. The third decision box detects a value of 0x30 (48 decimal). This corresponds to the switch on RB6 being pressed. If RB6 is pressed then we assign the variable 'channel' to have a value of 3. The next decision box detects a value of 0x40 (64 decimal). This corresponds to the switch on RB7 being pressed. If RB7 is pressed then we assign the variable 'channel' to have a value of Double click on the interrupt icon in the main program. In the 'Interrupt on:' text box select 'RB port change'. In the 'Will call macro:' text box enter 'Chandet' so that it will call the macro called 'Chandet'. 9. Select CHIP...CLOCKSPEED and select 1000 icons per second. The actual value is not important: a setting other than 'As fast as possible' will allow you to see the values of the variables in the system as your program simulates. 10. Run the program, and check that the variable 'channel' is assigned a channel number on the press of a switch. Setting Up the LCD The hi-fi system will make use of the LCD display. We want to make the display show the volume and also the radio channel. Later on we can make it display data on the CD player etc. To do this we will use the first line of the display to show the volume and the second line to show the radio channel. In this program we will separate all of the main routines into separate FACET by Lab-Volt 219

8 Microcontroller System Development macros to make the structure of the main program easier to read. The two macros we will set up here will initialize the display, and then refresh the display as the program runs. 1. Add an LCD display to your project. Connect it to port C. 2. Generate two new macros. One called 'updatedisplay' and the other called 'initializedisplay' 3. Develop the following program in the 'initializedisplay' macro: You should, by now, have a good grasp of what is happening here. The first icon starts the display, then the cursor is set to 0,0 or the top left. The third icon prints "Volume:" on the top line, and the next icon moves the cursor to the second line. The last icon prints "Channel:" on the bottom line. 4. Add a few new variables that you will need: 'volume' of type Byte, and 'ADCreading' of type Int. We will get to these later. 220 FACET by Lab-Volt

9 5. Develop the program shown here in the 'updatedisplay' macro: Here the first icon moves the cursor to a point after "Volume:". It prints three blank spaces to delete the old volume setting. The next two icons move the cursor back to the previous position and print the value of the volume variable. The next icon moves the cursor to the bottom line of the display after "Channel:", and the last icon prints the actual volume. 6. To the main program, add a macro icon which calls the 'initializedisplay' macro. 7. Next, add another Interrupt icon. To demonstrate that it is possible to use more than one type of interrupt in one program, we will use a timer interrupt to call the 'updatedisplay' macro. Change the properties of the 'Will call macro:' to 'updatedisplay'. Change the properties of the 'Interrupt on:' to 'TMR0 Overflow'. Click on the properties button and set the interrupt to go off as slowly as possible - prescale rate=1:256, 75 Hz with a 19MHz crystal. Your main program should look like the one shown next. FACET by Lab-Volt 221

10 Microcontroller System Development 8. Simulate your program. Note that both the port B change interrupt and the timer interrupt both simulate. You will have to wait some time during simulation before the timer interrupt is triggered. The interrupt rate is 75Hz and in this time the PICmicro can execute hundreds of instructions. Using the ADC So far you have set up two interrupts, and three macros which are called from the main routine. In this next part of the project you will set up a further macro that will measure the signal on an analogue input, by using an ADC (Analog-to-Digital Converter). 1. Add an ADC component onto your project workspace. Alter the connection so that it is connected to ADC 1 - this is the potentiometer on the Sensor board. 2. Add a new macro and call it 'ADCmeasure'. 3. Add two Component macros to the ADCmeasure macro. Set the first to sample the ADC, and the second to ReadAsInt the value of the ADC into the variable adcreading that you set up previously. 4. Drag a Calculation icon into the macro and enter 'volume = adcreading / 10' into the Calculation field. You should have something like the flowchart shown next. 222 FACET by Lab-Volt

11 The process of sampling the ADC is a two stage one: first sample the value and then read the value into a variable. The PICmicro we are using here actually has up to 8 ADC inputs - AN0 to AN8. Internally however the PICmicro only has one Analogue to Digital Converter. Each of the 8 inputs can be switched to the ADC using internal analogue switches. This gives you a clue as to why the process of getting an analogue input into your program is a two stage process: internally Flowcode sets up the analog switch to connect to the relevant input, then it takes a reading. The ADC inside the PICmicro has a resolution of 10 bits. This means that to make use of the full range of the ADC you need to use a variable of type Int: a 16 bit variable. The value you read will vary between 0 and However we are used to seeing Volume presented as 0 to 10, or 0 to 100. Because of this we can quickly divide the ADC reading by 10 to get a scale of 0 to 102 as our volume setting. You should by now be able to think of programming techniques to cap the value at 100. Your system is nearly complete. In the Main routine drag a Macro icon into the loop. In this icon call the ADCmeasure macro. Your project is now quite functional. If you simulate it then you should see that when you click on a channel button the LCD displays the channel number. When you move the Volume dial you should see the volume displayed. Of course there are some bits missing: you have not developed the hardware and software to connect to a radio system to actually alter the channel, and you have not made any use of the volume variable to alter the amplification of the output stage of your hi-fi. However you have completed a significant part of the overall design project. The following image shows the entire project. Hopefully in this section you have learned that it is really useful to break the stages of your development down into several individual programs - each in its own macro. This allows you to build and test one part of the program at a time. This also makes your main program very easy to read - although in this case readability is compromised slightly by the use of interrupts. It is not immediately obvious to someone who has FACET by Lab-Volt 223

12 Microcontroller System Development not written the program how the Chandet macro is called, although the two interrupt macro icons in the main program do give a clue. EXERCISE QUESTIONS 1. In this procedure, the PORTB Interrupt will trigger a when the RB0 line on the PICmicro device changed to a logic 1. a. macro b. string c. decision 2. The use of identifies the specific bits that are involved in any programming. a. masking b. macros c. decisions 3. In this procedure, the process of sampling the ADC includes the following: a. read the value into a variable. b. sample the value and then read the value into a variable. c. sample the value, read the value into a variable, compile the data. 224 FACET by Lab-Volt

13 EXERCISE CONCLUSION Sensors use the language of electrical signals, voltages, and currents to communicate information about the outside world to electronic control systems. Light sensors, temperature sensors, and sound sensors are some typical types of sensors. When there are changes in conditions, some sensors produce a change in resistance that is then converted to a change in voltage or current in order to be understood by the electronic control system. One way of making this conversion is by using a voltage divider. REVIEW QUESTIONS 1. An electronic control system requires to give it information about the outside world. a. capacitors b. electromagnets c. resistors d. sensors 2. An LDR and a photodiode are two types of a. humidity sensors. b. light sensors. c. sound sensors. d. temperature sensors. 3. In order for an electronic control system to understand the information from the sensor, the information must be turned into: a. electrical signals. b. voltages. c. currents. d. All of the above are correct. e. None of the above is correct. 4. A changes resistance into a change in voltage or current. a. voltage divider b. current divider c. voltage multiplier d. current multiplier 5. A thermister: a. is a temperature sensor. b. may be either NTC or PTC. c. is a type of LDR. d. Both a and b are correct. e. None of the above is correct. CMS AVAILABLE FAULTS AVAILABLE FACET by Lab-Volt 225