Fullscreen HTML5Med Res (??MB)HTML4

Over the years, many LED Dice circuits have been published – but none are as simple as this one! With just one PIC micro and a handful of other components it’s cheap and easy to build, too! By DOUG JACKSON F IRST OF ALL, let’s settle an argument before it starts. Die or Dice? Sure, the venerable Oxford would have us say one die, two dice. But every man and his dog uses the word “dice” for both singular and plural. So we’ll stick with Fido and use dice. But just in case you still want to argue, we’re correct either way with this circuit because it contains not one but two dice. So it’s perfect for all of those games which require the roll of two dice at once. By the way, if you only want a single version, that’s easy too: just leave out one set of LEDs and driver resistors. The PIC micro will never know! Ahh, the PIC micro. We were getting to that. Using a PIC allows us to significantly simplify our dice circuit. Previous designs have typically used at least two ICs, four or more transistors and many resistors and capacitors. And they’ve been fairly current 56  Silicon Chip hungry, discharging batteries far too quickly. Using a single microcontroller not only allows simplification, it also lets us add features that previously haven’t been available: the ability to recall the last roll, for example. This is the first in a short series of articles we hope to publish over the next few months which will use PICs in a variety of simple applications. What makes this series a little different is that we intend to guide you through the hardware and software design step-by-step so that you get a better idea of the design process. It’s an ideal way for a beginner in micros to get a grasp on the fundamentals. We are not planning to print de- tailed software descriptions, though – magazine space simply does not allow this. However, a web site has been set up to provide detailed software discussions of all the projects presented in the series. Before we start our design, let’s look at the basis for all of these projects, the PIC microcontroller. Pick a PIC The PIC microcontroller family covers a wide variety of devices incorporating embedded peripherals, such as: integrated timers; analog-to-digital converters; digital-to-analog converters; RAM and Electrically Erasable ROM (EEROM). Our project will use a Microchip PIC 16F84 microcontroller. This device has 1K of on-board flash programmable ROM, 68 bytes of RAM, 13 I/O lines and an internal counter/timer. Each I/O line can source or sink approxi- The PC board version of the LED Dice was housed in a zippy box with the LEDs and switch emerging through the front panel. The red LEDs form one dice while the orange LEDs form the second (yes, we know we said we used green ones!). The second version of the LED Dice is the same circuit but is built on Veroboard and this forms the lid of a zippy box. Some components are mounted on the other side of the board. Note that some component values have been altered. mately 50mA making it ideally suited terms are not interchangeable – a creep’ interfering with the completion to directly driving a LED display. microcontroller actually contains a of our project. The 16F84 has enjoyed significant microprocessor but it also contains The specifications for our project popularity in the hobbyist market re- memory, I/O (input/output) lines and are simple – we will design an eleccently. A major reason for this is that often other features. tronic simulation of two dice, using it has a flash ROM, making it easily 14 LEDs. A single pushbutton switch The project re-programmable. The advantage of will control the rolling of the dice in the flash ROM is that it doesn’t rethe following manner: Before we start designing our Dice, quire an ultraviolet eraser to erase we need to decide exactly what it does  When the button is pushed for a the device. and how it does it. In doing this, we short period (say less that 0.5 sec), the M i c r o c h i p ’ s w e b s i t e a t reduce the likelihood of ‘specification dice turn on and display the result of the last roll. (http://www.microchip.com)  If the button is pushed provides full documentation for greater than about for the entire range of PIC 0.5 seconds, both dice devices, as well as a full are cleared then roll indevelopment environment dependently, eventually (MPLAB). slowing and stopping afA simple PIC programmer ter the button is released. was published in the March  In all cases, the result 1999 issue of SILICON CHIP is displayed for 20 sec(back issues are available for onds and then the dice $7.00 including postage and turns itself off. packing [$7.70 after June]). It would be desirable to This programmer is suitable have no power switch, so for programming the devices we have to minimise curwe will use in this series. rent consumption while You may have noticed we the project is ‘off’. use