Abstract:

Motorola's HC08 and HC12 MCUs are widely used in ac line powered applications. In some cases, the MCU uses a non-isolated power supply. Performing in-circuit debugging on ac line powered systems can be tricky and sometimes dangerous due to differing ground connections, floating power supplies, etc. For example, Figure 1 . Microcontroller Circuit with Low Cost Power Supply shows an MCU controlling an ac motor. The MCU is powered from a low cost charge pump power supply that is connected directly to the ac line voltage. In relation to earth, the MCU Vdd and Vss pins can reach hundreds of volts. This type of power supply is common in white and brown goods applications, examples of which include vacuum cleaners, fridges, washing machines, light dimmers, etc. If a debugging tool was connected directly to the MCU in this circuit, two possibilities could occur depending on the construction of the tool's power supply.