MIDI Background Information The Musical Instrument Digital Interface (MIDI) is a standard allowing musical synthesizers and other electronic devices, of different makes and models, to communicate and control each other via a unidirectional link. For Example: A computer, or musical keyboard may control the notes played on a separate synthesizer.
MIDI History During the 70s the analogue synthesizer evolved from the electric organ and became popular by the end of the decade During this time, analogue control voltages were used for interconnectivity. Crude and Monophonic. In 1982 UMI (Universal Musical Interface) was proposed By 1983 it had been renamed to MIDI (Musical Instrument Digital Interface) Major contributors to the development of the standard were Roland (Japan) and Sequential Circuits (USA) source: http://www.mtsu.edu/~dsmitche/rim419/midi/HTMLs/MIDHIS~1.HTM
The MIDI Specification Hardware Specification –Asynchronous Serial Interface –Current Loop (current on = logic 0) Optical isolation & no common ground –31.25 Kbps (320us per byte)
The MIDI Specification Midi Messages (in general) –Each message can contain from 1 to 3 bytes –The first byte is always a status byte which specifies the TYPE (and channel) of the message. This byte always has the MSB set (i.e from 0x80 - 0xFF) –The following data bytes (if needed) contain information specific to the message type defined by the status byte
The MIDI Specification Some Status Bytes: 0x80 - 0x8F : Note Off 0x90 - 0x9F : Note On 0xA0 - 0xAF : Aftertouch 0xB0 - 0xBF : Program Change - where the lower nibble is the channel number
The MIDI Specification Example: The Note On status byte is followed by two data bytes which specify the note number and velocity (defined in the specification) The following byte stream will play note C6 (0x3C) at a velocity (volume) of 100 (0x64). 0x92, 0x3C, 0x64 Now, onto the convoluted contraption...
Device Programming Method –Download Separate Serial/Parallel PC interface Sysex Data Stream (through MIDI) –Code Generation Configuration data interpreted by Embedded Code Generation of instruction words of code Concept & Preliminary Design
Hardware Consideration Microcontroller Required: –Nonvolatile Memory –Sufficient Speed & Ram –Asynchronous serial input and output 31.25Kbps 1 start bit, 1 stop bit, 8 bit data byte
Hardware Consideration Microcontroller Chosen: –Microchip PIC16F876 8-bit RISC Structure (14 bit instruction word) Onboard Flash Memory (8K 14-bit words) 368 bytes of RAM USART Serial Communication 20MHz Maximum Operating Frequency –Allowing a 5MHz CPU clock 13 Interrupts
Hardware Consideration Configuration: –16Mhz Operating Frequency 4MHz CPU Clock –UART @ 31.25 Kbps 1 start/stop bits, 8 data bits, no interrupts enabled –Flash Memory Runtime Write Enable
Hardware Issues Supply voltage must be regulated to approximately 5V at varying current draw –In this case, a 5V regulator IC was used Opto-isolator and hex inverter must be capable of operation @ 31.25KHz With CPU Clock @ 4MHz, 1280 cycles are available between bytes
Software Production Software Production Phases –Begin Windows Application Software (C++) Basic User Interface. I.e. Menus, MDI Child Windows, Toolbars Drawing/Dropping routines –Create Embedded Firmware (PIC16 ASM) –Finish Windows Application Software Create Code Generation Routines Tidy up File Saving/Loading
Application Software Written wholly in ANSI C++ Using MinGW (Minimalist GNU for Windows) –collection of headers, libraries and compilers for Win32 Application creation –placed in the public domain & GNU licensed DJGPPs RHIDE used as editor
GUI Design STATEEVENT (Transition) TOOLS Pointer Draw Line Delete Edit Select Start State Actions within state Triggers within event
Embedded Code Written wholly in PIC16 ASM Using MPBLab IDE DJGPPs RHIDE used as editor Responsibilities –Receiving and organising MIDI bytes into messages –Calling Downloaded Code –Transmitting MIDI Output (from Output Buffer) –Allowing Downloading and Writing of State-Machine Code to Flash Memory
Full MIDI Message? Add byte to Message Buffer Embedded Code Call Downloaded Code With MIDI Message Byte Recieved? Transmit Next Byte (If Output buffer isnt Empty) Call Downloaded Code Without MIDI Message Y Y N N
Embedded Code Stats ROM –493 Word Program (1st Page) –2K Words Available for Download (2nd Page) RAM –Message Buffer - 4 bytes (1st Bank) –Output Buffer - 94 bytes (FIFO, 2nd Bank) Start Pointer, End Pointer & End Location –3rd Bank dedicated for Downloaded Code Ram
User Code Generation Code Generation Algorithm Users State Diagram(s) Machine Code (Performed Within Windows Application Software)
User Code Generation Each state diagram has a variable which defines the current state Every time the downloaded code is called it cycles through each of the state diagrams –For each diagram it jumps to the correct location in memory for the current state Each state location checks inputs to see if a change in state is necessary
User Code Generation Action1 Action2... Set State to STATE1 Return to root code STATE1AIf Event1 goto STATE2A... Return to root code STATE1B If Event2 goto STATE5A An Example for STATE 1. The root code will jump to STATE1B, but any other state will jump to STATE1A.
Resources Used MIDI Technical Fanatics Brainwashing Center MinGW, MPBLab & DJGPP Resources The Forgers Win32 API Tutorial Microchip product documentation MIDI Manufacturers Assoc. Homepage Programming and Customising PICmicro Microcontrollers –Myke Predko