I looked on silicon labs website for info. https://www.silab...1F34x.aspx I found a possible debugger https://www.silab...ter_UG.pdf What is odd is that they show their app note AN124 as being part of this device, but if you look carefully at the beginning the relevant devices do not include this particular chip. So, if you used this information they say to use some resistors after the 10 pin connector, but I do not think that applies to your particular chip. They do mention that you must have an external pull-up resistor for debugging and I saw that you indeed put an external resistor. I do not know for sure.... any thoughts guys? Another aspect is mentioned that the debugger can also power the device, but you mention using a linear power supply. Are you mixing the linear and the USB voltage? If so, maybe a diode is needed to make sure the voltage is not back-fed into the debugger as USB is usually slightly lower than a true 5V. In their debugger schematic there doesn't seem to be this type of protection. Maybe on the target board that they supply has this? They do mention that their target board can be configured to be USB powered or AC powered.