How to clear most significant bit in byte?

Robby Smet Published at Dev

Robby Smet

I want to parse a temperature from bytes.

The temperature consists of 2 bytes. The most significant bit of the first byte indicates if the temperature is positive or negative.

This is what I have so far:

public double parseTemperatureBytes(byte[] temperatureBytes) {
    // Must be divided by 10 (1 decimal)
    // First bit indicates sign (bit 1 = negative, bit 0 = positive)
    // Range [0x7FFF] : [-3276.7 … +3276.7]

    byte firstByte = temperatureBytes[0];
    int positiveOrNegative = ParseUtils.getMostSignificantBit(firstByte);
    boolean isPositive = positiveOrNegative == 0;

    String temperatureHex = ParseUtils.bytesToHex(temperatureBytes);
    int temperatureHexToInteger = Integer.parseInt(temperatureHex, 16);
    double temperature = temperatureHexToInteger / (double) 10;

    if (!isPositive) {
        temperature = -temperature;
    }

    return temperature;
}


// ParseUtils

public static int getMostSignificantBit(byte b) {
    return (b & 0xff) >> 7;
}

This works, but I still have to make sure I ignore the most significant bit of the first byte. It's just a flag and not part of the temperature.

Ex: If I pass in 0xFFFF it returns -6553.5 but it should be -3276.7

How can I achieve this?

Ramesh-X

You can check whether the given temperature is minus, using the following code..

public static int getMostSignificantBit(byte b) {
    return b & 0x80;
}

You can use the following code to get your temperature value..

public static double parseTemperatureBytes(byte[] temperatureBytes) {
    int firstByte = temperatureBytes[0] & 0x7F;
    int secondByte = temperatureBytes[1] & 0xFF;
    double temperature = ((firstByte << 8) | secondByte) / 10.0;

    if (getMostSignificantBit(temperatureBytes[0]) > 0) {
        temperature = -temperature;
    }

    return temperature;
}

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