Negative Temperatures reported incorrectly by getTemperature() in RTC_DS3231.cpp

[girishnand]

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;
}

From: https://datasheets.maximintegrated.com/en/ds/DS3231.pdf -

Temperature Registers (11h–12h)
Temperature is represented as a 10-bit code with a resolution of 0.25°C and is accessible at location 11h and 12h. The
temperature is encoded in two’s complement format. The upper 8 bits, the integer portion, are at location 11h and the
lower 2 bits, the fractional portion, are in the upper nibble at location 12h.

Although variable buffer[0] holds a signed value in twos complement format, it has been declared as uint8_t. Casting the value in buffer[0] to float does not handle negative values correctly, and we find in practice that for example a temperature of -1 deg C which is read from the DS3231's registers as {255,0}, is returned by this function as 255 deg C, and not as -1 deg C.

Changing the last line of the function to the following corrects this.

return (float) (*(int8_t *)buffer) + (buffer[1] >> 6) * 0.25f;

regards,