ds3231: Fix negative temperature conversion

I also verified my interpretation of the DS3231 datasheet and my implementation
of this formula by placing a DS3231 chip in the freezer section of my
refrigerator for an hour or two, then reading the temperature. I got -16C,
which is close enough to the -18C that the freezer was set to.

ds3231/ds3231_test.go

@@ -0,0 +1,76 @@
+package ds3231
+
+import (
+	"testing"
+)
+
+func TestPositiveMilliCelsius(t *testing.T) {
+	t1000 := milliCelsius(0, 0)
+	if t1000 != 0 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0, 0b01000000)
+	if t1000 != 250 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0, 0b10000000)
+	if t1000 != 500 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0, 0b11000000)
+	if t1000 != 750 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(1, 0b00000000)
+	if t1000 != 1000 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(2, 0b00000000)
+	if t1000 != 2000 {
+		t.Fatal(t1000)
+	}
+
+	// highest temperature is 127.750C
+	t1000 = milliCelsius(0x7f, 0b11000000)
+	if t1000 != 127750 {
+		t.Fatal(t1000)
+	}
+}
+
+func TestNegativeMilliCelsius(t *testing.T) {
+	t1000 := milliCelsius(0xff, 0b11000000)
+	if t1000 != -250 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0xff, 0b10000000)
+	if t1000 != -500 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0xff, 0b01000000)
+	if t1000 != -750 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0xff, 0b00000000)
+	if t1000 != -1000 {
+		t.Fatal(t1000)
+	}
+
+	t1000 = milliCelsius(0xfe, 0b00000000)
+	if t1000 != -2000 {
+		t.Fatal(t1000)
+	}
+
+	// lowest temperature is -128.000C
+	t1000 = milliCelsius(0x80, 0b00000000)
+	if t1000 != -128000 {
+		t.Fatal(t1000)
+	}
+}