the word “microconNow that we have detroller” where many people Fig.1: all six faces of a dice with the standard patterns cided (and written down) use “microprocessor”. The shown. May 2000  57 Fig.2: driving LEDs from a PIC is easy! All you need to do is limit the current from the PIC to a level which the LEDs can handle – and tell the PIC to light them up! what we will build, let’s start the fun stuff. The hardware Lets look at a good old-fashioned dice. As we all know, it has six sides, with one, two, three, four, five or six “spots” or dots on each. (Did you know that adding the opposite sides of a dice always equals 7?) If we analyse the various dot patterns in Fig.1, we can see that the following rules apply:  The central dot (7) operates inde-pendently.  Opposing corner dots (1) and (3) appear simultaneously.  Opposing corner dots (2) and (4) appear simultaneously.  Middle dots (5) and (6) appear simultaneously. Fig.3: providing an on/off switch is also simple with the right instructions in the program. Therefore we can actually drive all seven LEDs from only four I/O pins on the microcontroller. Remembering that our goal is to emulate the operation of a standard dice using LEDs, let’s start by connecting some LEDs to the microcontroller. Driving LEDs with a PIC microcontroller is a simple exercise. Because the PIC outputs can drive up to 50mA and LEDs typically require only 1020mA, we can drive each LED directly via a suitable series current limiting resistor. Fig. 2 shows typical connection details. But what are the values of the current limiting resistors? Ohm’s Law tells us that one: R = E/I We know that “I” is 20mA max. and that “E” in this case is the supply Fig.4: if timing accuracy is not important, a simple R/C circuit attached to the PIC’s “OSC” input is all you need. voltage (5.4V) less the forward voltage drop across each LED (typically 2.1V). So for a single LED: R = 3.3/.02 = 165Ω. Where there are two LEDs in series the forward voltage drop doubles so the formula becomes:      R = 1.2/.02 = 60Ω. To save drain on the battery (and therefore give it more life), we’ll be a bit conservative and go for slightly less current through the LEDs, resulting in resistor values of 220Ω for the single LEDs and 100Ω for the double LEDs. Now that we have designed the output, we need to consider our input; something to “roll” the dice. This can be done simply by connecting a pushbutton switch between the supply voltage (VCC) and one of the PIC inputs that provides an interrupt Fig.5: the PIC drives the LEDs for about 20 seconds and then goes to sleep to conserve the batteries. 58  Silicon Chip Parts List – PC Board Version 1 PC board, code 08105001, 58 x 73mm 1 130 x 67 x 44 plastic case (Jaycar HB-6013) 1 front panel label, 124.5 x 62mm 1 4 x AA square battery holder 1 PC-mount SPST pushbutton switch (Jaycar SP-0722) 4 9mm untapped spacers 4 M3 x 15mm CSK steel or nylon cheese-head screws 4 M3 nuts Fig.6: this code tells the PIC to determine a random number and store it in a certain location, then display the result. capability (we’ll look at interrupts later). Fig.3 shows an example. Note that the input is held low by a 4.7kΩ resistor to ensure that random noise picked up on the input pin does not cause an input to be recorded. Clock and power supply All that remains is to add a power supply and provide some sort of clock circuit to the microcontroller. A clock circuit, by the way, has little to do with telling the time. It provides pulses at a specific rate which cause the microcontroller to undertake certain tasks. First, though, the supply: the most simple power supply we can have is four AA batteries. This provides 6.0V (4 x 1.5V). If a series diode is placed between the batteries and the PIC, the available supply voltage drops to about 5.4V. This is due to the nominal 0.6V voltage drop across a forward-biased silicon diode. 5.4V is within the PIC’s rated input voltage range of 4-6V whereas 6V from the batteries would be right on the upper limit. The series diode also protects the PIC from damage if the battery is accidentally connected back to front. Traditionally, microcontroller systems have used some sort of 3-terminal voltage regulator to ensure that 5V is available to the CPU. We decided not to use a 78L05 or similar 3-terminal voltage regulator, as the 4mA standby current drawn by the regulator would swamp the sleep current of the PIC (about 7µA), giving poor battery life. So in theory, a set of four ‘AA’ alkaline batteries with a capacity of about 800mA.h should be able to last about 114,000 hours while in sleep mode. (That’s about 13 years . . . we suspect that the batteries will die of their own accord LONG before this time!). Of course, current consumption will increase to about 120mA during operation. PIC microcontrollers can use a variety of clock circuits, ranging from crystal controlled oscillators if accurate