
CHAPTER 14 USBU (UNIVERSAL SERIAL BUS UNIT)
User’s Manual U14579EJ2V0UM
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14.4.14 HC state transitions
The HC has four states. These states are UsbOperational, UsbReset, UsbSuspend, and UsbResume. The current
state is indicated in the HCFS area of the HcControl register. The HCD can execute the transitions between the USB
states shown in Figure 14-13. The HC can only execute the state transition from UsbSuspend to UsbResume during a
remote wakeup event.
Figure 14-13. HC State Transitions
Usb
Operational
Usb
Reset
Usb
Resume
Usb
Suspend
UsbOperational Write
UsbSuspend Write
UsbOperational Write
UsbReset Write
UsbResume Write
or
Remote Wakeup
UsbReset Write
Hardware Reset
In the UsbOperational state, the HC processes a list and issues an SOF token. At the same time that the HC
transitions to the UsbOperational state, the value of the FI area of the HcFmInterval register is loaded in the FR area
of the HcFmRemaining register.
The first SOF token that is sent after the HC enters the UsbOperational state is sent at the frame boundary for
which the FR area changed from 0 to the value of the FI area.
In the UsbReset state, the HC forcibly sends a reset signal to the bus. After a hardware reset, the HC is always in
the UsbReset state.
The UsbSuspend state indicates a state in which the USB is temporarily stopped. At this time, the HC monitors
USB wakeup actions. The HC is forced to transition to the UsbResume state by a remote wakeup condition. This
transition may conflict with a transition to the UsbReset state performed by the HCD. If this conflict occurs, the
transition to the UsbReset state that was performed by the HCD has priority. The HC cannot transition to the
UsbResume state for 5 ms after a transition to the UsbSuspend State.
In the UsbResume state, the HC forcibly sends a resume signal to the bus. During the UsbResume state, the root
hub always transmits USB resume signals to the downstream ports. A transition to the UsbResume state is started by
a remote wakeup signal from the HCD or root hub.