HX711.js

function HX711(options) {
  options = options||{};
  this.sck = options.sck;
  if (!this.sck) throw "Expecting sck";
  this.miso = options.miso;
  if (!this.miso) throw "Expecting miso";
  this.mode = options.mode||"A128";
  this.lsbGrams = options.lsbGrams||0.00103123388;
  /* We have to send different numbers of clock edges
    depending on the mode, so we actually use 'mosi'
    as the clock and send the data we want. */
  this.spi = new SPI(); // use software SPI
  this.spi.setup({miso:this.miso, mosi:this.sck, baud: 2000000/*ignored atm*/});
  this.sck.write(0); // idle, but on
  this.zero = 0;
  if (options.median)
    this.median = new Int32Array(options.median);
  // throw away first reading
  this.readRaw();
  if (options.median)
    this.median.fill(this.readRaw());
}

HX711.prototype.readRaw = function() {
  var finalClk = {
    A128:0b10000000, // 25
    B32:0b10100000, // 26
    A64:0b10101000, // 27
  }[this.mode];
  if (!finalClk) throw "Invalid mode";
  var c = 0b10101010;
  var d = this.spi.send(new Uint8Array([c, c, c, c, c, c, finalClk]));
  // mosi left as 0, so power on.
  function ex(x) { return ((x&128)?8:0)|((x&32)?4:0)|((x&8)?2:0)|((x&2)?1:0); }
  var val = (ex(d[0])<<20)|(ex(d[1])<<16)|(ex(d[2])<<12)|(ex(d[3])<<8)|(ex(d[4])<<4)|ex(d[5]);
  if (val&0x800000) val-=0x1000000; // two's complement
  // If we're using a median filter, do filtering here
  if (this.median) {
    this.median.set(new Int32Array(this.median.buffer,4), 0);
    this.median[this.median.length-1] = val;
    var m = new Int32Array(this.median);
    m.sort();
    var quarter = m.length>>2;
    var mid = new Int32Array(m.buffer, quarter*4, m.length>>2);
    val = E.sum(mid) / mid.length;
  }
  return val;
};
/// Set the current reading to be the zero
HX711.prototype.tare = function() {
  this.zero = this.readRaw();
};
/// Given a known weight on the scale (after tare) calculate a new 'lsbGrams'
HX711.prototype.calculateScale = function(weightInGrams) {
  return (this.lsbGrams = weightInGrams / (this.readRaw() - this.zero));
};
/// Read the ADC and return the result in grams (based on options.lsbGrams)
HX711.prototype.readGrams = function() {
  return (this.readRaw() - this.zero) * this.lsbGrams;
};
/// Is data ready for retrieval?
HX711.prototype.isReady = function() {
  return !this.miso.read();
};
/// Set whether the ADC is in standby mode or not
HX711.prototype.setStandby = function(isStandby) {
  this.sck.write(isStandby);
};
/// Work out the amount of grams the values in the median filter vary by
HX711.prototype.getVariance = function() {
  if (!scale.median) throw new Error("No Median Filter");
  var avr = E.sum(scale.median) / scale.median.length;
  return Math.sqrt(E.variance(scale.median,avr)) * 2 * this.lsbGrams / scale.median.length;
};

exports.connect = function(options) {
  return new HX711(options);
};