An apparatus (100, 200, 300, 400, 500, 600, 900) comprising: a power detector circuit (120, 276, 376, 430, 530, 614, 967) powered by a first voltage supply (104, 264, 364, 590, 949); and at least one voltage level-shifting device (116, 272, 372, 450, 550, 610, 965) coupled to a second voltage supply (108, 268, 368, 480, 580, 947) and configured to provide a test input (128, 612, 969) based on a second voltage of the second voltage supply (108, 268, 368, 480, 580, 947) to the power detector circuit (120, 276, 376, 430, 530, 614, 967), characterised in that the power detector circuit (120, 276, 376, 430, 530, 614, 967) is configured to: in response to a first voltage of the first voltage supply (104, 264, 364, 590, 949) not exceeding a first threshold, provide an output (124, 280) having a first logical level, and in response to the first voltage of the first voltage supply (104, 264, 364, 590, 949) exceeding the first threshold, provide the output (124, 280) having a logical level indicating whether the second voltage of the second voltage supply (108, 268, 368, 480, 580, 947) exceeds a second threshold, and wherein the first threshold of the first voltage supply (104, 264, 364, 590, 949) is less than the second threshold of the second voltage supply (108, 268, 368, 480, 580, 947). a power detector circuit (120, 276, 376, 430, 530, 614, 967) powered by a first voltage supply (104, 264, 364, 590, 949); and at least one voltage level-shifting device (116, 272, 372, 450, 550, 610, 965) coupled to a second voltage supply (108, 268, 368, 480, 580, 947) and configured to provide a test input (128, 612, 969) based on a second voltage of the second voltage supply (108, 268, 368, 480, 580, 947) to the power detector circuit (120, 276, 376, 430, 530, 614, 967), characterised in that the power detector circuit (120, 276, 376, 430, 530, 614, 967) is configured to: in response to a first voltage of the first voltage supply (104, 264, 364, 590, 949) not exceeding a first threshold, provide an output (124, 280) having a first logical level, and in response to the first voltage of the first voltage supply (104, 264, 364, 590, 949) exceeding the first threshold, provide the output (124, 280) having a logical level indicating whether the second voltage of the second voltage supply (108, 268, 368, 480, 580, 947) exceeds a second threshold, and the power detector circuit (120, 276, 376, 430, 530, 614, 967) is configured to: in response to a first voltage of the first voltage supply (104, 264, 364, 590, 949) not exceeding a first threshold, provide an output (124, 280) having a first logical level, and in response to the first voltage of the first voltage supply (104, 264, 364, 590, 949) exceeding the first threshold, provide the output (124, 280) having a logical level indicating whether the second voltage of the second voltage supply (108, 268, 368, 480, 580, 947) exceeds a second threshold, and in response to a first voltage of the first voltage supply (104, 264, 364, 590, 949) not exceeding a first threshold, provide an output (124, 280) having a first logical level, and in response to the first voltage of the first voltage supply (104, 264, 364, 590, 949) exceeding the first threshold, provide the output (124, 280) having a logical level indicating whether the second voltage of the second voltage supply (108, 268, 368, 480, 580, 947) exceeds a second threshold, and wherein the first threshold of the first voltage supply (104, 264, 364, 590, 949) is less than the second threshold of the second voltage supply (108, 268, 368, 480, 580, 947).