Core USB 2.0 Device Gateware

The LUNA gateware library provides a flexible base USB Device model, which is designed to provide the basis for creating both application-specific and general-purpose USB hardware.

USB devices are created using two core components:

  • A USBDevice instance, which provides hardware that handles low-level USB communications, and which is designed to be applicable to all devices; and
  • One or more endpoint interfaces, which handle high-level USB communications – and provide the logic the tailors the device to its intended application.

The USBDevice communicates with low-level transciever hardware via the FPGA-friendly USB Transceiver Macrocell Interface (UTMI). Translators can be used to transparently adapt the FPGA interface to other common bus formats; including the common ULPI low-pin-count variant of UTMI.

USB 2.0 architecture diagram

The overall architecture of a LUNA USB 2.0 device, highlighting the USBDevice components, their connections to the endpoint interfaces, and optional bus translator.

Conceptual Components

The USBDevice class contains the low-level communications hardware necessary to implement a USB device; including hardware for maintaining device state, detecting events, reading data from the host, and generating responses.

Token Detector

The Token Detector detects token packets from the host; and is responsible for:

  • Detecting start of frame packets, which are used to maintain consistent timing across USB devices.
  • Detecting the start of USB transactions.
  • Identifying the device and endpoint to which each transaction is addressed.

As each USB transaction starts with a token packet; it is implicitly the Token Detector’s responsiblity to notify endpoint interfaces of imminent incoming data (OUT transactions) and requests for data (IN transactions).

Handshake Detector

The Handshake Detector detects handshake packets from the host; and is responsible for identifying the host’s response to packets from the device – indicating whether the host successfully received a packet sent from the device.

Data Packet Receiver

The Data Packet Receiver is responsible for receiving data packets from the device – including the payloads of both OUT and SETUP transactions – and translating them to a simple data stream.

The Data Receiver handles error detection; and thus validates the checksums of each packet using the Data CRC Unit.

Device State Manager

The Device State Manager is responsible for storing global device state – primarily, the device’s current address and configuration. The device state manager accepts changes to the device’s address/configuration from each endpoint interface; and automatically resets the relevant parameters when a USB reset is received.

Handshake Generator

The Handshake Generator provides a simple, strobe-based interface that allows endpoints to easily emit handshake packets – allowing the device to acknowledge packets (ACK), issue stalls (STALL) , and to rate limit communications (NAK/NYET).

Data Packet Transmitter

The Data Packet Generator is responsible for generating outgoing USB packets from simple data streams; including emitting data packet IDs, sending data, and appending data CRCs. This class automatically appends the required data CRC-16s.

Data CRC Unit

The Data CRC Unit is shared among the packet receiver and packet generator; and handles computing the CRC-16 for USB data streams.

Interpacket Timer

The Interpacket Timer is responsible for maintaining maximum and minimum interpacket delays; ensuring that the device can correctly provide bus turnover times; and knows the window in which handshake packets are expected to arrive.

Reset/Suspend Sequencer

The Reset/Suspend Sequencer is responsible for detecing USB reset and suspend events; and where applicable, participating in the USB reset protocol’s high-speed detection handshake.

The sequencer:

  • Detects USB resets; and communicates to the Device State Manager that it should return the device to an un-addressed, un-configured state.
  • Performs the high speed detection handshake, which allows the device to switch to High Speed operation; and thus is necessary for the device to operate at high speed.
  • Manages the high-speed terminations; as part of the reset-handshake and suspend protocols.
  • Detects the periods of inactivity that indicate the device is being suspended; and automatically disengages high-speed terminations while the device is in suspend.

usb2.device Components

usb2.packet Components

usb2.reset Components