# MCU Boot Loader Troubleshooting

When MCU firmware load is interrupted, or the power-on firmware checksum test fails, the KX3 enters its bootstrap loader. You can also force the KX3 to enter its bootstrap loader by turning the KX3 on with a 10-second press and hold of the two POWER buttons (BAND- and ATU Tune). The MCU boot loader displays MCU LD on the KX3 front panel display. The display may not be backlit.

To load KX3 firmware, follow these steps in sequence:

- Turn your KX3 off by switching off or unplugging its power supply, including internal batteries. The KX3's microcontroller may continue to run at very low voltage, and it must start from a power-on reset. Disconnect any other connectors that have any possibility of providing power to the KX3. A very small leakage current from another device might be enough to keep the MCU from completely resetting. Wait 30-60 seconds before turning power back on.
- If the KX3 Utility is running, select its Port tab, then close the KX3 Utility, by clicking Exit or the red X at the top right of the main window. The Port tab provides the clearest indication of a successful connection when the KX3 Utility is subsequently started.
- If you are using a software sharing program such as N8LP's LP-Bridge or microHAM's Router between your personal computer and your KX3's serial port, change the cabling so that the PC is connected directly to the KX3 without an intervening software "forwarder". These programs send K3 and KX3 Programmer's Reference commands, and a KX3 in its bootstrap loader isn't able to process those commands. Some commands may put the KX3 bootstrap loader into an unresponsive state.
- Restart (reboot) the personal computer that will run the KX3 Utility. This isn't often necessary, but occasionally a serial port or USB device driver doesn't properly close. Rebooting reinitializes the device and its driver.
- Connect your KX3 to its power supply, then press and hold the two KX3 power buttons (BAND- and ATU TUNE) for 10 seconds, until you see MCU LD on the KX3 front panel display (without backlighting). The red Tx light should be blinking. Release the power buttons.
- Start the KX3 Utility and navigate to its Port tab. The KX3 Utility should discover the KX3 at 38,400 bit/s. If the Port tab has been selected, you should see a dialog that looks something like this:
  If you don't see this dialog, try clicking the Test Communications button. The KX3 Utility cannot load firmware if it does not receive responses. Information in USB to Serial Adapter Troubleshooting may help to resolve this issue. The problem might be serial port cabling, an incorrect COM port choice, a problem with the Personal Computer, or a damaged KX3 serial port. Incorrect COM port choice is a common problem here. Sometimes USB to Serial Adapter device drivers change the COM port number unexpectedly.
- When you see this dialog, press its OK button to continue.
- Firmware load can take quite a long time if the View Menu Trace menu item is checked. Uncheck Trace unless you require this diagnostic information.
- Select the Firmware tab, and click Send All New Firmware to KX3. If the send buttons are disabled, view Troubleshooting: Send Buttons Disabled.
- The KX3 Utility should send MCU firmware to your KX3, followed by any other "new" firmware required.