Building A Simple MIDI Controller:
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1. 6811 Interface O PD1 TxD Figure 1 The Interface Schematic 2 The MIDI standard requires an electrical current of a certain rating to transmit information Our power supply is 5 volts We will be using two 220 resisters in a series and all MIDI receivers add another 220Q for a total of 660 Q Also the two gates we use have enough resistance to bring our circuit s total impedance to about 1 000 Q According to Ohm s law we can determine that our circuit will allow a certain amount of current to flow The amount of current is equal to the voltage divided by the resistance How many thousandths of an ampere milliamps flow through the MIDI current loop 3 The MIDI standard states that a digital signal consisting of two states current flow and no current flow see exercise 1 is used like a kind of Morse code to transmit information about musical events This signal is transmitted according to a specific timing called a baud rate In our case this is the number of bits which are transmitted each second The 6811 is capable of transmitting at many different baud rates Using software we have selected the proper speed Look at the software listing near the end of this document Find where a constant is stated which expresses a MIDI baud rate Our M68HC11EVBU has an 8 Megahertz MHz clock Using the tables located on in section 9 of the M68HC11 Reference Manual determine the standard MIDI baud r2. Building A Simple MIDI Controller A Laboratory Exercise Charles Repetti May 1992 A dissertation in the Department of Computer Science submitted to the faculty of the Graduate School of Arts and Sciences in partial fulfillment of the requirements for the degree of Masters of Science at New York University Approved Bubhaneswar Mishra Research Advisor Building a Simple MIDI Controller A Laboratory Exercise INTRODUCTION This laboratory illustrates a simple method for building a Musical Instrument Digital Interface MIDI controller from the ground up It is intended for music students who would like to build a musical instrument of their own design but who have very little electrical engineering or computer science background With proper guidance an ambitious student should be able to complete the lab in a single three or four hour session although the motivated student will want to spend much more time implementing his or her own specific design In this experiment the student learns to use the MC68HCIIE9 to load and edit code assemble it and download the result Also some simple wire wrapping is demonstrated and the use of a Hall Effect Transducer to activate a MIDI synthesizer is accomplished Once the activation of a sensor is shown to be capable of driving a synthesizer it is up to the student to imagine the type of movement he or she would like to make to create music THEORY and BACKGROUND All moder
3. address lines as input selectors might be suggested for those students interested in extending the number of sensors used The 6811 Serial Peripheral Interface might also be mentioned as a means of linking multiple processors Students should be encouraged to explore the application of the 6811 in musical instruments of their own design rather than simply extending the lab to include more specific experiments Music students tend to be a creative bunch and hopefully the more inspired amongst them will be willing to take on the discipline required to complete a large scale project if the goal is perceived as an instrument of their own design PC s should be set up to run Microsoft Windows with an editor an assembler and PCBUG11 as tasks Note that we have found that PCBUG 11 must be in full screen mode under Windows 3 0 to avoid dropping characters Windows 3 1 should solve this problem although this release was not available when this lab was first performed Also it is likely that some students will become confused by the wire wrapping process It would probably be helpful to check each student s wire wrap plan before work begins to be sure it is correct and to be sure that each student understands that once the board is turned over a different pin orientation will present itself NARA ARAL ANA ENN AANA ARAMA AARNE EAA ENE nD Neotentlox DS Neotantlox x ne ga Reg a da2 1972 horiec Rayetti Atl riynteo rasonvad 27 1942 Chov
4. ate 92S S 12 2 Ground CF C Vee 5V PBE A14 C 634SS2 PB6 A14 Ground Figure 2 Hall Effect Transducers LABORATORY INSTRUCTIONS The first thing which must be done is to assemble the interface to the sensor and to the synthesizer Place the two integrated circuit chips into wire wrapping sockets and place them on the 6811 board in the arrangement you created on the last page of this document Note that the top of the IC s are marked with an indentation be sure of the way the chip is seated or you will do everything backward You can also use sockets to attach the resisters and the sensor to the board Be sure to note the difference in the pins orientation once you turn the board over everything is upside down Be very careful here as there is no room for error in these wire connections Once everything is connected plug the board into the computer and to the synthesizer using the cable your instructor supplies and direct your attention to the computer s screen There are a number of windows displayed The first one to notice is the editor window This contains the program that will be run on the 6811 First fads head ory ba ng 322 092 Chores Rayettx AIL piynto rasosvad one saves the program then assembles it then downloads it to the 6811 computer and executes it By using the mouse press on the editor window and you should be able to browse through the program Beir with the Pa
5. ctor for help this is a tricky lab Once everything is working though you should have a better idea about how to go about designing and building your own musical instruments Building a Simple MIDI Controller A Laboratory Exercise INSTRUCTOR NOTES It is probably too much to expect that the students of this lab understand everything about the theory behind this experiment It might be useful to discuss the difference between analog and digital signaling A presentation involving Ohm s law might provide a helpful introduction to what is required to working in an engineering domain It might also be interesting to review the Hall Effect as well as photo optical and radio based alternatives for position sensing Some amount of assembly language programming will be required in a larger project as well If the final project is to include a MIDI input or thru port then the topic of interrupt programming for the Asynchronous Serial Communications Interface must be addressed One interesting project might involve using the sensor to generate pitch or modulation controller information for one of the many household keyboards that were manufactured without such a controller The controller could be hooked up without building a thru or input port Since only a small number of sensors would be required no off chip multiplexing would be required The use of the Precision Monolithics Mux O8 as an analog multiplexer with the 6811 data
