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By Clive Seager* Last month, we assembled our Schools Experimenter board, installed the Programming Editor software and ran a simple test program. This month, we’ll look at how to write programs that respond to input signals. In this article you will learn: how to write a program that responds to digital inputs; • the difference between a digital and analog input; and • how to write a program that responds to analog inputs. • Inputs and outputs The PICAXE-08M microcontroller has five pins available for use in your circuits (see Fig.1). Of these, pins 1, 2 and 4 can be used as outputs, digital inputs or analog inputs. On the experimenter board, pins 1 and 2 are used as outputs to drive the yellow and green LEDs, whereas pin 4 is used as an analog input for the light dependant resistor (LDR). * About the author: Clive Seager is the Technical Director of Revolution Education Ltd, the developers of the PICAXE system. 76  Silicon Chip Pin 0 can only be used as an output. In addition to driving the red LED, it is used for communications when downloading a program from your computer into PICAXE memory. It is useful to remember that this output toggles rapidly (as is evident by the flickering of the red LED) during program downloads. Lastly, pin 3 can only be used as a digital input. On the experimenter board, this input is connected to the pushbutton switch (SW1). Important: in the PICAXE system, the physical pins of the microcontroller are often referred to as “legs”. On the other hand, port inputs and outputs are called “pins”. For example, on the PICAXE-08M, pin 2 is input2 (or output2 or ADC2) and appears on leg 5 (see Fig.1). Getting started Even those of us who don’t drive a Fig.1: the pinouts for the PICAXE-08M microcontroller, as used in the Schools Experimenter board described in Pt.1 last month. siliconchip.com.au Fig.2: a pushbutton switch generates a digital signal with the aid of a 10kW “pulldown” resistor. Fig.3: when connected in a simple potential divider circuit, an LDR generates an analog signal proportional to light intensity. motor vehicle will be familiar with the red - green - orange - red sequence of traffic lights. The BASIC program to simulate a traffic light sequence on the PICAXE experimenter board is shown in Listing 1. Of course, we’ve used the yellow LED in place of orange and we acknowledge that where you live, the sequence might be slightly different, so jump right in and change it to suit! Note the use of the symbol command at the start of the program. Symbol can be used to make a program easier to understand, as you do not have to remember which LED is connected to which output. As you would expect for a traffic light simulator, the program runs continuously in a loop, starting as soon siliconchip.com.au Fig.4: As light intensity decreases, the resistance of the LDR increases, so a greater portion of the supply voltage appears across its terminals. Conversely, the voltage at the PICAXE’s analog input decreases, as a smaller portion is dropped across the 10kW resistor. as the battery is connected. But what if you only want the outputs to come on when a switch is pushed? A realworld example of this can be seen in a washing machine, where it’s necessary to push the “Start” button to initiate a wash cycle. Digital inputs A miniature pushbutton switch is included on the experimenter board and it’s connected to input3 of the micro. Fig.2 shows the components used in the switch circuit. As you can see, it’s very simple; just the switch and a 10kW resistor connected in series between the 4.5V and 0V power supply rails. The 10kW resistor performs an important function. Without it, the PICAXE input would not be connected to any electrical signal when the switch is open, causing it to “float” to an indeterminate logic state. How- ever, with the 10kW resistor in place, the input has two definite states: 0V when the switch is not pushed and 4.5V when the switch is pushed. In digital electronics, these two states are referred to as a “low” (logic 0) and a “high” (logic 1), respectively. A BASIC program that demonstrates how to respond to the switch input is shown in Listing 2. In this program, the green LED will come on every time the switch is pushed (closed). Task – write a program to make the LED come on when the switch is open (rather than closed). Responding to multiple inputs Making the program react to two (or more) switches is also quite straightforward. By way of example, Listing 3 adds a second (hypothetical) switch on input4. As shown, the LED will be illuminated when either of the switches is closed. June 2005  77 Program Listings Listing 1 symbol red = 0 symbol yellow = 1 symbol green = 2 main: high red pause 500 low red high green pause 500 low green high yellow pause 500 low yellow goto main Listing 2 loop: if input3 = 1 then main goto loop main: high 2 pause 500 low 2 goto loop Listing 3 loop: if input3 = 1 or input4 = 1 then main goto loop main: high 2 pause 500 low 2 goto loop Listing 4 loop: if input3 = 1 and input4 = 1 then main goto loop Listing 4 shows how the program is easily modified to react only when both switches are closed at the same time. TASK – write a program to make the LED come on when two switches (on input3 and input4) are pressed together or when a switch on input1 is pressed by itself. Waiting until a switch is released Sometimes it is necessary to wait 78  Silicon Chip the switch has been pushed and then released. Adding switch debouncing