[sdiy] Seb's Fat Controller (was DAC via Mux)

[Seb Francis]

Justin Owen wrote:
> -----Original Message-----
> From: Eric Brombaugh [ebrombaugh1 at cox.net]
>
>   
>> Check Seb Francis' Fat Controller MIDI2CV schematic for a good example of how this is done:
>>     
>
> http://burnit.co.uk/sdiy/index.php?page=fatcontroller
>
> Thanks for this link Eric.
>
> I'm not sure if Seb is reading this - but the one query I have about his S&H circuit is that there doesn't seem to be any switching/sampling part to it. The Paia Fatman S&H uses a 4016 switched by the micro but Seb's Mux output seems to go straight into the S&H cap/op amp combo.
>
> Anyone care to explain that to me? Seb?
>   

As Neil hinted at, the Mux also acts as a switch.  It's necessary to 
give some consideration to the timing of everything.  Since I can't lay 
my hands on the notes I made at the time, here's a the relevant bit of 
source code, with my comments at the time...

nop2 and wait_us are macros - I expect you can guess what they do!
Update frequency is 51.2us per channel (768us overall = 1.302 KHz sample 
rate)
PIC is running at 20MHz, so that's 0.2us per instruction

UpdateCVFromRAM
    <snip>
   
    ; Set DAC to voltage
    bcf    PORTC,DACCS    ; Set !CS low
    movwf    SSPBUF        ; Transmit MSB
    nop2            ; Wait 9 cycles before sending next byte
    nop2
    nop2
    nop
    bcf    MUXE        ; While waiting: Turn off MUX (takes max 300ns)  
; Maybe move this to earlier to eliminate digital feedthrough from the 
SPI signal???
    movfw    TEMP1        ; While waiting: Load LSB into w
    movwf    SSPBUF        ; Transmit LSB and sub-bits
    nop2            ; Wait 8 cycles
    rlf    TEMP3        ; While waiting: Shift MUX bits to left
    btfsc    TEMP3,4        ; While waiting: Copy bit 4 to left
    bsf    TEMP3,5
    movfw    TEMP3
    movwf    PORTA        ; While waiting: Select MUX channel
    nop
    bsf    PORTC,DACCS    ; Set !CS high (loads DAC)
   
    wait_us 5        ; Wait 5us for DAC and opamps to settle
                ; Maybe less wait with faster opamp?
   
    bsf    MUXE        ; Turn on MUX (takes typ 105ns, max 600ns)
   
    ; S&H now has until next call of UpdateCVFromRAM to settle

    bsf    STATUS,RP0    ; Select RAM bank 1
   
    return