Using the AN0042 bootloader on EFM32 STK's can be a bit tricky. Here are the complete steps required to do so:
Open the Studio Launcher perspective with the kit connected. Select [Change] for the Debug Mode:
Under [Adapter Configuration], Choose Debug Mode: [MCU]:
Under the [Documentation] tab, scroll through the [Application Notes] list until you reach AN0042. Select this, and a new window will open with the application note selected. Hit [Open Folder] here.
This folder contains images for the bootloader under the ‘binaries’ folder. This is where you will find the pre-compiled hex image for the bootloaders. For the giant gecko, the file you want is bl-usartusb-gian-v2.0.4.hex. Make a note of this folder:
Back in Studio, open the tools menu:
Open the Flash Programmer:
Browse to the location of the hex image listed before. Hit Program.
If programming fails, hit [Unlock Debug Access] and try again.
Back in Studio, change the debug mode to [IN].
On the STK, connect SWCLK (Pin 9, debug header) to VMCU (pin 2, expansion header).
Connect the mini USB cable at the bottom of the board to your computer. Press the Reset button.
Device appears in the Device Manager under Ports (COM & LPT):
If the device does NOT appear, please install the driver located in the Studio Install directory under: developer\sdks\gecko_sdk_suite\v1.1\app\mcu_example\EFM32GG_STK3700\usbdcomposite\Silabs-CDC_Install.zip