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ds3231: Document incorrect leap year 2100

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The current code interprets the 'century' flag as the year 2100. However the
DS3231 hardware does not incorporate this flag in its leap year calculation, so
will incorrectly consider the year 2100 as a leap year and increment from
2100-02-28 to 2100-02-29 instead of the correct 2100-03-01.

The 'century' bit is not useful for anything as far as I can tell. But instead
of removing the code that uses the 'century' bit, I thought it would be less
intrusive to just document the current behavior.
This commit is contained in:
Brian Park
2023-03-03 07:54:35 -08:00
committed by Ron Evans
parent 94017b5437
commit b0a971573b
1 changed files with 15 additions and 1 deletions
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ds3231/ds3231.go
+15 -1
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@@ -73,7 +73,17 @@ func (d *Device) SetRunning(isRunning bool) error {
return nil
}
// SetTime sets the date and time in the DS3231
// SetTime sets the date and time in the DS3231. The DS3231 hardware supports
// only a 2-digit year field, so the current year will be stored as an offset
// from the year 2000, which supports the year 2000 until 2100.
//
// The DS3231 also supports a one-bit 'century' flag which is set by the chip
// when the year field rolls over from 99 to 00. The current code interprets
// this flag to be the year 2100, which appears to extend the range of years
// until the year 2200. However the DS3231 does not incorporate the 'century'
// flag in its leap year calculation, so it will incorrectly identify the year
// 2100 as a leap year, causing it to increment from 2100-02-28 to 2100-02-29
// instead of 2100-03-01.
func (d *Device) SetTime(dt time.Time) error {
data := []byte{0}
err := d.bus.ReadRegister(uint8(d.Address), REG_STATUS, data)
@@ -92,6 +102,10 @@ func (d *Device) SetTime(dt time.Time) error {
data[2] = uint8ToBCD(uint8(dt.Hour()))
year := uint8(dt.Year() - 2000)
// This code interprets the centuryFlag to be the year 2100. Warning: The
// DS3231 does not incorporate the centuryFlag in its leap year calculation.
// It will increment from 2100-02-28 to 2100-02-29, which is incorrect because
// the year 2100 is not a leap year in the Gregorian calendar.
centuryFlag := uint8(0)
if year >= 100 {
year -= 100