Fullscreen HTML5Med Res (??MB)HTML4

Make it with Micromite Phil Boyce – hands on with the mighty PIC-powered, BASIC microcontroller Part 23: Analogue inputs and servos T hroughout this series we have shown you how to interface different types of hardware (components and modules) to the Micromite. The Micromite is then programmed to act as ‘an intelligent controller’ to make the attached hardware behave as we want. The exact behaviour of the hardware is ultimately determined by software, or to be more specific, by the sequence of MMBASIC commands that are used within the program code. Along the way we have demonstrated how to use buttons, infrared receivers, and GPS modules for inputs, and have used LEDs, motors and piezo sounders for outputs, as well as using various types of display modules. These have all used MMBASIC commands that are essentially based on digital signals; for example, standard on/off control of inputs and outputs (such as buttons and LEDs), or UART, SPI, or I2C protocols to interact with devices (such as a GPS module, LED matrix or IPS display). This month, we want to go right back to the basics and cover one area that has been requested by several PE readers; and that is to explore how to use the Micromite to read (and respond to) an analogue input. We will be demonstrating this by using a potentiometer to feed a variable voltage (between 0V and 3.3V) to an analogue input pin; and then use the relevant MMBASIC commands so that the voltage level can be measured. To make this a bit more interesting, we will use the input voltage (ie, the rotational position of the potentiometer) to control the position of a servomotor ‘actuator arm’ – see Fig.1. Just two low-cost items are required to follow the topics this month: a linear Questions? Please email Phil at: contactus<at>micromite.org 50 Fig.1. This month, we show how to read an analogue input (a voltage supplied by the potentiometer) and use it to control the actuator arm on a mini servomotor. potentiometer and a small servomotor (often just called a ‘servo’). You may well have one, or both, of these items in your ‘spare parts’ drawer, but if not, they can be purchased online at minimal cost from many different sources – more on these items shortly. Available pins Before we can start to connect anything to the Micromite we first need to understand which Micromite pins are available for use as an analogue input. Referring to Fig.2, you will see a representation of the I/O headers on our Micromite Keyring Computer. For now, ignore the servomotor at the top and the potentiometer at the bottom. The position (and pin number) of the available analogue input pins are highlighted in grey. As you can see, there are ten analogue input pins available: pins 2-7, and pins 23-26. Also highlighted in Fig.2 are the five PWM pins. The reason these are labelled is that each of these PWM pins can be used to control a servomotor instead of outputting a PWM signal. This is achieved in code by using the SERVO command instead of the PWM command – more on this later when we come to control our servomotor. The 0V, 3.3V, and 5V header positions are also labelled. We’ll need these when we connect the potentiometer and the servomotor (as you can see in the top and bottom of Fig.2). Analogue voltage source One of the first things learnt in electronics is Ohm’s Law. We are not going to go into great detail here other than to say that we are using the concept of a potential divider (ie, two resistors connected in series) to effectively generate our variable voltage which we will then feed into our Practical Electronics | December | 2020 P W M individual values of R1 and R2 will both vary, but they will always add Serv o motor up to a total of 10kΩ (or whatever is the value of the potentiometer). Assume that our potentiometer has a 26 25 24 23 22 21 19 18 17 16 15 spindle that is turned – 0 V 5 V much like a traditional M K C volume control on an 0 V 3 .3 V amplifier. At one end of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 spindle travel (eg, fully anti-clockwise), R1 will 10kΩ be 10kΩ, and R2 will be 0Ω, in which case Point B = 0V. At the other end of spindle travel (fully clockwise), then R1 = 0Ω, Fig.2. The position (and pin numbers) of the ten available and R2 = 10kΩ, in which analogue input pins are highlighted above, along with the case Point B = 3.3V. five available PWM pins. Also shown are the connections to I did say that we the potentiometer and servomotor. would go back to basics, the reason being that I just wanted to analogue input pin. Fig.3a shows a simple demonstrate how a potentiometer can be example of a potential divider based on used as the source of our variable analogue two 5kΩ resistors, connected in series, voltage. Fig.2 shows how to connect the between 0V and 3.3V. potentiometer across the 3.3V supply, and If we were to measure the voltage at the wiper (ie, point B in the explanation Point A (relative to 0V), then without above) to analogue input Pin 5. even using Ohm’s Law