4 RTC chips — pinouts, specs, and datasheets.
DS1307
DS1307 I2C real-time clock (8-pin DIP) with battery backup and 1Hz/SQW output.
DS3231
DS3231 temperature-compensated I2C RTC (SO-16), high accuracy.
MCP79410
MCP79410 I2C RTC/calendar (8-pin DIP) with SRAM/EEPROM and battery switchover.
PCF8563
PCF8563 low-power I2C RTC/calendar (SO-8) with alarm and CLKOUT.