This application report describes the design of a software I2C slave that can run up to 100-kbps using an MSP430 MCU. This software I2C uses a small amount of MCU resources so that it can be implemented on lower-end MSP430 devices. The interface to the I2C bus uses two I/O pins; and the code size, including flash memory and RAM, is small. Low power is built into the software design. The application of this software I2C is not just confined to low-end products.