DS1302 containing a real-time clock / calendar and 31 bytes of static RAM, via a simple serial interface to communicate with the microcontroller.
Real-time clock / calendar circuit provides seconds, minutes, hours, days, weeks, months, years of information, the number of days per month and leap year automatically adjust the number of days. Clock can be done by the AM / PM or 12/24 hour format.

To communicate with DS1302 and the microcontroller can simply use the synchronous serial communication mode, used only three port lines: (1) RST Reset (2) I / O data lines (3) SCLK serial clock. Clock / RAM read / write data to a byte or up to 31 bytes of character set.

DS1302 low-power work to maintain the data and clock information when power is less than 1mW
31 8-bit  temporary data storage RAM
Serial I / O port pin makes the least number of ways
Wide range of operating voltage 2.0 5.5V
Current less than 300nA @ 2V
Read / write clock or RAM data transmission when there are two single-byte transmit and multi-byte character sets transfer mode
simple 3-wire interface
Vcc = 5V and TTL-compatible
Crystal 32.768KHz, matching capacitance 6pF
VCC → +5 V/3.3V
GND → GND