Real Time Clocks (RTCs) provide time and/or date information in almost any electronic device which needs to keep accurate time. The clock types are binary counter, clock/calendar/supervisor, elapsed time counter, phantom time chip, portable system controller, temperature recorder, time event recorder, and timer clock peripheral.  They contain memory sizes ranging from 2 B to 2 MB and have interface options of I2C, Parallel, Serial, SPI, 1-Wire, 2-Wire, 3-Wire, and 4-Wire.