1 Introduction Bootloader is a small program put into a device that allows user application codes to be programmed to the device. USB Host bootloaders using USB mass storage device (MSD) class were built for Freescale 32-bit ColdFire and Kinetis MCU families. User application codes can be programmed to the MCUs by plugging a USB memory stick into the system. This application note uses MCF51JM128, MCF52259 and MK60N512VMD100 MCUs to demonstrate how the bootloaders work in ColdFire and Kinetis MCUs. 2 Bootloader overview Bootloader is a small application used to erase flash and load user applications to a device. The USB Host MSD bootloader provides an easy and reliable way to load user applications to devices. After a USB memory stick containing a valid s-record or binary file has been plugged into the system, the bootloader loads the user application code and programs it to the device. The new user application can then be run in the device. This application note helps the readers gain an insight into the bootloader driver and build capabilities to develop their own applications using the bootloader.