6. em a sort of a music supercomputer But once again the first step is to get one sensor working SUGGESTED READING The M68HCIIEVBU comes with a box full of books explaining its operation These books become essential as one desires to build more complex systems You will need portions of the User s Manual and the Reference Manual for this lab A good introductory programming text would be helpful for reading and writing 6811 code although this is not absolutely required for our lab The offerings vary greatly It is best to go to your local computer bookstore and look for something that you feel you can work with Also each of the components used in this lab are included in a data book published by the manufacturer which gives great detail as to the component s use Anyone who works with MIDI will want a copy of the current MIDI standard which is available from the International MIDI Association 11857 Hartsook Street North Hollywood CA 91607 Many other books are also available which describe the MIDI standard in depth Again check with your local bookstore Some other suggested sources of information are 1 Micro Switch a Honeywell Division Hall Effect Transducers Freeport Illinois 61032 Micro Switch 1982 2 Horowitz and Hill The Art of Electronics Cambridge Cambridge University Press 1989 3 Precision Monolithics Incorporated Linear_and Conversion Products Santa Clara CA 95050 PMI Inc 1984 4 Texas Inst
7. geUp and PageDown E Sense seo arice keys Save the program from the amp File menu by pressing Alt Figure 3 The Edit Screen F1 then use the arrow keys to navigate to the Save option This writes the program to the computer s disk start c wit cute Start aF Laud rato o 2865 KL contro reyianer Now go to the assembly window by pressing on it with the mouse Type an a and the PC will assemble the program you just saved to disk The text from the editor is translated to the binary format required by a Motorola 6811 and can now be transferred over to the EVBU board Now press the hardware reset button on the EVBU board Use your User s Manual to locate this button Press on the PCBUG11 window Type run and press enter The system will now reset the software portion of your 6811 load the program from the personal computer the one you just assembled into location one hundred hexadecimal in the 6811 s on chip memory and run that program Now take a magnet possibly taped to a pencil or other apparatus and bring it close to the sensor You should hear the synthesizer play a note louder and louder as the magnet gets closer to the sensor Truth be told it is unlikely that this entire setup is going to work perfectly the first time If you are having problems carefully recheck all of your work from the beginning Don t be embarrassed to ask your lab assistant or instru
8. leo Rayoter AUL wbynts rasonvad start cf mitt cute J cou 30218 stast cf mitt cute 3 gt weds vetautes osi bawl cos rigat NGCHE 9 osl bowl con 4BBIIPBBG aud vato GG xj sULIPGBE Laud rato VORIOIG 2enGnd SUL contro reyiancy F i AO YORAM vanond SUL consyod raqdaney 90a ON BOE Fiaure 4 The Assembler Window 7 Fiaure 5 The PCBUG11 Screen 1 1401 Quad Two Input NOR Gate 1 7405 Open Collector Hex Inverter 2 220 O Resistors 1 92S812 2 Hall Effect Transducer or 634SS2 1 Rare Earth Magnet 1 Wire Wrapping Tool and Wire 1 PC running MS Windows an Editor a 6811 Cross Assembler and PCBUG 11 1 MIDI Sound Generator no keyboard required 1 5 Pin MIDI Connector and a DIN Socket with soldered leads 1 Any type of Sound System for the Sound Generator 1 5 Volt Power Supply 1 Motorola M68HC11 EVBU Student Evaluation Board
9. n music synthesizers are capable of receiving and acting upon the same type of signal according to the MIDI standard Until recently building a controller such as a keyboard was quite difficult But with the advent of the Motorola 6811 microcontroller the process is finally within the reach of the performing musician The 6811 is capable of receiving a output of a typical analog device such as a turnable knob or a pressable lever and translating that signal all the way to a MIDI signal In our example we will pass a magnet near a sensor This generates an increased voltage which the 6811 picks up and deciphers The 6811 is then free to send the position information to a synthesizer in a digital format which resembles an advanced Morse code There is an inexhaustible amount of theory concerning this process The interested reader is directed to the Suggested Reading section for more information about this The important thing to keep in mind is that the placement of a magnet near a sensor can be detected and used to play musical notes The 6811 is designed so that more than one sensor can be hooked up Uncomplicated methods exist for hooking up any number of sensors to one 6811 but the first step is just to get one working As many sensors are attached to one 6811 though the device can become overloaded Fortunately it is easy to hook up a number of 6811 s to one another so they can share the work This forms a parallel processing syst
10. ruments The TTL Data Book for Design Engineers Houston TX TI Inc 1981 Motorola M68HC11 EVBU Laboratory PRE LAB REQUIREMENTS EXERCISES 1 If you could build an instrument that could be played in any way that you desired what such an instrument look like Consider simplicity in your design Can solid materials be used instead of gears or pullies to make your design work How could you place magnets and sensors see figure 2 within the design to make it work 2 The schematic diagram below represents the way in which the parts must be connected to make the MIDI interface work Using this diagram make a drawing of where you will place your parts using the worksheet included on the last page of this text Consult chapter 2 of the M68HC11EVBU Universal Evaluation Board User s Manual for pin assignments on P4 connector and for more information about the wire wrap area Once you have decided where to locate your parts on the wire wrap area draw in lines between the exact place on each part where wire connections must be made Be sure to also include a sensor see Figure 2 in your design The legs of these sensors fit into the holes just like normal integrated circuits One of the sensors lies flat and responds from a magnet nearing it from above the other stands up and is exited from one side 2202 _ MIDI Pin 4 i l 2200 SM AA AN to 8 VVV MIDI Pin s ee a OOOUOOO Aa T T 507 04 MIDI Connector MIDI
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