An apparatus comprising: a first circuit configured and arranged to operate under a first power domain (120, 420, 520) having a first voltage range; a second circuit configured and arranged to operate under a second power domain (130, 430, 530) having a second voltage range; and an interface circuit (110, 410, 510) connected to the first circuit and to the second circuit, the interface circuit being configured and arranged to: in a first mode, convert a voltage level of communications passed between the first circuit and the second circuit, and in a second mode, pass communications directly between the first circuit and the second circuit at a common voltage level, wherein the first circuit operates between a first high power rail and a first low power rail that operates at a voltage level that is lower than a voltage level of the first high power rail, the second circuit operates between a second high power rail and a second low power rail that operates at a voltage level that is lower than a voltage level of the second high power rail, characterised in that the interface circuit is configured and arranged to: in the first mode, couple the first low power rail to the second high power rail, thereby recycling power from the first circuit into the second circuit, and in the second mode, couple the first high power rail to the second high power rail, and couple the first low power rail to the second low power rail, thereby operating the first circuit and the second circuit over a common voltage range. a first circuit configured and arranged to operate under a first power domain (120, 420, 520) having a first voltage range; a second circuit configured and arranged to operate under a second power domain (130, 430, 530) having a second voltage range; and an interface circuit (110, 410, 510) connected to the first circuit and to the second circuit, the interface circuit being configured and arranged to: in a first mode, convert a voltage level of communications passed between the first circuit and the second circuit, and in a second mode, pass communications directly between the first circuit and the second circuit at a common voltage level, in a first mode, convert a voltage level of communications passed between the first circuit and the second circuit, and in a second mode, pass communications directly between the first circuit and the second circuit at a common voltage level, wherein the first circuit operates between a first high power rail and a first low power rail that operates at a voltage level that is lower than a voltage level of the first high power rail, the second circuit operates between a second high power rail and a second low power rail that operates at a voltage level that is lower than a voltage level of the second high power rail, characterised in that the interface circuit is configured and arranged to: in the first mode, couple the first low power rail to the second high power rail, thereby recycling power from the first circuit into the second circuit, and in the second mode, couple the first high power rail to the second high power rail, and couple the first low power rail to the second low power rail, thereby operating the first circuit and the second circuit over a common voltage range. in the first mode, couple the first low power rail to the second high power rail, thereby recycling power from the first circuit into the second circuit, and in the second mode, couple the first high power rail to the second high power rail, and couple the first low power rail to the second low power rail, thereby operating the first circuit and the second circuit over a common voltage range.