始めましてkkumaxと申します。ほぼ同じ構成でDACを構築しているようで
質問したいことがあります。良かったら教えてください。
写真をみると
特にPIC基板、マイコン関連等が見あたらないのですが
自作プログラムとは？XMOSの中身をいじっているのでしょうか？
最初はその方向で自分は検討したのですが．．．
英語力不足で挫折しました(笑)
今はArduinoをやってみようかと検討中です。

はじめまして、kkumaxさん。
このP2D基板自体にマイコンは搭載されていますよ。
XMOS自体は、単体モデルで中身は変更していません。
AK4490用のファームウェアが存在しているとは思うのですが、DIYINHKさんが公開してくれていないのでなんともなりませんねぇ。

XMOSUSB基板との信号はPCM・DSD識別信号ですが、このP2D基板の識別入力にてそのまま識別可能です。

ここで気づいたのですが、もしやAK4490についてでしょうか？

であれば、AK4490はくろ豆工房さんのところで頒布されているLCD一体型のArduino搭載キットがあり、それをLCDの状態表示とAK4490をソフトウェアモードにて制御しています。
ただ、私も自作プログラムは初めてで、状態表示とPCM・DSD自動切り替えの機能をもたせた程度の単純でなんとか動作できている程度のプログラムしか出来ていません。データシートを睨みつけながら1ヶ月試行錯誤でなんとかなりました。（^^;）

私の作成したなんとか動作したレベルのプログラムです。

/*
* Controler for AK4490EQ
* 2015/08/31
* V1.6
*/

// LIBRARIES

#include <LiquidCrystal.h> // For LCD
#include <Wire.h> // For I2C

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(2, 3, 15, 14, 17, 16);

// CONSTANT DEFINITION

// (The digital volume for Velvet is 0 to -127 db in .5 db steps)

#define DEFAULTATTNU 0xFF //-50 dB this is 50x2=100 or 0x64
#define MAXATTNU 0x01 //-99dB this is 99X2=198 or 0xC6
#define MINATTNU 0xFF //- 0 dB

// analog input Pin number

//#define PNDPin 10 //Velvet PND Pin Output
#define DSDModePin 11 //PCM・DSD mode signal
int swVal = 0;

#define VOLUPPIN 5 // Button to increase volume or RotEnc A terminal
#define VOLDOWNPIN 7 // Button to decrease volume or RotEnc B terminal
#define SELECTPIN 8 // Switch to select function

#define INTERVAL_BOUNCE 2 // Time in milliseconds to debounce the rotary encoder
#define INTERVAL_SWITCHBOUNCE 250 // Time in milliseconds to debounce switch
#define INTERVAL_SELECT 5 // Time to exit select mode when no activity

#define VOL 0 // The order of selection when clicking the select switch
#define FIL 1
#define SC 2

// VARIABLE DECLARATION

byte regVal; // Variable to pass register value
byte regAddr; // Variable to pass register value
byte currAttnu=DEFAULTATTNU; // Variable to hold the current attenuation value

unsigned long displayMillis = 0; // Stores last recorded time for display interval
unsigned long debounceMillis = 0; // Stores last recorded time for switch debounce interval
unsigned long selectMillis = 0; // Stores last recorded time for being in select mode

byte select; // To record current select option

boolean SHARP; // FIR filter mode
boolean ON; // SOUND CONTROLL
boolean selectMode;

// Rotary encoder interrupt service routine
static boolean rotating=false;
void rotEncoder()
{
rotating=true; // If motion is detected in the rotary encoder, set the flag to true
}

void writeVelvetReg(byte regAddr, byte regVal)
{
Wire.beginTransmission(0x11); //Hard coded to the the Velvet device address
Wire.write(regAddr); // Specifying the address of volume register
Wire.write(regVal); // Writing the volume value into the register
Wire.endTransmission();
}

void setVelvetVolume(byte regVal)
{
writeVelvetReg(0x03, regVal); // set up volume in DAC-L ATT
writeVelvetReg(0x04, regVal); // set up volume in DAC-R ATT
}

void printVol(byte regVal)
{
lcd.setCursor(11,0);
if (regVal<10) // transition between two digit and one digit display
{
lcd.print("0"); // Add a leading zero
lcd.print(regVal, DEC);
}
else
{
lcd.print(regVal, DEC);
}
}

void printSelectBar(){ // PRINTING SELECTION INDICATOR
lcd.setCursor(5,1);
lcd.print("");
lcd.setCursor(15,1);
lcd.print("");
}

void clearSelectBar(){ // CLEARING SELECTION INDICATOR
lcd.setCursor(5,1);
lcd.print("");
lcd.setCursor(15,1);
lcd.print("");
}

