Real-time clock (RTC) and crystal selection criteria and proper layout techniques for connecting a 32kHz crystal to a RTC
A real-time clock (RTC) provides a binary-coded-decimal (BCD) representation of the time and the date to the system's main processor. RTC accuracy is dominated by the characteristics of the external 32.768kHz “watch crystal.” The main processor communicates with a Maxim RTC through a serial interface connection.
The RTC operates from the main supply and often uses a backup battery to power itself and keep accurate real time while the main power is absent. Most RTCs are designed to operate from a lithium backup battery for up to 10 years. Some RTCs have their own internal power switchover from the main supply to the backup battery. Simpler RTCs rely on microprocessor supervisors to provide the power switchover. Because the RTC runs from a 32.768kHz crystal, it requires currents typically under 1µA to run the RTC while in battery backup mode.
There are three main areas that need to be taken into account in the selection and the use of an RTC for a specific application. System, hardware, and software considerations will each be analyzed here to provide a checklist for optimum RTC selection.
Read more: http://www.maxim-ic.com/appnotes.cfm/an_pk/617
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