TheThingsNetwork Feather LearnGuide

TheThingsNetwork_Feather/ttn_decoder.js

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+function Decoder(bytes, port) {
+  // Decode an uplink message from a buffer
+  // (array) of bytes to an object of fields.
+  var decoded = {};
+  
+  // temperature 
+
+  rawTemp = bytes[0] + bytes[1] * 256;
+  
+  decoded.degreesC = sflt162f(rawTemp) * 100;
+  
+  // humidity 
+  rawHumid = bytes[2] + bytes[3] * 256;
+  decoded.humidity = sflt162f(rawHumid) * 100;
+  
+  return decoded;
+}
+
+function sflt162f(rawSflt16)
+	{
+	// rawSflt16 is the 2-byte number decoded from wherever;
+	// it's in range 0..0xFFFF
+	// bit 15 is the sign bit
+	// bits 14..11 are the exponent
+	// bits 10..0 are the the mantissa. Unlike IEEE format, 
+	// 	the msb is transmitted; this means that numbers
+	//	might not be normalized, but makes coding for
+	//	underflow easier.
+	// As with IEEE format, negative zero is possible, so
+	// we special-case that in hopes that JavaScript will
+	// also cooperate.
+	//
+	// The result is a number in the open interval (-1.0, 1.0);
+	// 
+	
+	// throw away high bits for repeatability.
+	rawSflt16 &= 0xFFFF;
+
+	// special case minus zero:
+	if (rawSflt16 == 0x8000)
+		return -0.0;
+
+	// extract the sign.
+	var sSign = ((rawSflt16 & 0x8000) != 0) ? -1 : 1;
+	
+	// extract the exponent
+	var exp1 = (rawSflt16 >> 11) & 0xF;
+
+	// extract the "mantissa" (the fractional part)
+	var mant1 = (rawSflt16 & 0x7FF) / 2048.0;
+
+	// convert back to a floating point number. We hope 
+	// that Math.pow(2, k) is handled efficiently by
+	// the JS interpreter! If this is time critical code,
+	// you can replace by a suitable shift and divide.
+	var f_unscaled = sSign * mant1 * Math.pow(2, exp1 - 15);
+
+	return f_unscaled;
+	}