foo.cpp

const int SCK_PIN = 5;
const int RCK_PIN = 9;
const int NOE_PIN = 8;
const int DIN_PIN = 6;
const int WTF_PIN = 3;

static void cycle_sck(void) {
  digitalWrite(SCK_PIN, HIGH);
  //delay(1);
  digitalWrite(SCK_PIN, LOW);
  //delay(1);
}

static void cycle_rck(void) {
  digitalWrite(RCK_PIN, HIGH);
  //delay(1);
  digitalWrite(RCK_PIN, LOW);
  //delay(1);
}

/*
CREG bits

QA/0: ?
QB/1: ?
QC/2: ? display
QD/3: ?
QE/4: !status led
QF/5: backlight
QG/6: ? display
QH/7: ? display
 */
uint8_t writeCtrl(uint8_t b) {
  // write bits to shift-register
  writeData(b);

  // copy from shift register to 
  // backing register
  cycle_rck();

  return b;
}

void writeData(uint8_t b) {
  for (int i=0; i < 8; i++) {
    if (b & 1) {
      digitalWrite(DIN_PIN, HIGH);
    } else {
      digitalWrite(DIN_PIN, LOW);
    }
    cycle_sck();
    b >>= 1;
  }
}

/*
 * assumption: RGB interface. 
 * from SPFD54122B datasheet (p.10) we have the following pins:
 * - HSYNC
 * - VSYNC
 * - DE (HIGH=valid data on DB[0;7])
 * - PCLK
 * 
 * PCLK might be WTF_PIN, DEN
 * " So, controller (host) must always transfer PCLK, VS, HS and DE signals to SPFD54122B"
 * 
 * Signal description on p.137
 * 
 * - The host changes D[17:0], VS, HS and DE lines [on] falling edge of PCLK
 * - The driver read the D[17:0], VS, HS and DE lines [on] rising edge of PCLK
 * 
 * Detailled description on p.143 to write a new frame (see also p.138 for HS/VS details):
 * 
 * 0. HSYNC=1, VSYNC=1, DE=0
 * 1. toggle VSYNC low - high
 * 128 times:
 *   1. toggle HSYNC low - high
 *   2. set DE=1
 *   3. write pixels via DB for 128 PCLK cycles
 *   4. set DE=0
 *   
 * For each change, PCLK low first, after change, PCLK up.
 */
void rgbtest(void) {
  const int PCLK = WTF_PIN;
  
  // bit assignments in CREG
  // available: 0, 1, 5
  // H V D 
  // 0 1 5 x
  // 1 0 5 x
  // 0 5 1 x none of this works
  // 1 5 0 x
  // 5 0 1 x
  // 5 1 0 x
  // alternative: 2, 6, 7
  const int HSYNC = 0;
  const int VSYNC = 1;
  const int DE = 5;
  const int LED = 3;
  int CREG = 0b0000100;

  // CREG:    
  // 0b00100011
  //     ^   ^^-.
  //    d1  d2  d3

  // step 0; init
  digitalWrite(PCLK, LOW);
  writeCtrl(CREG | (1 << HSYNC) | (1 << VSYNC));
  digitalWrite(PCLK, HIGH);

  // step 1; toggle VSYNC
  digitalWrite(PCLK, LOW);
  writeCtrl(CREG | (1 << HSYNC));
  digitalWrite(PCLK, HIGH);
  digitalWrite(PCLK, LOW);
  writeCtrl(CREG | (1 << HSYNC) | (1 << VSYNC));
  digitalWrite(PCLK, HIGH);

  for (int i=0; i < 128; i++) {
    // toggle status LED every second row
    if (i % 2 == 0) {
        CREG ^= (1 << LED);
    }
    
    // step 1: toggle HSYNC
    digitalWrite(PCLK, LOW);
    writeCtrl(CREG | (1 << VSYNC));
    digitalWrite(PCLK, HIGH);
    digitalWrite(PCLK, LOW);
    writeCtrl(CREG | (1 << HSYNC) | (1 << VSYNC));
    digitalWrite(PCLK, HIGH);

    // step 2: set DE=1
    digitalWrite(PCLK, LOW);
    writeCtrl(CREG | (1 << HSYNC) | (1 << VSYNC) | (1 << DE));
    digitalWrite(PCLK, HIGH);

    // step 3: write pixels
    for (int j=0; j < 128; j++) {
      digitalWrite(PCLK, LOW);
      writeData(0xAA);
      digitalWrite(PCLK, HIGH);
    }

    // step 4: set DE=0
    digitalWrite(PCLK, LOW);
    writeCtrl(CREG | (1 << HSYNC) | (1 << VSYNC));
    digitalWrite(PCLK, HIGH);
  }
}


void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(NOE_PIN, OUTPUT);
  pinMode(DIN_PIN, OUTPUT);
  pinMode(SCK_PIN, OUTPUT);
  pinMode(RCK_PIN, OUTPUT);
  pinMode(WTF_PIN, OUTPUT);

  digitalWrite(DIN_PIN, LOW);
  digitalWrite(SCK_PIN, LOW);
  digitalWrite(RCK_PIN, LOW);
  digitalWrite(WTF_PIN, HIGH);

  // disable output
  digitalWrite(NOE_PIN, HIGH);

  delay(100);

  // enable output
  digitalWrite(NOE_PIN, LOW);

  // toggle backlight off, on
  writeCtrl(0b0000000);
  delay(100);

  writeCtrl(0b0000100);
  delay(100);

  while (true) {
    rgbtest();
  }
}