we can see that it Go ahead and connect a potentiometer would be 3.3V. Likewise, Point C would as shown by using whatever method be 0V. However, it is Point B that we are works best for you. I prefer to plug the interested in, as this will be a voltage potentiometer directly into a breadboard, somewhere between 0V and 3.3V. and then use jumper wires to the MKC Since the two resistors are of equal (see Fig.1). However, you may prefer to value, then we can quickly conclude solder wires onto your potentiometer. that the voltage at Point B would be half The only important point to mention of the 3.3V applied across both resistors; is to ensure that you correctly identify ie, 1.65V. This can indeed be verified by the potentiometers wiper contact out of calculation by using the familiar equation the three choices. This wiper-contact is V = IR. For completeness, we will calculate the one connected to the Micromite’s it as follows: analogue input pin. Current through both resistors: I=V/R = Note that it is safe to use a potentiometer 3.3/(5,000 + 5,000) = 0.00033A (= 0.33mA) with any value between 220Ω and Voltage across R2 = I × R = 0.00033 × 100kΩ; hence you will probably have 5,000 = 1.65V something in your spares drawer that The reason for showing this calculation you can use straight away. If you have is that if we were to vary the value of various potentiometer values to choose either R1 or R2, then the voltage at point from then we suggest using one closest B would change. to 10kΩ as you can’t do much damage So, let us now consider a potentiometer, with 0.33mA. which is effectively a variable potential divider – refer to Fig.3b. Here, R1 + R2 = 10kΩ, and it is fixed by the actual MMBASIC AIN parameter value of the potentiometer. However, Now that we have an analogue voltage as the potentiometer’s mechanical source connected to the MKC, let’s proceed spindle (or slider) is adjusted, then the with how to measure it from within 2 B P W M P W M P W M P W M 2 A 1 1 C 1 B 1 A A 3 .3 V 3 .3 V 3 .3 V 5kΩ V O U T= (3 .3 – 0 ) × B 2 R 2 R 1 + R 2 5kΩ 0 V = 3 .3 2 = 1 .6 5 V C B 10kΩ 0 V 0 V A V O U T C Fig.3 a) A potential divider comprisies two resistors; b) a potentiometer is effectively a variable potential divider. Practical Electronics | December | 2020 MMBASIC. Next, start your Terminal program so that you have a connection to your MKC, and get yourself to the command prompt (possibly press Ctrl-C should you have an auto-running program). The first command we will require is SETPIN. We saw this early on in this series when we used it to configure an I/O pin to be either a digital input, or a digital output. As a reminder, we used: SETPIN 3,DIN for configuring pin 3 as a digital input (ie, detect button press) SETPIN 4,DOUT for configuring pin 4 as a digital output (ie, control an LED) So to configure pin 5 as an analogue input, we simply follow the above format and use: SETPIN 5,AIN Type this at the command prompt (then press the Enter key). You won’t actually see anything happen apart from the cursor moving down to the next line – if you see an error message then simply correct anything mis-typed. Now that we have configured Pin 5 as an analogue input, we can proceed with reading its value. This is really simple, as all you need to do is type: PRINT PIN(5)and press Enter. MMBASIC will then return the voltage value and display it on the terminal screen – try it now. Next, adjust your potentiometer, and repeat the PRINT PIN(5) command – you should now see a different value. There are two more things to check, and that is the measured voltages at each extremity of the potentiometer. So, turn it fully clockwise and check the voltage; and repeat for fully anti-clockwise. In one position you should see 0V, and in the other, 3.3V. Note that fully anticlockwise may read 3.3V rather than 0V – this is not an error, it just depends on which way round you have the 0V and the 3.3V connected to the potentiometers end contacts. One important point to note is that the MKC’s analogue input pins can only read a voltage between 0V and 3.3V. Should you wish to read a higher analogue voltage then you will need to use external hardware (such as an op amp) to scale it down to between 0V and 3.3V. Analogue voltage reader We have just seen how to configure an analogue input pin, and also how to read the voltage level on the pin. However, this was all done directly from the command prompt. So let’s now write a program that continually displays the voltage on the Terminal screen so that as we adjust the potentiometer we see the voltage value change on the screen. This is very easy to achieve, but we will need to use some VT100 Escape codes to help format the terminal display nicely. Now type the following sevenline program into your MKC (remember 51 to save any work first, or alternatively, just insert these lines before the start of your existing code). SETPIN 5,AIN PRINT CHR$(27)+“[2J” DO PRINT CHR$(27)+“[2;2H”; Vin=PIN(5) PRINT STR$(Vin,1,2); LOOP Before you run the