MIDI Input Example

[Anonymous]

Hi Gang,

Here's a MIDI input example in Great Cow Basic. It's purpose is simply to respond to MIDI Start, Stop and Continue commands. This is more a demo than anything, but could indeed be used as-is to control digital recorders, sequencers, lights and whatever else is out there that responds to those signals.

More importantly, it illustrates the use of the analog comparators, the USART and interrupts. And you get to see how to implement a circular queue. The code is very short and sweet for all that, and the circuit can be built up in minutes, it's so simple.

Here's the code. The schematic follows in the next post.

Thomas Henry

;Project 66
;A program to detect MIDI Start, Stop and Continue.

;----- Configuration

#chip 16F88, 20                 ;PIC16F88 running at 20 MHz
#config mclr=off                ;reset handled internally
#config osc=hs                  ;use resonator

;----- Constants

#define USART_BAUD_RATE 31250
#define USART_BLOCKING 1
#define start_LED   b'00010000' ;LED bit patterns
#define cont_LED    b'01000000'
#define stop_LED    b'10000000'
#define dud         h'F9'       ;a never-used dummy MIDI value
#define midi_start  h'FA'       ;MIDI status byte: Start
#define midi_cont   h'FB'       ;MIDI status byte: Continue
#define midi_stop   h'FC'       ;MIDI status byte: Stop

;----- Variables

dim midi, head, tail, temp, current as byte
dim queue(80)

;----- Program

dir PortA in                    ;all inputs
ANSEL = 0b11111111              ;and all analog
dir PortB 0b00000100            ;B.2 is input

head = 0                        ;initialize queue
tail = 0

CMCON = 0b00010101              ;set up comparator
on interrupt Comp1Change call fetch ;enable interrupts

do
  if head <> tail then          ;anything in queue?
    midi = queue(head)          ;get the byte
    head ++                     ;bump to next position
    if head > 80 then           ;or wrap around to start
      head = 0
    end if

    select case midi
      case midi_start           ;MIDI Start received
        PortB = start_LED       ;so turn on proper LED
      case midi_cont
        PortB = cont_LED        ;MIDI Continue received
      case midi_stop
        PortB = stop_LED        ;MIDI Stop received
    end select                  ;all other bytes ignored
  end if
loop                            ;repeat forever

;----- Subroutines

sub fetch
  if C2OUT = on then            ;comparator went high
    HSerReceive queue(tail)     ;read a byte in
    tail ++                     ;prep for next time
    if tail > 80 then           ;wrap around if needed
      tail = 0
    end if
  end if
end sub

[Anonymous]

And here's the schematic...

[polyconnect]

nice. Thanks.
Can you explain the basic theory of the queue ?