A hard disk drive enters a low power mode to reduce power consumption. To maintain communication with a host device, a communication interface remains energized along with a circuit portion storing configuration data for the communication interface. To energize the communication interface and the circuit portion, low power voltage regulators provide suitable reference voltages. One low power voltage regulator is dedicated to this purpose. Another voltage regulator is converted from an active, switching mode to a low power, linear mode to provide the necessary reference voltage. Also, unique handshaking signals are used to control entry and exit from the low power mode by the hard disk drive.