Hi,

I'm working on a basic MIDI controller based on an LPC936 and a single pot between +3.2V and ground, with the wiper attached to pin 24 (port 0.3). This works, but with a couple of minor issues.

A minor problem is that the values output jump around a fair bit. Reading some threads here suggested using moving averages, which I've implemented (I think), which has improved things enormously.

The main problem is that I'm still continually sending MIDI messages that differ by one even when the pot is still. This isn't desirable - I only want to send messages on a "genuine" change. Is there anything I can add to the circuitry to stabilize things a bit? I have a 100nF cap across the pot already.

Resolution isn't really a problem on a device like this, as I'm only sending 7 bits of information. I suspect I need to filter out relatively short term changes (blips and spikes), and only send relatively long term changes (intentional control movements).

Any suggestions appreciated.

;******************************************************************************
;
;       MIDI controller based on Linear pot & LPC935
;
;******************************************************************************
;
;

;       Primary controls

$TITLE(BYTE MIDI_1)
$PAGEWIDTH(132)
$DEBUG
$OBJECT
$NOPAGING
;
;
;  Variable declarations

$include(C:\asm51\LPC936.inc)

    
;
;   Data memory

    slot1 data 30h
    slot2 data 31h
    slot3 data 32h
    slot4 data 33h
    average data 34h
    old_average data 35h
    
;   Bit memory
    
    AD_Complete         Bit     00h
    
;   Equates

    slot_pointer equ R1

;   Buffers for data storage


    
    ORG     00H
    
    CSEG
    
    LJMP INIT
        
    ORG 0073h
    
    LJMP ADC_INTERRUPT
    
        
;##############################################################################
;
;   Initialisation routines
;

    
INIT:

;//////////////////////////////////////////////////////////////////////////////
;
;   configure port pins
    
    mov P1M1, #00h      ;P1 in QBD mode as we're using TxD
    mov P2M1, #00h      ;Ditto for P2
    
;//////////////////////////////////////////////////////////////////////////////
;
;   configure uart for BaudRate,8,n,1 
    
    mov brgcon, #00h
    mov brgr0, #0dch    ;This line for 31250 (Midi)
    ;mov BRGR0, #30h    ;This line for 115200 (PC)
    mov brgr1, #00h
    mov brgcon, #03h
    mov scon, #50h      ;mode 1
    mov pcon, #0c0h     ;enable fe bit & smod = 1
    


    
;//////////////////////////////////////////////////////////////////////////////
;
;       Configure ADC
    
    clr AD_Complete     ;Init AD complete flag
    
    mov ADINS, #10h     ;Select ADI12 - Port 0.3 Pin 24
    
    mov ADCON1, #45h    ;Enable ADCI1, En ADC1, immediate mode
    
    mov ADMODB, #40h    ;ADC Clock divisor 3
    
    setb EAD            ;enable AD complete interrupt
    
    setb EA             
    
;//////////////////////////////////////////////////////////////////////////////
;
;       Init variables
    
    mov slot_pointer, #30h  ;Address of slot1
    mov a, #00h
    
;//////////////////////////////////////////////////////////////////////////////
;
;  Main loop
;       

LOOP1:

    orl ADCON1, #01h    ;Immediate start A/D conv
    
    jnb AD_Complete, $  ;Wait for interrupt
    
    mov a, AD1DAT0      ;Store ADC reading
    
    clr c
    rrc a               ;Divide by 2 to convert reading from 0-255 to 0-127
        
    mov @slot_pointer, a    ;Store our reading in the current slot
    
    inc slot_pointer        ;Increment pointer to the next slot
    cjne slot_pointer, #34h, carry_on
    mov slot_pointer, #30h  
    
carry_on:

    push acc            ;save a/d reading   
    
;//////////////////////////////////////////////////////////////////////////////
;
;   Calculate moving average  
;   
    mov a, #0h
    
    addc a, slot1
    addc a, slot2
    addc a, slot3
    addc a, slot4       ;Add all 4 values together
    

    rrc a               ;First rotate needs to include carry
    clr c
    rrc a               ;div by 4 to get average
    
    mov average, a      ;Store the average
    
    clr AD_Complete     ;Clear ADC Complete flag
    
    cjne a, old_average, diff   ;Are the old and new averages different?
    
    pop acc             ;dummy pop
    
    jmp loop1
    
diff:

    mov old_average, a  ;Store average as old average
    
;//////////////////////////////////////////////////////////////////////////////
;
;  Send data
;   
    pop acc             ;get a/d reading back

    CLR TI
    mov sbuf, #0B0h     ;MIDI CC
    jnb ti, $
    
    CLR TI
    mov sbuf, #04h      ;CC number 4
    jnb ti, $
    
    clr ti
    mov sbuf, a         ;Current A/D reading
    jnb ti, $

    mov R7, 0ffh        ;Wait for a bit
TimerLoop:
    nop
    djnz R7, TimerLoop 
    
    jmp  LOOP1                  ; endless loop

;++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
;
;
;           ADC interrupt.
        
ADC_INTERRUPT:

    anl ADCON1, #0F4h   ;Clear conversion complete, set to stop mode
    setb AD_Complete    ;Set ADC Complete flag

RETI

;
;           End of ADC interrupt
;++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    
END