main: high 2 pause 500 low 2 goto loop Listing 5 loop: if input3 = 1 then loop1 goto loop loop1: pause 10 if input3 = 1 then loop1 main: high 2 pause 500 low 2 goto loop When most mechanical switches close, two sprung metal contacts move closer together and then eventually touch. Unfortunately, these contacts do not move precisely and quite often “bounce” against each other a couple of times before coming to a stop. This means that the electrical connection opens and closes rapidly a number of times whenever the switch is activated. A PICAXE microcontroller processes much faster than a mechanical switch can operate and so will detect the switch “bouncing” as legitimate on/off switch action. By adding a 10ms delay into the loop (the pause 10 command in Listing 5), we provide the switch contacts with time to settle before the program reads the switch input and makes the on/off decision. Analog inputs Listing 6 main: readadc 4,b1 debug b1 pause 100 goto main Listing 7 symbol action = 80 loop: readadc 4,b1 if b1 < action then main goto loop main: high 2 pause 500 low 2 goto loop until a switch is pushed and then released before continuing the program. In this case, the program in Listing 5 can be used. As in the previous examples, the program waits in a loop until the switch is pushed. However, it then jumps to “loop1” where it waits until the switch is released again before continuing. This means that the “main” section of the code is processed only after As we’ve seen, a pushbutton switch is essentially a digital device, as it has only two states (on or off). However, some sensors, such as light and temperature sensors, generate a continuously varying signal. These varying signals are called analog signals. Input4 on the Schools Experimenter board is connected to an LDR and 10kW resistor (see Fig.3). These two components are connected in series between the +4.5V and 0V power supply rails, forming a “potential divider”. This term refers to the fact that each of the components has a fraction of the 4.5V supply across it, in effect dividing the supply voltage. As more light falls on the LDR, its resistance decreases, meaning that a smaller percentage of the 4.5V supply will appear across it. Therefore, it follows that the voltage reading at the PICAXE input will vary according to how much light falls on the LDR. The general idea is explored in Fig.4, where three arbitrary light levels produce different resistance values and correspondingly different voltage levels. The PICAXE chip can measure this varying voltage using the readadc command. Readadc is shorthand for “read-analog-to-digital-converter”. This command instructs the PICAXE to read the analog voltage value and then save that value as a number in memory. As the PICAXE works with siliconchip.com.au byte values, the result will always be a whole number between 0 and 255. In the simplest possible terms, if you connect 4.5V to the input, you will get the number 255 in your program. Connect 0V to the input and you will get the number 0. Connect any voltage between these two values and you will get a number between 0 and 255, which in our case can then be used as the “light level” reading. Task – what values would be returned by the readadc command with input levels of 2V, 3V and 4V? The program in Listing 6 reads the analog level on input4 and stores the value in variable byte 1 (b1). The debug command then transmits this value via the serial cable to your computer screen every 100ms. Run this program and then vary the light levels reaching the LDR using your hand. You should see the value of variable b1 change as the light falling on the sensor changes. Make a note of the “bright” light level value (sensor exposed) and the “dark” value (sensor obscured). Use these values to decide on an “action threshold”, which should be about halfway between these two values. Silicon Chip Binders REAL VALUE AT $12.95 PLUS P & P This is the basic Schools Experimenter board described in Pt.1 last month. The program in Listing 7 uses an action threshold value of 80, which you can change to suit your experimental value. When the light level is less than the action level, the green LED will light. Task – write a program to make the LED come on when the light level is below your action value and the pushbutton switch is pressed. What’s coming That’s all for this month. Next month, we’ll look at a more sophisticated sensor for temperature measurement and have some fun with tunes SC using the piezo sounder. H Each binder holds up to 12 issues H S ILICON C HIP logo printed on spine & cover H Heavy board covers with mottled dark green vinyl covering Price: $A12.95 plus $A5 p&p each (available only in Australia). Buy five and get them postage free. Just fill in the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. From the publishers of SILICON CHIP PERFORMANCE ELECTRONICS FOR CARS NOT A REPRINT: More than 160 pages of new and exciting projects never published before – all designed to get top performance from your car. FASCINATING ARTICLES: 7 chapters explaining your car – engine management, car electronics systems, etc ADVANCED PROJECTS: You’ll build controllers for turbo boost, nitrous, fuel injection and much more! We explain the why as well as the how to! Available direct from the Publisher ($22.50 inc postage): Silicon Chip Publications, PO Box 139, Collaroy NSW 2097. Ph (02) 9939 3295; Fax (02) 9939 2648; email silchip<at>siliconchip.com.au or via our website: www.siliconchip.com.au siliconchip.com.au June 2005  79