C# signed fixed point to floating point conversion

Question

I have a temperature sensor returning 2 bytes. The temperature is defined as follows :

What is the best way in C# to convert these 2 byte to a float ?

My sollution is the following, but I don't like the power of 2 and the for loop :

static void Main(string[] args) { byte[] sensorData = new byte[] { 0b11000010, 0b10000001 }; //(-1) * (2^(6) + 2^(1) + 2^(-1) + 2^(-8)) = -66.50390625 Console.WriteLine(ByteArrayToTemp(sensorData)); } static double ByteArrayToTemp(byte[] data) { // Convert byte array to short to be able to shift it if (BitConverter.IsLittleEndian) Array.Reverse(data); Int16 dataInt16 = BitConverter.ToInt16(data, 0); double temp = 0; for (int i = 0; i < 15; i++) { //We take the LSB of the data and multiply it by the corresponding second power (from -8 to 6) //Then we shift the data for the next loop temp += (dataInt16 & 0x01) * Math.Pow(2, -8 + i); dataInt16 >>= 1; } if ((dataInt16 & 0x01) == 1) temp *= -1; //Sign bit return temp; }

Populate a const or static double[] with the values from 2^-8 through 2^6 once, then do (dataInt16 & 0x01) * bitTemps[i] instead of re-calculating the power every time — Mathias R. Jessen
– Mathias R. Jessen, Commented Sep 24, 2021 at 10:56
You could also go from MSB to LSB, starting with temp = 64 and then divide by two for each step. — 500 - Internal Server Error
– 500 - Internal Server Error, Commented Sep 24, 2021 at 10:58

Matthew Watson · Accepted Answer · 2021-09-24 11:23:40Z

This might be slightly more efficient, but I can't see it making much difference:

static double ByteArrayToTemp(byte[] data) { if (BitConverter.IsLittleEndian) Array.Reverse(data); ushort bits = BitConverter.ToUInt16(data, 0); double scale = 1 << 6; double result = 0; for (int i = 0, bit = 1 << 14; i < 15; ++i, bit >>= 1, scale /= 2) { if ((bits & bit) != 0) result += scale; } if ((bits & 0x8000) != 0) result = -result; return result; }

You're not going to be able to avoid a loop when calculating this.

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