timing is required, through to simple RC (resistor/capacitor) networks. In our application, we are not concerned about speed and clock accuracy, so we use an RC oscillator. This is shown in Fig.4. This works simply by charging the 100pF capacitor through the 10kΩ resistor until the microcontroller’s threshold voltage is reached, at which time the capacitor discharges quickly through the microcontroller. When the voltage falls to the micro’s lower threshold it goes high, allowing the capacitor to start charging once again. The final circuit Tying all of this together, we come up with the circuit for the hardware of our LED Dice simulation. This is shown in Fig.5. Semiconductors 1 PIC16F84 programmed microcontroller (IC1) 1 1N4004 diode (D1) 7 5mm red LEDs (LED1 - LED7) 7 5mm LEDs, another colour (LED8 - LED14) Capacitors 1 10µF 16VW PC electrolytic 1 .001µF ceramic disc Resistors (0.25W, 5%) 2 10kΩ 1 4.7kΩ 6 100Ω 2 220Ω Parts List – Veroboard Version 1 piece of Veroboard or other strip board, 107 x 57mm 1 112 x 60 x 27mm plastic case 4 AA batteries 1 PC-mount SPST pushbutton switch (Jaycar SP-0722) Semiconductors 1 PIC16F84 programmed microcontroller (IC1) 1 1N4004 diode (D1) 7 5mm red LEDs (LED1 - LED7) 7 5mm LEDs, another colour (LED8 - LED14) Capacitors 1 10µF 16VW PC electrolytic 1 .001µF ceramic disc Resistors (0.25W, 5%) 2 10kΩ 1 4.7kΩ 6 100Ω 2 220Ω Miscellaneous Hook-up wire, bubble-wrap plastic or other suitable insulation. May 2000  59 Fig.7: here’s how to mount the PC board to the front panel. Note the distance from the board to the LEDs and also the fact that the electrolytic capacitor will need to be bent over to allow clearance. Now you can see the simplicity of using a single chip microcontroller. The total circuit contains just one IC and a handful of discrete components! Random numbers One item that we will look at from the software is the generation of a random number. Mathematically, generating a truly random number is a very complex exercise. In our simple PIC circuit, we can generate a random-enough number in a couple of ways:  A seemingly random number can be obtained by timing how long the button is held down, using a timer that is incremented VERY quickly. (It would be a very rare person who could hold the button down for exactly 2243ms every time).  Alternatively, we could imple­ment a mathematical pseudo-random number generator. This requires the use of multiplication and division. A pseudo-random generator generates a very long sequence of numbers that eventually repeats, after many cycles. In our project, we use the first method. We sample the internal timer (TMR0) which is constantly increment­ ing at one quarter of the clock speed (about 256kHz) and store the sample in a variable, as long as the button is held down. A short code routine to perform this function is shown in Fig.6. As previously mentioned, the microcontroller will be spending most of its time in sleep mode (especially while it is sitting majestically on the mantelpiece!). In sleep mode, the internal oscillator is stopped and the device consumes about 7µA. Interrupts In order to wake up from sleep mode, we need to have an ‘interrupt’ Fig.8: the front panel for the PC board occur. Interrupts can be effected from version mates with the PC board a variety of sources but they always underneath. signal some external change. The LED Dice project that we are building has the pushbutton connected to bit 1 of Port B (RB0). This pin also functions as an ‘interrupt’ input. When the voltage level on this pin changes, an interrupt is generated, causing the PIC to stop whatever it was doing and to do something else. It is this interrupt that causes the PIC to wake up from its sleep mode. Interrupts in the PIC can be ‘global’ in nature (Global Interrupt Enable [GIE] bit set) or localised. In our example, we would like to continue executing instructions immediately following the ‘sleep’ command, so we need to ensure that the GIE bit is clear. Global interrupts cause program execution to branch to location 4, which is useful for a more traditional Fig.9: this is the component overlay for the PC board version. Compare this with the vectored interrupt approach photograph alongside. Note that two of the LEDs (labelled LED3 and LED13) mount the which we will cover in later other way around to the rest. The second colour LEDs can be green, orange or yellow. 