The internal AREF voltage generators have short circuit protection. If the STK600 measures the AREF0/AREF1 to be 0.3V or more below the set-point, AREF will be shut off. When this happens, the status LED will blink red. The AREF0 and AREF1 will also be shut down by the Master MCU if a short circuit is detected on VTarget (in addition to shutting down VTarget). In this case, the status LED will blink red.