Readout distance from a Sndway Laser Distance Meter SW-A40 with Arduino Mini

sndway_simple_readout.ino

/**
 * Reading actual measured distance from LCD of a Sndway SW-A40 laser distance meter.
 *  
 * Developed and tested on Arduino Mini 328P@16MHz.
 * See https://www.mictronics.de/2018/02/laser-distance-meter-hack/
 * Mictronics 2018 @ www.mictronics.de
 */

/** 
 * Pin connection to LCD SPI bus. 
 */
const byte clkPin = 2; // LCD clock signal = IO2
const byte dataPin = 4; // LCD data signal = IO4
const byte csPin = 3; // LCD chip select signal = IO3

const word lutNumber[] = {'8',    '0',    '9',    '6',    '5',    '3',    '2',    '4',    '7',    '1'};
const word lutFrame0[] = {0x0006, 0x0006, 0x0004, 0x0006, 0x0004, 0x0004, 0x0002, 0x0004, 0x0004, 0x0004};
const word lutFrame1[] = {0x0006, 0x0004, 0x0006, 0x0002, 0x0002, 0x0006, 0x0006, 0x0006, 0x0004, 0x0004};
const word lutFrame2[] = {0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0004, 0x0000, 0x0000, 0x0000};
const word lutFrame3[] = {0x0006, 0x0006, 0x0006, 0x0006, 0x0006, 0x0004, 0x0004, 0x0002, 0x0004, 0x0000};

const word digit5 = 0x0002;
const word sign = 0x0001;
const word unit_ft = 0x1000;
const word unit_in = 0x4800;
const word unit_m = 0xC000;
const byte laserOn = 0x08;
const byte reference = 0x04;

byte lcdFrameBuffer[70]; // Holding a single LCD frame, containing 4 sub-frames
byte data = 0; // Holding a single data byte
byte bitCount = 8; // Counting bits in each data byte
byte byteCount = 0; // Counting bytes in lcdFrame
char serialOutBuffer[20];
byte serialOutIndex = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("Sndway Rangefinder v1.0");  
  pinMode(clkPin, INPUT);
  pinMode(dataPin, INPUT);
  pinMode(csPin, INPUT);
  /* External interrupt 0&1 enable on rising edge. */
  EICRA = _BV(ISC11) | _BV(ISC10) | _BV(ISC01) | _BV(ISC00) ;
  EIMSK = _BV(INT1) | _BV(INT0);
}

void loop() {
  word frame0 = 0, frame1 = 0, frame2 = 0, frame3 = 0;
  word mask0 = 0, mask1 = 0, mask2 = 0, mask3 = 0;

  /* Skip LCD init frame and start reception inbetween two frames */
  if((byteCount > 2) && (lcdFrameBuffer[0] != 0x10) && (lcdFrameBuffer[1] != 0x00)){
    delay(50);
    byteCount = 0;
  }
  
  /* Process a complete LCD frame */
  if(byteCount == 68){
    noInterrupts();
    // Check for correct frame order
    if((lcdFrameBuffer[0] == 0x10) &&(lcdFrameBuffer[1] == 0x00) &&(lcdFrameBuffer[2] == 0xC0)){
      // Save last LCD line data to work variables
      frame0 = lcdFrameBuffer[10];
      frame0 <<= 8;
      frame0 += lcdFrameBuffer[11];
      frame1 = lcdFrameBuffer[27];
      frame1 <<= 8;
      frame1 += lcdFrameBuffer[28];
      frame2 = lcdFrameBuffer[44];
      frame2 <<= 8;
      frame2 += lcdFrameBuffer[45];
      frame3 = lcdFrameBuffer[61];
      frame3 <<= 8;
      frame3 += lcdFrameBuffer[62];
    }
    /* Get ready to receive new frame */
    byteCount = 0;
    interrupts();
    // Process last frame, get distance and unit from LCD data
    // Sign
    if((frame0 & sign) == sign){ serialOutBuffer[serialOutIndex++] = '-';}
    // Digit 5
    if((frame2 & digit5) == digit5){ serialOutBuffer[serialOutIndex++] = '1';}
    // Remaining 5 digits
    for(byte d = 0; d < 5; d++){      
      for(byte n = 0; n < 10; n++){
        mask0 = lutFrame0[n];
        mask1 = lutFrame1[n];
        mask2 = lutFrame2[n];
        mask3 = lutFrame3[n];
        mask0 = (lutFrame0[n] << (d*2));
        mask1 = (lutFrame1[n] << (d*2));
        mask2 = (lutFrame2[n] << (d*2));
        mask3 = (lutFrame3[n] << (d*2));
        if(((frame0 & mask0) == mask0) && ((frame1 & mask1) == mask1) && ((frame2 & mask2) == mask2) && ((frame3 & mask3) == mask3)){
          serialOutBuffer[serialOutIndex++] = lutNumber[n];
          break;
        }
      }
      // Decimal points
      if((d == 1) && ((frame2 & 0x0020) == 0x0020)){ serialOutBuffer[serialOutIndex++] = '.'; }
      else if((d == 2) && ((frame2 & 0x0080) == 0x0080)){ serialOutBuffer[serialOutIndex++] = '.'; }
      else if((d == 3) && ((frame2 & 0x0200) == 0x0200)){ serialOutBuffer[serialOutIndex++] = '.'; }
    } 
    // Unit
    if((frame2 & unit_ft) == unit_ft){
      serialOutBuffer[serialOutIndex++] = 'f';
      serialOutBuffer[serialOutIndex++] = 't';
    } else if((frame2 & unit_in) == unit_in){
      serialOutBuffer[serialOutIndex++] = 'i';
      serialOutBuffer[serialOutIndex++] = 'n';
    } else if((frame2 & unit_m) == unit_m){
      serialOutBuffer[serialOutIndex++] = 'm';
    }
    serialOutBuffer[serialOutIndex++] = '\n';
    Serial.write(serialOutBuffer, serialOutIndex);
    serialOutIndex = 0; 
  }
}

/**
 * Reads the data pin on CLK rising edge.
 */
ISR(INT0_vect) {
  if(digitalRead(csPin) == LOW) {
    if(bitCount == 8) {
      // Skip data/command identifier bit
      --bitCount;
      return;
    }
    asm volatile("nop;");
    data |= (digitalRead(dataPin) << bitCount);
    --bitCount;
  }
}

/**
 * Save a single data byte to LCD frame buffer on CS rising edge.
 */
ISR(INT1_vect) {
  lcdFrameBuffer[byteCount] = data;
  ++byteCount;
  data = 0;
  bitCount = 8;
}