Enhanced Host Controller Interface
The Enhanced Host Controller Interface (EHCI) is the single method of interfacing with USB 2.0. This makes life a lot easier than with USB 1.0, where two incompatible standards (OHCI and UHCI) both survived. This is because the USB-IF insisted on a single implementation, produced by Intel.
Interface
On a PC you will normally find the EHCI USB controller on the PCI bus - in fact it is the only access method specified in the specification. USB 2.0 supports interfacing with USB 1.0 devices. However, EHCI is NOT expected to support them. Instead, you will find an UHCI or OHCI companion controller. Therefore you must implement OHCI and UHCI as well. Note there are NOT companions of both types, it is one type or another.
The EHCI controller will always have a PCI Class ID of 0x0C, a Subclass ID of 0x03, and an Interface value of 0x20. These values can be used to find the configuration space of the controller, which contains the memory mapped address of the EHCI registers. The address information can be found in the BAR0 PCI configuration space register.
Capability Registers
The capability registers can be found in memory at the address specified in the PCI configuration space BAR0 register.
| Offset (Hex) | Name | Description |
|---|---|---|
| 00 | CAPLENGTH | Capability Register Length |
| 01 | Reserved | |
| 02 | HCIVERSION | Interface Version Number (BCD) |
| 04 | HCSPARAMS | Structural Parameters |
| 08 | HCCPARAMS | Capability Parameters |
| 0C | HCSP-PORTROUTE | Companion Port Route Description |
Operation Registers
The operation registers can be found after the capability register area in memory. Add the Capability Register Length value above to the BAR0 address to find the operation register base address.
| Offset (Hex) | Name | Description |
|---|---|---|
| 00 | USBCMD | USB Command |
| 04 | USBSTS | USB Status |
| 08 | USBINTR | USB Interrupt Enable |
| 0C | FRINDEX | USB Frame Index |
| 10 | CTRLDSSEGMENT | 4G Segment Selector |
| 14 | PERIODICLISTBASE | Frame List Base Address |
| 18 | ASYNCLISTADDR | Next Asynchronous List Address |
| 40 | CONFIGFLAG | Configured Flag Register |
| 44 | PORTSC[1-N_PORTS] | Port Status/Control Register |
USB Command Register
| Bits | Name | Description |
|---|---|---|
| 31-24 | Reserved | |
| 23-16 | Interrupt Threshold | Number of micro frames to process between interrupts |
| 15-12 | Reserved | |
| 11 | Async Schedule Park Mode Enable | |
| 10 | Reserved | |
| 9-8 | Async Schedule Park Mode Count | |
| 7 | Light Host Controller Reset | Resets the controller without affecting connected devices |
| 6 | Interrupt On Async Advance Doorbell | Allows software to trigger interrupt |
| 5 | Async Schedule Enable | |
| 4 | Periodic Schedule Enable | |
| 3-2 | Programmable Frame List Size | |
| 1 | Host Controller Reset | |
| 0 | Run |
USB Status Register
| Bits | Name | Description |
|---|---|---|
| 31-16 | Reserved | |
| 15 | Async Schedule Status | |
| 14 | Periodic Schedule Status | |
| 13 | Reclamation | |
| 12 | Halted | |
| 11-6 | Reserved | |
| 5 | Doorbell Interrupt | |
| 4 | Host System Error | |
| 3 | Frame List Rollover | |
| 2 | Port Change Detect | |
| 1 | USB Error Interrupt | |
| 0 | USB Transfer Interrupt |
USB Interrupt Enable Register
| Bits | Name | Description |
|---|---|---|
| 31-6 | Reserved | |
| 5 | Async Advance Interrupt Enable | |
| 4 | Host System Error Interrupt Enable | |
| 3 | Frame List Rollover Interrupt Enable | |
| 2 | Port Change Interrupt Enable | |
| 1 | USB Error Interrupt Enable | |
| 0 | USB Transfer Interrupt Enable |
Port Status/Control Register
| Bits | Name | Description |
|---|---|---|
| 31-23 | Reserved | |
| 22 | Wake On Overcurrent Enable | |
| 21 | Wake On Disconnect Enable | |
| 20 | Wake On Connect Enable | |
| 19-16 | Port Test Control | |
| 15-14 | Port Indicator Control | 0 = Off, 1 = Amber, 2 = Green |
| 13 | Companion Port Control | 0 = Local, 1 = Companion Host Controller |
| 12 | Port Power | |
| 11-10 | Line Status | |
| 9 | Reserved | |
| 8 | Port Reset | |
| 7 | Suspend | |
| 6 | Force Port Resume | |
| 5 | Overcurrent Change | |
| 4 | Overcurrent | |
| 3 | Port Enabled Changed | |
| 2 | Port Enabled | |
| 1 | Connect Changed | |
| 0 | Connected |
Asynchronous Queue List
The asynchronous queue list is a ring of Queue Head records that point to the next Queue Head in the ring, and the first Transmit Descriptor in the queue. As long as the Asynchronous Schedule flag is set in the Command register, the EHCI controller will start with the first Queue Head located at the address in the Next Asynchronous List Address register, and process all Queue Heads until it encounters a Queue Head with the Reclaim Head flag set. The reclaim head flag is not checked on the very first Queue Head record, so you can set it's reclaim head flag, and the controller will process all Queue Heads until it reaches the first Queue Head again.
The Queue Head contains a copy of the current TD that the controller uses to process each Transfer Descriptor. This area should initially contain the first TD of the queue in the Next TD field of the TD working area.
Queue Head
| Offset (Hex) | Name | Description |
|---|---|---|
| 00 | Horizontal Link Pointer | |
| 04 | Endpoint Characteristics | |
| 08 | Endpoint Capabilities | |
| 0C | Current TD Address | |
| 10 | Current TD Working Area | This area contains as copy of the current TD data. |
Horizontal Link Pointer
| Bits | Name | Description |
|---|---|---|
| 31-5 | Next Queue Head | Address of the next Queue Head in the ring |
| 4-3 | Reserved | |
| 2-1 | Next Queue Type | 0 = Isochronous TD, 1 = Queue Head, 2 = Split Transaction Isochronous TD, 3 = Frame Span Traversal Node |
| 0 | Terminate | Set if this is the last Queue Head in a Periodic List. Not used for Asynchronous List. |
Endpoint Characteristics
| Bits | Name | Description |
|---|---|---|
| 31-28 | NAK Reload | |
| 27 | Control Endpoint | Not used for High Speed devices |
| 26-16 | Maximum Packet Length | |
| 15 | Head of Reclamation List | Set if this is the first Queue Head in an Asynchronous List |
| 14 | Data Toggle Control | Set if data toggle should use value from TD |
| 13-12 | Endpoint Speed | 0 = Full Speed, 1 = Low Speed, 2 = High Speed |
| 11-8 | Endpoint Number | |
| 7 | Inactivate | Only used in Periodic List |
| 6-0 | Device Address |
Endpoint Capabilities
| Bits | Name | Description |
|---|---|---|
| 31-30 | High Bandwidth Pipe Multiplier | Must be greater than zero |
| 29-23 | Port Number | Used for split transactions |
| 22-16 | Hub Address | Used for split transactions |
| 15-8 | Split Completion Mask | Used for split transactions |
| 7-0 | Interrupt Schedule Mask | Used for split transactions |