RTC_DS3231: handle negative temperatures

RTC_DS3231::getTemperature() assumes the temperature register stores a
positive integer whereas, according to the datasheet,[1] it is a signed
number in two’s complement format.

Fix the method by transferring this register into a signed 16-bit
integer. As the CPU uses two’s complement natively, no further
conversion is needed other than scaling by a factor (1/256.0).

Fixes adafruit#287.

[1] https://www.analog.com/media/en/technical-documentation/data-sheets/DS3231.pdf#page=15

src/RTC_DS3231.cpp

@@ -114,7 +114,8 @@ void RTC_DS3231::writeSqwPinMode(Ds3231SqwPinMode mode) {
 float RTC_DS3231::getTemperature() {
   uint8_t buffer[2] = {DS3231_TEMPERATUREREG, 0};
   i2c_dev->write_then_read(buffer, 1, buffer, 2);
-  return (float)buffer[0] + (buffer[1] >> 6) * 0.25f;
+  int16_t temp = uint16_t(buffer[0]) << 8 | buffer[1];
+  return temp * (1 / 256.0);
 }
 
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