A switchable supply network for powering multiple digital islands. In one embodiment, a first digital island includes a first power collapsible circuit and a first retention circuit, and a second digital island includes a second power collapsible circuit and a second retention circuit. In a normal mode of operation, the first digital island is provided a first supply voltage and a second digital island is provided a second supply voltage higher than the first supply voltage. In a transition mode the second power collapsible circuit is powered down and the second supply voltage is lowered and provided to the second retention circuit. When the second supply voltage falls below the first supply voltage, the first power collapsible circuit is powered down. The second supply voltage is now provided only to the retention circuits, and is furthered lowered in a retention mode to a final retention voltage.