The LED display is equipped with a USB port to provide the possibility of using a PC to display IceCube event data not embedded in the device's firmware, or make use of nicer effects that aren't so easy to produce using only the microcontroller. A full-speed USB 2.0 compliant interface is provided, using 0x1CE3 as a vendor ID. The vendor ID is not registered with the USB-IF, as this would be rather expensive for a small scale project like this.

The full-speed interface provides a bandwidth of 12Mbps, enough to run thousands of LEDs at 25 FPS. The display interface is provided in a very generic fashion, so any application that can render (low resolution) RGB data at video frame rates, can show this on the display.

USB device details

The interface presented by the device consists of two endpoints:

EP0: default control endpoint for status reporting and control
- Default control endpoint
- Display metadata reports
EP1: bulk endpoint for frame data transfer to the device
- Frame data transfer over USB

After enumeration is completed, the device can be used by the user. Unless it is known beforehand which device will be present, the display has to be queried to check which LED type it supports, how many LEDs are present, and what kind of data that can be displayed. Querying the display is achieved by obtaining a display metadata report. Once the buffer structure and content is determined based on the display properties, a display frame buffer can be filled with appropriate data and transmitted to the display. This is achieved by sending the buffer data to the bulk endpoint.

Multi-segment display facilities

It is possible to construct a single large display, e.g. to represent IceCube, consisting of multiple segments each driven by its own microcontroller. Since every microcontroller represents a single USB device, these actually behave as independent displays. When displaying fast moving images on these composite displays however, care has to be taken to prevent tearing. This occurs when one segment is displaying frame data ahead of an adjacent segment, possilby resulting in visible discontinuities. To prevent tearing, different segments should display data from the same frame as close together in time as possible. Some initial synchronisation mechanism has been provided, described in Display synchronisation.