60  Silicon Chip articles. Code to implement the interrupt functionality would look like that shown in Fig.11. Note that once the microcontroller has received an interrupt, it wakes and immediately disables any further interrupts. Multiple levels of interrupts can cause unexpected program errors, so we stop any further interrupts from occurring. Now that we have examined how to implement input, output, random num­ber generation and interrupts, we can tie all of this together and produce the code that will actually run the dice. There is a small amount of ‘glue code’ around these functions to produce actual running code. I recommend that you obtain the program listings and study them for more information. When you study the listings, you may find that there are faster, more elegant ways to do what has been done. Remember that there are commercial realities as to the time spent on producing a particular solution and that some times, doing something the ‘no brain’, long way is actually faster to develop. This is an embedded system and in a simple system like this, the emphasis is on producing a result, not on producing the most elegant code available. (Have you actually looked at the code in your microwave oven controller? Believe it or not, many of these Fig.10: full-size PC board pattern for those wishing to make their own boards. Otherwise, use this pattern to check commercial boards before commencing construction. This photo of the Veroboard version is reproduced slightly larger than actual size, so you can see exactly where the components go. Note that some of the components are on the other side of the board. The black object below the IC is a header pin set with a shorting link, used as an on-off switch in the prototype. However, this is considered unnecessary and has not been specified in the parts list. contain microcontrollers!) As previously mentioned, in an article of this length it is not appropriate to include bulk source code listings, so the source code and corresponding hex file to supply to the PIC programmer are available on my web site (http://www.dougzone.com). PIC programming To make the LED Dice operate you need to load the LED Dice program into a PIC. You can either purchase a pre-programmed PIC or you can program one yourself. Programming one yourself allows you to enter the world of PIC software design. In order to program the PIC, you need some basic tools. First, you need the Microchip assembler and simulator (MPLAB), available as a 9MB download from the Microchip web site (http://www.micro-chip.com). This is a HUGE download but you only need it once. Remember to make a backup. In addition to the assembler, you need a programmer. The PIC programmer that I use is based on a design Fig.11: this code will implement the interrupt function. May 2000  61 with a multimeter to minimise errors. When bending component leads, remember that using a pair of needle nose pliers will minimise stress while performing the bend. Continue the assembly by soldering in the 18-pin IC socket, ensuring that the indentation on the socket agrees with the position shown on Fig.9. Next, solder in the 14 LEDs. Be careful with their orientation, as they will not operate if they are installed backwards. The short leg is the cathode. Note that two of the LEDs are mount­ ed the opposite way around to the rest! Mount the pushbutton switch directly to the PC board, ensuring its straight edge is aligned as shown. Finally, connect the battery holder, ensuring that the batteries are not installed. Veroboard version Here’s what it looks like assembled and opened out. The batteries were simply soldered together and placed in the bottom of the case, with a piece of bubblewrap plastic to stop them moving around or shorting to the copper tracks. by Michael Covington, which was described in the May 1999 issue of SILICON CHIP. Initially, I had a some trouble getting the published programmer to operate with my particular parallel port, so I built the NOPPP-2 (Experimental) version that used a 74HC08 in place of the diode logic that was present in the initial version. It work­ed flawlessly. The programmer software (noppp) is available from the SILICON CHIP web site or from Michael’s web site (http://www.covingtoninnovations. com/noppp/). Once you have the tools, you need to create a .hex file to feed to the programmer. Start by loading up the MPLAB software and creating a project by selecting ‘Project’, ‘New Project’ from the menu and typing the name of the project (LED Dice) into the file name box, ensuring that the default directory is in a reasonable location for your system. You need to add a source (.asm) file by clicking on the ‘Add files’ button in the ‘Edit Project’ menu. Now that the project has a source file associated with it, you can assemble it by pressing F10. The build 62  Silicon Chip process will start and a .hex file will be produced in the default directory specified above. Once the program has been assembled, exit the MPLAB environment and start the programmer (noppp). Specify the type of PIC (16F84) and load the .hex file. Insert the PIC into the programmer and select Program. The PIC will be programmed in about six seconds. Exit the programmer and remove the PIC from the socket. Construction Two versions are presented, one on a PC board and the other on Veroboard. In the first, all the components mount directly on the PC board, which