When the controller starup sequence encounters some error condition, it transitions to bootloader mode and sets GPIO_23 (connected to an LED) HIGH to indicate a low-level fault condition. See the debug message printed in the UART debug terminal or via the Get Boot Hold Reason command to get more details about the cause of error. When the ARM processor detects one of the data abort, pre-fetch abort or undefined instruction exceptions, it starts blinking GPIO_23 in specific hex code format. During this error condition, communication with the controller is not possible. The only way to communicate with the controller is to analyze the hex error code from the pattern and then debug the hardware to fix the issue.