App note(PDF!) on Microchips’ USB hubs suspend indicator feature.
The USB251xB / USB3x03 / USB253x / USB3x13 / USB46x6 product lines are a family of feature-rich, low power USB 2.0 hubs optimized for use in embedded systems where high performance and advanced functionality are critical design requirements. The USB251xB / USB3x03 / USB253x / USB3x13 / USB46x6 product lines feature a suspend indicator which can be used to signal to an embedded controller the suspend status of the hub. This feature is useful for signaling when a system should enter or exit a low power state.