Fix twos complement

src/Q2HX711.cpp

@@ -12,42 +12,71 @@ Q2HX711::Q2HX711(byte output_pin, byte clock_pin) {
 Q2HX711::~Q2HX711() {
 }
 
+/**
+ * Puts the chip in power down mode by setting the clock signal high.
+ *
+ * The HX711 will enter power down when PD_SCK is high for more then 60μs.
+ */
+void Q2HX711::powerDown() {
+    digitalWrite(CLOCK_PIN, HIGH);
+}
+
+/**
+ * Wakes the chip from power down mode by setting the clock signal low.
+ *
+ * When PD_SCK returns to low, the HX711 will reset and enter normal operation mode.
+ * After a reset or power-down event, input selection is default to Channel A with a gain of 128.
+ */
+void Q2HX711::powerUp() {
+    digitalWrite(CLOCK_PIN, LOW);
+}
+
 bool Q2HX711::readyToSend() {
   return digitalRead(OUT_PIN) == LOW;
 }
 
 void Q2HX711::setGain(byte gain) {
   switch (gain) {
-    case 128:
+    case 128: // Input Channel A
       GAIN = 1;
       break;
-    case 64:
+    case 64: // Input Channel A
       GAIN = 3;
       break;
-    case 32:
+    case 32: // Input Channel B
       GAIN = 2;
       break;
   }
 
-  digitalWrite(CLOCK_PIN, LOW);
+  powerUp();
   read();
 }
 
+/**
+ * Reads a value from the chip and returns the raw value.
+ */
 long Q2HX711::read() {
    while (!readyToSend());
 
+  // Read 24 bit data, most significant bit first.
   byte data[3];
-
   for (byte j = 3; j--;) {
-      data[j] = shiftIn(OUT_PIN,CLOCK_PIN, MSBFIRST);
+      data[j] = shiftIn(OUT_PIN, CLOCK_PIN, MSBFIRST);
   }
 
-  // set gain
+  // Set gain and input for next conversion by pulsing the clock signal 1 to 3 times.
   for (int i = 0; i < GAIN; i++) {
     digitalWrite(CLOCK_PIN, HIGH);
     digitalWrite(CLOCK_PIN, LOW);
   }
 
-  data[2] ^= 0x80;
-  return ((uint32_t) data[2] << 16) | ((uint32_t) data[1] << 8) | (uint32_t) data[0];
+  // Collect the bytes in a signed integer
+  long value = ((uint32_t) data[2] << 16) | ((uint32_t) data[1] << 8) | (uint32_t) data[0];
+  // The value read from the HX711 is in two's complement.
+  // Normally the most significant bit of a 24 bit number represents 2^23.
+  // But for a 24 bit two's complement number, the bit represents -2^23 (a negative value).
+  if (value & (1L << 23)) {
+      value -= (1L << 24);
+  }
+  return value;
 }

src/Q2HX711.h

@@ -9,9 +9,12 @@ class Q2HX711
     byte OUT_PIN;
     byte GAIN;
     void setGain(byte gain = 128);
+
   public:
     Q2HX711(byte output_pin, byte clock_pin);
     virtual ~Q2HX711();
+    void powerDown();
+    void powerUp();
     bool readyToSend();
     long read();
 };