Given that software-only extensibility is not possible on low-tier mobile phones, we are exploring a combined hardware-software approach to achieve this. In this report we present the architecture and design of a battery-powered, microcontroller-based programmable accessory for cell phones. The idea is to use the accessory for general purpose computation and leverage the cell phone for IO and communications. The accessory connects to a cell phone over the phone’s data port and communicates using a serial protocol.