/*************************MAIN PROGRAM*************************************************************/
void setup() {

// Set up the LCD's number of columns and rows:
lcd.begin(16,2);

// Join the I2C bus as a master
Wire.begin();

//Velvet PND Pin setup (Reset)
// pinMode(PNDPin,OUTPUT);
// digitalWrite(PNDPin,LOW);

// Attach Interrupts
attachInterrupt(0, rotEncoder, CHANGE); // ISR for rotary encoder

// Set up the pin modes
pinMode(VOLUPPIN, INPUT); // Button for Encoder pin for volume up
digitalWrite(VOLUPPIN, HIGH); // If H/W debouncing is implemented, set to LOW

pinMode(VOLDOWNPIN, INPUT); // Button for Encoder pin for volume down
digitalWrite(VOLDOWNPIN, HIGH); // If H/W debouncing is implemented, set to LOW

pinMode(SELECTPIN, INPUT); // Button for encoder switch
digitalWrite(SELECTPIN, HIGH); // Enable pull-up resistor

pinMode(DSDModePin,INPUT); //DSD mode Pin setup

//AK4490EQ reset
Wire.beginTransmission(0x11);
Wire.write(0x00); // move pointer to control1 address
Wire.write(0x00); // sends 0x00 (hex) 00000000 (binary)
Wire.endTransmission();

//32bitI2S mode
// Wire.beginTransmission(0x11);
// Wire.write(0x00); // move pointer to control1 address
// Wire.write(0x8F); // sends 0x8F (hex) 10001111 (binary)
// Wire.endTransmission();

// Print the welcome message and other labels to the LCD

lcd.clear();
lcd.setCursor(2,0);
lcd.print("AK4490EQ DAC");
lcd.setCursor(5,1);
lcd.print("Ver 1.5");

delay(1000);
lcd.clear();
delay(500);

lcd.home();
lcd.setCursor(10,0);
lcd.print("-");
lcd.setCursor(12,0);
lcd.print("- dB");
lcd.setCursor(1,1);
lcd.print("FIL:SHARP");
lcd.setCursor(11,1);
lcd.print("SC:1");

SHARP=true; // This is the default value of roll-off filter (sharp)
ON=true; // This is the default value for SOUND CONTROLL (ON)

setVelvetVolume(DEFAULTATTNU); // Set volume at startup otherwise
// default is full volume
printVol(DEFAULTATTNU/2-127);
}

void loop() {

//AUTO INPUT MODE SELLECT
swVal = digitalRead(DSDModePin);

if (swVal == HIGH) {
Wire.beginTransmission(0x11); // Set to DSD Mode / L out to R in/R out to L in
Wire.write(0x02); // move pointer to control3 address
Wire.write(0x82); // sends 0x80 (hex) 10000010 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to RESET
Wire.write(0x00); // move pointer to control1 address
Wire.write(0x01); // sends 0x01 (hex) 00000001 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to DSD Data Mute/DSD Mute Control/DSD Mute Release
Wire.write(0x06); // move pointer to control5 address
Wire.write(0x99); // sends 0x99 (hex) 10011001 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to DSD Sampling Speed Control
Wire.write(0x09); // move pointer to control8 address
Wire.write(0x01); // sends 0x01 (hex) 00000001 (binary)
Wire.endTransmission();
lcd.setCursor(0,0);
lcd.print("DSD");
lcd.setCursor(4,0);
lcd.print("");
}
if (swVal == LOW) {
Wire.beginTransmission(0x11); // Set to PCM Mode
Wire.write(0x02); // move pointer to control3 address
Wire.write(0x00); // sends 0x80 (hex) 00000000 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to Master Clock Frequency Auto / 24bit I2S Mode
Wire.write(0x00); // move pointer to control1 address
Wire.write(0x8F); // sends 0x8F (hex) 10001111 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to De-emphasis Response 44.1 / Sharp roll-off filter
Wire.write(0x01); // move pointer to control2 address
Wire.write(0x00); // sends 0x00 (hex) 00000000 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to De-emphasis Response 44.1 / Sharp roll-off filter
Wire.write(0x02); // move pointer to control3 address
Wire.write(0x00); // sends 0x01 (hex) 00000000 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to L out to L in/R out to R in
Wire.write(0x02); // move pointer to control3 address
Wire.write(0x00); // sends 0x00 (hex) 00000000 (binary)
Wire.endTransmission();
lcd.setCursor(0,0);
lcd.print("PCM");
lcd.setCursor(4,0);
lcd.print("I2S32");
}

