bug: sensor (DS18S20) is considered disconnected when reporting the lowest temperature (-55 deg C)

[bonnyr]

When a sensor is reporting -55°C, the library treats the sensor as disconnected and reports temperature readings as -127°C.

The issue appears to lie with the library since it defines the following (in DallasTemperature.h): #define DEVICE_DISCONNECTED_RAW -7040

And has the following to say about that value in the implementation (DallasTemperature.cpp):

// returns temperature in 1/128 degrees C or DEVICE_DISCONNECTED_RAW if the
// device's scratch pad cannot be read successfully.
// the numeric value of DEVICE_DISCONNECTED_RAW is defined in
// DallasTemperature.h. It is a large negative number outside the
// operating range of the device
int32_t DallasTemperature::getTemp(const uint8_t* deviceAddress) {

	ScratchPad scratchPad;
	if (isConnected(deviceAddress, scratchPad))
		return calculateTemperature(deviceAddress, scratchPad);
	return DEVICE_DISCONNECTED_RAW;

However, -55 deg C is exactly -7040 (if you multiply it by 128 which the code does all over)

Thus, when you request a reading the next time, the value matches a DEVICE_DISCONNECTED_RAW sensor when in reality it is not.

This was tested with the latest version in master.