Electronic – Microchip 9808 temperature sensor – Incorrect calculation result

I am using a Microchip MCP9808 I2C temperature sensor (https://learn.adafruit.com/adafruit-mcp9808-precision-i2c-temperature-sensor-guide/overview).

I'm having problems being able to calculate the temperature using the values I'm reading from the sensor.

The temperature is 13 bits wide. According to the datasheet, I need to send two read commands, the first read to return the upper byte and the second read will return the lower byte. 3 of the remaining bits are used to indicate temperature alarms, and 1 bit is used to indicate the sign of the reading (positive or negative).

I'm able to read the two bytes, by using the sample code from the datasheet as a starting point. However, the values I'm getting from it are incorrect. I'm also unable to get negative temperatures, so I think my code has something wrong with it.

I'm using a USB->I2C adaptor, and I've written a C# implementation of the datasheet sample code to calculate the temperature.

And then my code (which apart from some extra necessary castings due to the way C# handles data types) I think is identical.

    public static double CalculateAmbientTemperature(byte upperByte, byte lowerByte)
    {
        const byte ALERT_MASK_VALUE = 0x1F;
        const byte CLEAR_SIGN_MASK = 0x0F;

        double temperature = 0.00;

        upperByte = (byte)(upperByte & ALERT_MASK_VALUE);

        // Ambient temperature less than 0 degrees
        if((upperByte & (byte)(upperByte & 0x10)) == 0x10)
        {
            upperByte = (byte)(upperByte & CLEAR_SIGN_MASK);
            temperature = 256.0 - ((double)upperByte * 16.0 + (double)lowerByte / 16.0);
        }
        else
        {
            temperature = ((double)upperByte * 16.0 + (double)lowerByte / 16.0);
        }

        return temperature;
    }

So to test the code, I've been sending the bytes: START_BIT 0x3E (the address) 0x05 (the address of the temperature register) This tells the sensor to point the register pointer at the temperature register, so that when I send the two read commands, it reads from that register.

And when I read the data back, I'm getting 0xC1 0x24. When I pass these values to the function, I'm getting 17.0625 degrees C. Bearing in mind, I'm holding an ice cube on the sensor. (Currently don't have a more precise way). So basically, my calculation is faulty but I don't know where to begin with fixing it.

Any help would be appreciated.

(EDIT: To check I'm able to read the chip correctly, I read the Manufacturer ID register and get back the expected 0x54)