The purpose of this paper is to design and simulate a Protocol Conversion Unit (PCU) for seamless communication between the two widely accepted serial communication protocols SPI and I2C. Design given in this paper takes data from a sender device working on SPI protocol and sends it to a receiver device working on I2C protocol, which otherwise without such design would not be possible. SPI supports full duplex communication unlike I2C which is half duplex. Also SPI is faster than I2C. I2C on the other hand is just a two wire interface as unlike SPI it does not use a dedicated Slave Select line. Instead I2C relies on Address and Acknowledgement scheme to communicate with slave. Thus in areas where the controlling device needs to communicate with a lot of peripheral devices, it is essential for the controller to send commands and data to the concerned peripheral device quickly using high speed of SPI and at the same time save on dedicated pins for each peripheral device using a rather simple Two Wire Interface of I2C, a design capable of providing conversion between SPI and I2C formats becomes essential. In this paper support for just one peripheral device is given. The design in this paper can be upgraded to support large no of peripherals by providing a First In First Out (FIFO) Queue for storing commands and data along with corresponding addresses of peripheral devices in the Protocol Conversion Unit (PCU).