measures 58 x 73mm. It is always wise to carefully examine any PC board prior to assembly to ensure that there are no shorts, or breaks present. It saves a significant amount of time to spot them now. A component layout for the board is shown in Fig.9. Start the assembly by installing the passive components first, such as the resistors and capacitors. You may find that it is beneficial to measure the values of the resistors The Veroboard version is designed to mount on the top of a medium sized plastic zippy box, replacing the lid. This is to allow the simplicity of the circuit to be displayed to any curious onlookers. If desired, the project can be mounted inside a slightly larger case, with the LEDs and pushbutton mounted on the lid in a more conventional manner. Building on Veroboard also allowed a fast development time to be achieved on the hardware. If you use Veroboard, be very careful to support the board while cutting the hole for the pushbutton switch, otherwise, the board will snap in half (been there, done that . . .). File the edges if the board is slightly too large (you will probably have to file the corners round, too). No component overlay is shown for the Veroboard version but the photographs will give a very good idea of component placement. Some of the components are mounted on the copper (strip) side of the board. Take care when cutting the Vero­ board tracks that the cut is complete and no copper swarf shorts to an adjacent track. The easiest way to cut Veroboard tracks is to take a twist drill bit about 5mm or so and simply twist it in the hole to be cut with your fingers. If the drill is sharp it results in a clean, quick hole. You may like to install a small piece of clear Perspex sheeting over the top of the project to protect it from small which incidentally, is where those 265 other dice went. Have fun. And remember, unless you create some code to allow you to cheat, it is very hard to force the dice to roll a particular way. Remember also that the one disadvantage of this project over the real dice is that it isn’t built to survive 20G’s of deceleration, so throwing it would be bad. Troubleshooting There wasn’t room for a battery hold­ er: a piece of bubble-wrap held the batteries in place and stopped any possibility of shorts. prying fingers. This can be mounted on 12mm brass standoffs on the top of the Veroboard, with a suitable hole for the pushbutton. Testing Examine the PC board or Veroboard to ensure there were no shorts created during assembly and then install the batteries. Note that the PIC microcontroller is NOT installed yet. Verify that +5.4V is present on pins 4 & 14 (with respect to pin 5 [GND]). Finally, disconnect the batteries, install the pre-programmed PIC (16F84) and re-install the batteries. (Don’t insert the PIC with the power applied!). You should be rewarded with a self-test pattern. Verify that the unit operates when the button is pressed as described earlier in this article. When you release the button, the display should ‘slow down’ and then display the result for approximately 20 seconds before turning itself off. Quickly pushing and releasing the pushbutton should recall the last roll. If the unit operates correctly, carefully mount the PC board in the top of the zippy box. All that remains now is to instruct the kids on how to operate it and to chain it to the table so that it doesn’t end up at the bottom of the toy box, If for some reason the project fails to work, check all soldering carefully. Verify that all the LEDs have been installed correctly. You can check the hardware by removing the PIC and placing a 10Ω resistor between pin 14 (VCC) and each of the LED drive lines (pins 1, 2, 10, 11, 12, 13, 17 and 18) one at a time. The LEDs should light. You can verify that the pushbutton switch operates correctly by monitoring pin 6 with a logic probe, or multimeter while pushing the button. It should go to +5.4V when the button is down. Finally, if you have a CRO, you can verify that the internal PIC oscillator is running by examining pin 15 (CLK­ OUT). This pin is not used by our circuit but from it you should see a 1MHz square wave for three seconds after the device is powered up and for 20 seconds after the button is pressed. Remember that the device spends most of its time in sleep mode, with the CPU clock turned off to conserve power. If all of the hardware checks out, you should try re-programming the PIC. Perhaps it has the wrong code installed. Good Luck. And remember that this SC is supposed to be fun! Want to know more? As mentioned in the text, source code for the PIC microcontroller and other information is available for those interested in this project. You can log in direct to: www.dougzone.com or you can access it via the SILICON CHIP website, www.siliconchip.com.au and follow the link from the selection bar on the left side of the opening page. May 2000  63