if((digitalRead(SELECTPIN)==LOW)&&((millis()-debounceMillis)>INTERVAL_SWITCHBOUNCE)) {
selectMode=true; // Now we are in select mode
printSelectBar(); // Indicate we are in select mode
debounceMillis=millis(); // Start debounce timer
selectMillis=millis(); // Start being in select mode timer

select++; // Move to next select option
switch(select%3){
case VOL:
printSelectBar();
break;
case FIL:
printSelectBar();
lcd.setCursor(5,1);
lcd.print("");
break;
case SC:
printSelectBar();
lcd.setCursor(15,1);
lcd.print("");
break;
} // End of switch
}

if(rotating)
{
delay(INTERVAL_BOUNCE); // debounce by waiting INTERVAL_BOUNCE time

switch(select%3){
case VOL:
if (digitalRead(5) == digitalRead(7)) // CCW
{
if (currAttnu<MAXATTNU) // Check if not already at maximum attenuation (minimum Volume)
{
currAttnu+=2; // Increase 1 dB attenuation (decrease volume 1 db)
setVelvetVolume(currAttnu); // Write value into registers
printVol(currAttnu/2); // Divide by 2 to print in whole dBs
}
}
else // If not CCW, then it is CW
{
if (currAttnu>MINATTNU) // Check if not already at minimum attenuation (max volume)
{
currAttnu-=2; // Decrease attenuation 1 dB (increase volume 1 db)
setVelvetVolume(currAttnu); // Write value into registers
printVol(currAttnu/2); // Divide by 2 to print in whole dBs
}
}
break;
case FIL: // Toggle between Sharp and Slow filter
if (SHARP){ // If currently in sharp, toggle to slow roll off filter
Wire.beginTransmission(0x11); // Set to Slow roll-off filter
Wire.write(0x01); // move pointer to control2 address
Wire.write(0x00); // sends 0x00 (hex) 00000000 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to Slow roll-off filter
Wire.write(0x02); // move pointer to control3 address
Wire.write(0x01); // sends 0x01 (hex) 00000001 (binary)
Wire.endTransmission();
lcd.setCursor(5,1);
lcd.print("SLOW ");
SHARP=false; // Now in slow roll off filter
}
else { // Toggle to sharp roll off filter
Wire.beginTransmission(0x11); // Set to Sharp roll-off filter
Wire.write(0x01); // move pointer to control2 address
Wire.write(0x00); // sends 0x00 (hex) 00000000 (binary)
Wire.endTransmission();
Wire.beginTransmission(0x11); // Set to Sharp roll-off filter
Wire.write(0x02); // move pointer to control3 address
Wire.write(0x00); // sends 0x01 (hex) 00000000 (binary)
Wire.endTransmission();
lcd.setCursor(5,1);
lcd.print("SHARP");
SHARP=true; // Now in sharp roll off filter
}
selectMillis=millis(); // Update being-in-select-mode timer
break;
case SC:
if (ON){ // If SOUND CONTROLL is on, turn off
Wire.beginTransmission(0x11);
Wire.write(0x08); // move pointer to control8 address
Wire.write(0x01); // sends 0x03 (hex) 00000001 (binary)
Wire.endTransmission();
lcd.setCursor(14,1);
lcd.print("2");
ON=false; // Now SOUND CONTROLL is off
}
else {
Wire.beginTransmission(0x11);
Wire.write(0x08); // move pointer to control8 address
Wire.write(0x00); // sends 0x01 (hex) 00000000 (binary)
Wire.endTransmission();
lcd.setCursor(14,1);
lcd.print("1");
ON=true; // Now SOUND CONTROLL is on
}
selectMillis=millis(); // Update being-in-select-mode timer
break;
} // End of switch

rotating=false; // Reset the flag

} // End of while(rotating)

// When the being-in-select mode timer expres, we revert to volume mode
if(selectMode&&millis()-selectMillis > INTERVAL_SELECT*1000){
selectMode=false;
clearSelectBar();
select=VOL; // Back to volume mode
}
} // End of loop()