Called Shim RTC, the clock board is also compatible with A, B and B+ models, and is designed to be added to Raspberry Pi without obstructing other expansion boards.“It will fit under most add-on boards. The Shim adapter is a Raspberry Pi GPIO extender, making it easy to connect additional add-on accessories, work with breadboards, attach wires or access to pin testing all at the same time,” claimed Element 14. “With one set of holes placed over the Raspberry Pi headers the other set of holes on the Shim adapter can accommodate header pins and wires to support the user’s application while not covering up the header pins for additional add on boards.”Timekeeping comes from a Microchip MCP7940N, backed by an on-board CR1220 (20×1.2mm) lithium coin cell, and connected via I2C.
Existing boards up-dated to work with Model B+ as well as B and A are: PiFace Control & Display and PiFace Digital, now both at version 2.
For more detail : RTC anchors Raspberry Pi in the now, and I/O boards for Model B plus