program, let’s first take a quick look at how it works (if you have been following this series then there won’t be anything here you don’t recognise). The first line, as we have just seen, configures pin 5 as an analogue input. The second line uses an Escape code to clear the terminal screen. Then we have a DO…LOOP comprising three lines. The first of these lines positions the cursor 2 lines down the screen, and 2 characters along the line (ensure you type this exactly as shown above – case sensitive and with the semi-colon at the end of the line). This Escape code is used so that the cursor position is moved away from the topleft corner of the screen and hence makes it easier to read the voltage that we are about to display. The second line in the DO…LOOP loads a variable (that we have called Vin) with the measured analogue voltage on pin 5. We are storing the voltage value in a variable as we will not only be displaying the value on the screen, we will also be using it for a calculation when we add the servo (discussed shortly). The last line in the DO… LOOP displays the value on the screen and uses the STR$() command to format it to 1 leading digit along with 2 decimal places. The STR$() command is used because we need to convert a number (ie, Vin) into a string (something that can be displayed), so that we can then format it to the required number of characters (here x.xx). Doing this ensures that the displayed voltage appears to remain in a static position rather than jumping about. Now RUN the program and check it works. As you adjust the potentiometer, you should see the displayed value vary. If not, check your wiring, and also that your code has been entered correctly. Do check that at one end of the potentiometer’s travel the value is 0.00, and at the other end it is 3.30; however, do note that if you do not get exactly to 0.00 or 3.30 this will not be a fault of anything you have done; it will simply be down to the quality of the potentiometer. A simple servo Now that we have a method of adjusting an analogue voltage (between 0V and 3.3V), and also have the ability to read the voltage within our code, we can use it to control a mini servo. So what is a servo? Essentially, it is a motor with built-in positional control. It has a spindle that is typically limited to a rotational movement of 180° (half a turn). Onto the spindle you can attach an actuator arm, which in turn can be attached to something mechanical. For example, in a toy car, a servo’s actuator arm may be attached to the front-wheel mechanism allowing the servo to steer the car. A servo has three wires, two for power (5V in this case), and one for a control line. The control line requires a signal that is a square wave (within a certain frequency range). The square wave’s duty cycle (the ratio of on-time to off-time) determines the position of the servo’s spindle. MMBASIC makes it very easy to control a servo thanks to the SERVO command – this eliminates the need to worry about the signal timing, as we will see shortly. If you don’t have a servo in you spares drawer, then many are available online at a very low cost and we would recommend obtaining a few as they can be a lot fun to use. Please see Fig.4 for the type of mini servo we are using here. 52 Servomotor cable colour code Red +5V Brown GND Orange PWM Fig.4: A low-cost mini servomotor is readily available online. It’s a perfect match for MMBASIC’s SERVO command. MMBASIC SERVO command You may recollect that we used the PWM pins earlier in the series (along with the PWM command) to drive a piezo buzzer. Essentially, the PWM command was used to adjust the frequency of a square wave that in turn was driving the piezo sounder; the end result enabled us to play different musical notes. For the purpose of music generation, the PWM command used a duty cycle of 50% – ie, a precise square wave. However, as mentioned above, a servo uses a specific frequency, and it is simply the duty-cycle that is adjusted in order to move the servo’s spindle to a specified position. The duty-cycle value is represented as a time (in ms) with a typical value range from 0.8ms to 2.2ms for most servos. MMBASIC’s SERVO command has the following syntax: SERVO channel, freq, duty-timeA[, duty-timeB, duty-timeC] Here, channel is set to 1 or 2 (depending on which Micromite pin is used – see pinout in Fig.1); freq is set to an appropriate value for the servo used (we will be using a frequency of 100Hz); and duty-time is the time (in milliseconds (ms)) as described above. So, if you have a servo available, now is the time to connect it to your MKC. Simply connect the three leads as shown in Fig.2 – in other words, connect the servo’s red lead to +5V, the servo’s brown lead to 0V, and the orange lead to Pin 26. Do a quick check you have it connected correctly, and then start your Terminal app so that you have a connection to your MKC and can see the command prompt. Next, type: SERVO 2,100,1.2 and check that the servo moves. Note that it is better to add an actuator