Nuts and Volts - September/October 2018

a PIC16F648A with associated crystal, SRAM, optocoupler input, and double-inverter output (or equivalent).

The PIC16F648A MIDI Controller — A Very Demanding Boss

This chip has a very difficult job to do and needs to work like greased lightning. Figure 11 provides an overview. In many systems, you would expect the user interface to be the master, but here this MIDI controller chip is the ‘boss’ out of necessity.

The USART (Universal Asynchronous Receiver-Transmitter) receives and transmits MIDI data via FIFO (First in First Out) buffers in internal RAM.

The ISR (Interrupt Service Routine) handles the reading and writing to the buffers. Separate to this is the main program loop that runs a variety of tasks.

The thing that matters above all else in our system is that we must never ever lose any received data. The PIC16F648A has a one byte receive register in the USART that receives the MIDI data. As soon as this is filled, the byte must be transferred to the receive buffer in RAM. Otherwise, it will be overwritten.

Another essential is that any note messages are handled accurately and promptly. If they are being recorded, then they must be time-stamped ideally within 10 milliseconds of the ‘real’ time to maintain accuracy. If they are output (either echoed from the input or played back from recorded data), this must be fast to avoid noticeable delay. Technically speaking, there’s a need for low jitter and low latency.

All code has been carefully crafted and all unnecessary steps eliminated while keeping overall functionality. The ‘Pro’ (paid monthly) version of compiler-optimization is used in MPLAB to ensure maximum speed (and minimum code size). Tricks such as bit-shifting instead of slower multiply have been used. The PIC16F648A is driven at its maximum operating frequency of 20 MHz.

It was decided to write all software in fully-optimized C without any handwritten assembler. This was purely a practical decision, realizing that highly optimized code can still be extremely fast and much easier to write.

The MIDI protocol has a trick up its sleeve called running status that lets you only send a status byte when the status changes from the previous message. This means that, for example, you can send a string of ‘note on’

messages but only send one status message to indicate they are of type ‘note on.’ Running status can reduce note data by a third, so is fully utilized in this design. The MIDI controller PIC not only processes MIDI messages, but also applies the logic of transposing, tempo management, and playable ranges. It adjusts recorded tempo using a timer1 pre-scalar lookup, plus it communicates data with SRAM and interfaces to the other microcontrollers! Because this chip is so busy and Resources

History of multi-track recording https://en.wikipedia.org/wiki/History_of_multitrack_recording Bruce Springsteen’s Nebraska albums made on a four-track Portastudio http://tascam.com/news/display/226

Wiki’s comprehensive summary of MIDI https://en.wikipedia.org/wiki/MIDI SparkFun’s very readable summary of MIDI https://learn.sparkfun.com/tutorials/midi-tutorial/all Harmony Central’s fascinating history of MIDI www.harmonycentral.com/articles/a-brief-history-of-midi The MIDI Association’s summary of MIDI messages https://www.midi.org/specifications-old/item/table-1-summary-of- midi-message A review of embedded scheduling techniques https://www.embeddedrelated.com/showarticle/969.php A general overview of memory management https://en.wikipedia.org/wiki/Memory_management A nice explanation of interfaces and ‘encapsulation’ that can be an analogy of MIDI talking to all sorts of devices www.eeng.dcu.ie/~ee553/ee402notes/html/ch01s04.html Kaiser Chiefs MIDI Tech tells of ‘MIDI Madness’ during live performance (at end of article) https://www.soundonsound.com/people/roger-lyons-kaiser-chiefs-midi-tech Hitachi HD44780 LCD controller https://en.wikipedia.org/wiki/Hitachi_HD44780_LCD_controller Overview of scheduling techniques https://www.embeddedrelated.com/showarticle/969.php Parallel and distributed computing https://en.wikipedia.org/wiki/Distributed_computing#Parallel_and_ distributed_computing