usb_embedded

An Ada USB stack for embedded devices

This project and its design are inspired by the USB device stack of the libopencm3 project and TinyUSB.

How to use the USB Embedded Device Stack:

What you need:

• A USB Device Controller (a.k.a UDC), this is the low level driver for the USB controller of your microcontroller. It should be provided by the HAL (Hardware Abstraction Layer) or BSP (Board Support Package) for your microcontroller.

  If you are developing a new HAL or BSP for a microcontroller, see the Porting USB Embedded section for writing a UDC implementation.

• One or more Device Classes. These are the features that your USB device will provide (mouse, keyboard, serial port, MIDI, etc.). This crate provides a few of the most common classes that can be used out-of-the box, but you can also implement your own class (see doc).

• A USB Embedded Device Stack, this is what implements the USB control protocol and links the UDC and classes together.

For example with an UDC driver for samd51 and two classes:

   UDC : aliased SAM.USB.UDC
     (Periph          => SAM_SVD.USB.USB_Periph'Access,
      EP_Buffers_Size => 256,
      Max_Packet_Size => 64);

   Class_Serial : aliased USB.Device.Serial.Default_Serial_Class (...);
   Class_HID    : aliased USB.Device.HID.Default_HID_Class (..);

   Stack : USB.Device.USB_Device_Stack;

Initialization

There are 4 major steps in the USB Device stack initialization process:

• Initialize the UDC: This step depends on the microcontroller and might require:

  • Setting up IO pins
  • Setting up clocks
  • Setting up Direct Memory Access (DMA) Please follow the documentation the of HAL/BSP for your microcontroller.

• Register classes: This is when the application decides what features will be provided by the USB Device. It can be serial, HID (mouse/keyboard), both or more.

• Initialize the stack: During this step the classes will request resources from the Stack and the UDC. If the classes cannot get the resources they need, an error is returned and the stack cannot be started.

• Start the stack: After this step the stack is ready to work and receive the first packets from the USB Host.

begin

   --  TODO: UDC Initialization here

   --  Register classes
   Stack.Register_Class (Class_Serial'Access);
   Stack.Register_Class (Class_HID'Access);

   --  Initialize the stack
   if Stack.Initialize (UDC'Access,
                        USB.To_USB_String ("Wee Noise Makers"),
                        USB.To_USB_String ("Noise Nugget"),
                        USB.To_USB_String ("v1.0"),
                        UDC.Max_Packet_Size) /= Ok
   then
      raise Program_Error with "Cannot initialize USB Stack";
   end if;

   --  Start the stack
   Stack.Start;

Polling or interrupt driven:

There are currently two ways to use the USB stack:

Polling

With this method, the application must call the Poll sub-program of the stack at a regular interval. The stack will then ask the UDC if any event occurred on the USB bus since the last call and handles those events.

Interrupt driven

With this method the application will still call the Poll sub-program of the stack, but only in the interrupt handler of the UDC.

Writing a custom Device Class

Initialization

If a class is registered, its Initialize procedure will be called during initialization of the stack. The body of the procedure Initialize is the only place where the classes can request resources such as end-points and end-points buffers.

Requesting End-Points

Classes use end-points to send or receive data to/from the host.

Unfortunately every UDC only has a limited number of end-points available for classes, so an end-points request can potentially fail.

Requesting End-Point Buffers

Some UDC have internal DMA controller to access any data buffers in RAM (samd51), others can only access special memory banks attached to the controller (RP2040).

In theory the internal DMA controller allow a design with one less copy of memory during the transfers as data doesn't have to be copied to a special memory area. In practice there are constraints on what the DMA control can do, for instance data has to be aligned on 4 bytes or data must not be in flash (samd51). The result is that USB class drivers that will work on any hardware will not benefit always from this optimization (less copy) and handling the different cases will increase the complexity.

For the design of this USB stack the choice is to ignore DMA controllers and always use memory allocated by the UDC. The stack provides a way for classes to request buffers that will be used with end-points. UDC will either return a pointer in RAM (samd51) or in dedicated memory, regardless the provided memory must be accessible by the CPU to read/write data from/to it.

Classes must always use memory allocated from this API with end-points and nothing else. To that effect the EP_Ready_For_Data and EP_Send_Packet do not have a buffer address argument, the buffer address is always the one return by Request_Buffer.

Configuration

WORK IN PROGRESS

Setting up End-Points

WORK IN PROGRESS

Host to Device transfers (OUT)

WORK IN PROGRESS

Device to Host transfers (IN)

WORK IN PROGRESS

Porting USB Embedded

Porting the USB Embedded stack to a new microcontroller mostly means implementing the USB.HAL.Device.USB_Device_Controller interface for the USB Device controller.

WORK IN PROGRESS

Design

It is highly recommended to read the invaluable "USB in a NutShell" website to understand the following design documentation.

Device Control Transfers

The main part of the USB Stack is the handling of control transfers (Setup Requests). The control part is mostly a state machine that handles Setup Requests and the data payload associated with them. The payload can be either sent by the Host with the Setup Packet (bmRequestType = Host to Device), or sent by the Device as an answer to the Setup Packet (bmRequestTyoe = Device to Host). A zero-length-packet (ZLP) is sometimes used for acknowledgement of the Setup Request. If the Device doesn't support a given Setup Packet sent by the Host, the control end-points are stalled to indicate an error.

The control state machine starts in an Idle state. When a Setup Packet is received, it is decoded and analyzed.

• If the Setup Packet is of type Host to Device with a payload, the state machine is set to Data_Out/Last_Data_Out. These states receive the payload from the Host by chunks. The Last_Data_Out state is used when the next chunk is expected to be the last one for the payload. Once the full payload is received, the Setup Packet is processed.

• If the Setup Packet is of type Host to Device but without a payload, packet is processed immediately.

• If the Setup Packet is of type Device to Host, the packet is processed immediately. If the answer to the packet has a payload (Req.Length > 0), The state machine is set to Data_In/Last_Data_In. These states send the payload to the Host by chunks. The Last_Data_In state is used when the next chunk is expected to be the last one for the payload.

Dispatching of Setup Requests

WORK IN PROGRESS