arm onto the servo’s spindle so that you can clearly see it moving. If you don’t have an actuator arm, just use a small piece of tape and attach it to the spindle to simulate a ‘pointer’. If it does not move, then recheck your three connections, and also check that you have typed the command exactly as shown. Now repeat the above but with a duty-time of 0.8 (instead of 1.2), and then try 2.2. You should then see the servo move near to its extreme positions covering an angle of close to 180°. Potentiometer control of the servo Now that we have connected up the servo, and seen how MMBASIC can be used to directly control the position of the servo, we will modify our program code from earlier so that turning the potentiometer adjusts the position of the servo’s spindle. As always, this is much easier than it sounds; so go ahead and make the following changes by adding the four lines of code highlighted in bold: Practical Electronics | December | 2020 MinVal=0.8 MaxVal=2.2 SETPIN 5,AIN PRINT CHR$(27)+“[2J” DO PRINT CHR$(27)+“[2;2H”; Vin=PIN(5) PRINT STR$(Vin,1,2); SerPos=((MaxVal-MinVal)*(Vin/3.3))+MinVal SERVO 2,100,SerPos LOOP Before running the program, we’ll quickly explain what the four lines of code do that have just been added. The first two lines simply set two variables that we will use in a calculation within the DO…LOOP. If you look at the names and values, you should recognise that they represent the minimum and maximum values for the servo’s duty time (in ms, as discussed earlier). Note that in the real world, different quality servos have different performances and specifications. It may be that your specific servo can work beyond these values, hence setting them in the first two lines of code makes it very easy to try different values that may well work with your servo. For now, though, leave them set to the values of 0.8 and 2.2. The third line added may look complex, but it is just a calculation that maps (‘translates’) a voltage value between 0 and 3.3 (from the potentiometer) to a value between 0.8 and 2.2 (ie, between MinVal and MaxVal) to be used by the SERVO command. The fourth line then uses the mapped/calculated value (which we have stored in the variable SerPos) and passes that to the servo. Now RUN the program and make sure that the servo moves proportionally to the turning of the potentiometer. As usual, if anything does not work as expected, check your connections, and also check your code. Challenges There are many modifications that you could make to this month’s code, and we would certainly encourage you to experiment. Here are two ideas to try: Change the MinVal and MaxVal values to find the optimum values that give you as near as 180° of spindle movement. Be careful doing this as the servo mechanism can be damaged if a relatively big (or little) value is used in error. I advise applying 0.1ms step changes at a time. Get the spindle to move in the opposite direction. This could be achieved by simply swapping the two 0V and 3.3V wires on the potentiometer (try it and see!). However, imagine the scenario where you were part of the hardware design team and you created a PCB and you don’t want to have to modify the PCB. Instead, you want to swap the direction by modifying the software – this is your challenge! I hope this month’s topic of analogue inputs and servos has inspired you to explore things further. If you have built the Micromite Robot Buggy, then how about trying to make a remote control based on two potentiometers: one for speed, and the other for turning (much like a radio-controlled car). Alternatively, use two potentiometers along with two servos and make a pan/tilt mechanism onto which you could attach a distance module so that you can measure the distance to any object that you are pointing it at. Next month One further topic that we have been asked to cover is pulse counting, so next month that is exactly what we will be exploring. Until then, have fun coding! ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. 3D Printing • Cable • CCTV • Connectors • Components • Enclosures • Fans • Fuses • Hardware • Lamps • LED’s • Leads • Loudspeakers • Panel Meters • PCB Production • Power Supplies • Relays • Resistors • Semiconductors • Soldering Irons • Switches • Test Equipment • Transformers and so much more… JTAG Connector Plugs Directly into PCB!! No Header! No Brainer! Monday to Friday 08:30 - 17.00, Saturday 08:30 - 15:30 Our patented range of Plug-of-Nails™ spring-pin cables plug directly into a tiny footprint of pads and locating holes in your PCB, eliminating the need for a mating header. Save Cost & Space on Every PCB!! Station Road Cullercoats North Shields Tyne & Wear NE30 4PQ Tel: 0191 2514363 sales<at>esr.co.uk www.esr.co.uk Practical Electronics | December | 2020 Solutions for: PIC . dsPIC . ARM . MSP430 . Atmel . Generic JTAG . Altera Xilinx . BDM . C2000 . SPY-BI-WIRE . SPI / IIC . Altium Mini-HDMI . & More www.PlugOfNails.com Tag-Connector footprints as small as 0.02 sq. inch (0.13 sq cm) 53