The DS1904 RTC iButton is a module that functions as a reliable real-time clock and is easy to configure as it is accessible with very minimal hardware.

The DS1307 serial real-time clock (RTC) is a lowpower, full binary-coded decimal (BCD) clock/calendar plus 56 bytes of NV SRAM.

The DS1501/DS1511 RTC is a low-power clock/date device with a programmable day of week/date alarm. The DS1501/DS1511 is accessed through a parallel interface. The clock/date provides seconds, minutes, hours, day, date, month, and year information. The date at the end of the month is automatically adjusted for months with fewer than 31 days, including corrections for leap year.