Here is the original MIDI circuit, designed to send static rhythms to a MIDI synth. It took a while to get the output working properly using Tom Igoe's notes. Tetsu Kondo was a big help in finally debugging the circuit; one would do well to follow a few of his suggestions:

• Try removing Tom's suggested twin capacitors from oscillator to ground;
• Make sure that pin 8 is grounded (for the PIC16F876, at least);
• Don't forget the 220 ohm resistor into the MIDI power input.

Once that circuit was up and running, I attached a basic bank of eight switches to simulate a single row of squares on my percussion chessboard. The PIC scrolls through each switch and based on whether it is on or off plays a conga beat or a different percussion sound.

The potentiometer at left was attached to control the tempo. It is read after each beat to determine the wait until the next beat. Anything less than 200 seemed to clog the PIC, so I made that value a base threshold. Obviously, some optimization will have to be done before I can attach eight banks of sensors to the chip.

Circuit Schematic

Pic Basic Pro Code

According to Tom, there is a way to call port registers with variables using the format PORTA.x or the like. This returned a lot of "bad variable modifier" errors, so I tackled the loop with a more direct approach. The Pic Basic Pro manual seems to indicate that the proper format for variable modifiers is PORTA.0[x] instead, but I haven't had a chance to check that yet.

INCLUDE "modedefs.bas"

define OSC 20

define HSER_TXSTA 20h  ' enable the transmit register
define HSER_BAUD 31250 ' set the baud rate

DEFINE  ADC_BITS        10
DEFINE  ADC_CLOCK       3
DEFINE  ADC_SAMPLEUS    10

tempo VAR WORD

TRISA = %11111111
ADCON1 = %10000010
TRISB = %11111111

pause 500

main:

        if PORTB.0 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.1 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.2 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.3 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.4 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.5 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.6 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif
        if PORTB.7 = 1 then
            gosub playNote
        else
            gosub playLowNote
        endif

goto main

playNote:
        hserout [$99, $40, $40]
        gosub setTempo
        pause tempo
        hserout [$89, $40, $000]
return

playLowNote:
        hserout [$99, $4C, $40]
        gosub setTempo
        pause tempo
        hserout [$89, $4C, $000]
return

setTempo:

        ADCIN 0, tempo
        tempo = (tempo + 100) / 2
        if tempo < 200 then
            tempo = 200
        endif

return