Universal Serial Bus, widely known as USB, has revolutionized PC connectivity with its hot plug-and-play simplicity. Not only has USB greatly reduced the complexity of adding peripherals to a PC, it has also fulfilled the need for a high-speed expansion port for many data-rich applications. Invented mainly as a cable connection for out-of-the-box I/O expansion, USB has begun making its way into embedded systems for integrating peripherals inside the box. The same features that made USB popular as a cable also make it a system bus of choice in the embedded world. Those features include high speed, low pin-count, well-defined system architecture, and widely available software and silicon.

Fig. 1. The photo printer as dual-role USB host/device system

The modern printer stands as the perfect example of the creative utilization of USB, which has not only replaced the parallel port as the most popular method of connecting a printer to a PC, but also has transformed the printer from a dumb PC peripheral into an intelligent home appliance. The catalyst for this transformation was digital photography. Growing out of a need for the easy transfer of photographs from a digital still camera to a printer, designers added USB to the printer, since USB is the most commonly found connectivity protocol on every camera.

 

Fig. 2. USB as a system bus in a photo printer.

Unlike connecting a printer to a PC, where the printer is the USB device and the PC is the USB host, connecting a camera directly to a printer requires that the printer be the host since the camera is always a USB device. To resolve this role reversal conflict, the printer has evolved into a dual-role USB system, capable of behaving both as a USB device and a USB host.

The addition of a USB host controller in a printer allows a digital still camera to be connected directly to the printer, bypassing the PC while still maintaining printer’s traditional role of being a USB device and PC peripheral. This architectural change not only made printing photographs a less PC-centric and a more user-friendly task, but the printer also gained the valuable bonus of implementing USB as an internal system bus as a result.

Fig. 3. Sharing embedded USB peripheral with the outside world is less straightforward and more cumbersome to implement via software.

A concept just as compelling as connecting a camera directly to a photo printer is the idea of having a flash media interface on the printer for accepting flash cards as digital film. This makes sense given that many users have more than one flash card in use in their cameras and the ability to print directly from any flash card is very appealing. Printing directly from flash cards also does not require a USB cable, which is often misplaced over time. However, adding a multi-format flash media interface to a printer is no easy task. The competing flash media formats require a large number of I/O pins on the printer ASIC.

The perpetual evolution of flash media standards also makes frequent silicon revision necessary to accommodate new cards of ever increasing capacity and speed demanded by the latest multi-mega pixel cameras. Both are financially prohibitive propositions for printer ASIC designers. Instead of throwing dollars at the challenge, many printer designers have turned to USB to overcome this hurdle. By embedding a USB flash media reader as a separate chip off the printer ASIC, a full-featured digital film interface is easily and economically implemented.

Now that the printer exists with its own ecosystem of USB connectivity inside the box, it should also become accessible to the outside world. Furthermore, it is also desirable for a camera or flash card attached to the printer to be “viewed” by a PC that is also connected to the printer so that photographs temporarily stored on digital film can be relocated to a more permanent home on the PC. From the USB topology standpoint, this should be possible since the PC is a USB host while the camera and flash media reader are USB devices. But the data path going through the printer ASIC is treacherous.

One can always choose to manually switch the camera’s USB cable from the printer to the PC, but switching the “built-in” USB flash media reader is less intuitive. The printer ASIC must route the PC bound data from its built-in USB host controller to the USB device controller on-chip, as well as bear the burden of emulating a USB Mass Storage Class device.

Such software emulation is cumbersome to implement, creating a traffic jam in the USB data path and limiting the possibility of any future peripheral additions inside the printer. (See Fig. 3). The result is a compromise between functionality and performance. The printer is able to present both itself and attached media cards to the PC, but the card reading speed may be severely penalized.

Fig. 4. USB MultiSwitch enables easy sharing of embedded peripheralswith a PC.

A USB hub with port switching capability that allows two USB hosts sharing a common set of peripherals would simplify this complicated scenario. The SMSC USB MultiSwitch hub (USB2524) is such a device, combining a high-speed USB hub and switching functionality in a single component. Equipped with two upstream ports and four downstream ports, the USB MultiSwitch enables USB device sharing and switching between two USB hosts without cable swapping. Requiring no more than standard hub driver support, the USB MultiSwitch appears as a 4-port USB hub to either USB host system and supports standard USB topology. For photo printer designers, the USB MultiSwitch provides a unique tool for adding USB ports inside the box for integrating peripheral functions as required by the rapidly developing multi-function printer market.

In addition, the USB MultiSwitch gives every built-in USB peripheral an opportunity to interact with an external USB host where sharing “usage rights” presents interesting scenarios. Through a simple 4-pin “port assignment” input, the USB MultiSwitch hub connects one or more sources of digital photography to the printer USB host where direct printing takes place.

At a push of a button, or under the control of the printer ASIC, the sources of data can be easily re-directed to a PC for photo editing and storage as the USB MultiSwitch hub electronically “detaches” and “re-attaches” the USB peripherals from the printer host to the PC host (Fig. 4). Peripherals will perform at their intended maximum speed, bypassing the potential bottleneck created inside the printer ASIC. Gone are the complicated software emulation and the lack of expandability of an ASIC-based system.

Fig. 5. Stationary USB infrastructure surrounding a desktop PC.

To embed a new peripheral, such as a hard disk drive, CD-RW drive, or wireless connectivity, to the printer and allow it to be shared by a PC, one could simply add the USB peripheral at one of the downstream ports of the USB MultiSwitch hub, and the attached peripheral will connect to either the printer or the PC as instructed. Similar peripheral sharing opportunities can be found elsewhere. Many multi-function USB systems can take advantage of the USB MultiSwitch to increase accessibility. Those systems include a PC, direct-copy HDD/DVD-RW drive, or multi-function LCD display.

The desktop PC is an interesting environment, and one naturally suited to the USB MultiSwitch hub. Cable swapping is a common chore that many PC users face on a daily basis, especially in a multi-PC environment where obtaining “usage rights” to common USB peripherals requires frequent cable swapping. With the growing number of computers, and even more USB peripherals in the same household, there is a strong desire to make USB peripherals shareable among multiple computers without switching cables.

One of the most common multi-PC settings in a home involves a stationary PC where equally stationary peripherals reside, while a mobile PC regularly comes in and out of the environment. In such a situation, it would be useful if there was a one-cable mechanism able to provide the mobile PC instant access to the stationary USB ecosystem. This can be easily implemented by embedding a USB MultiSwitch hub into the desktop PC media drive bay. The media drive bay on a desktop PC is an IO and storage expansion slot that usually offers an easily accessible front-facing panel (Fig. 5). This, therefore, is the most convenient location for attaching and detaching the second PC.

With such a setup, a user could simply connect the mobile PC to the desktop PC via a single USB cable and the USB MultiSwitch hub would instantly grant the mobile PC usage rights to all peripherals that are connected to the switching ports on the desktop PC. This includes not only peripherals externally connected to the desktop PC, but embedded peripherals, such as a flash media reader or a removable hard drive that are internally connected via USB. Since most mobile PCs do not have a built-in flash media card reader at all, or have only limited flash media format support, sharing the drive bay on a desktop PC gives the mobile PC a full multi-format flash media interface on demand.

As USB continues to proliferate outside of its traditional realm of PC connectivity, there are ample opportunities for multiple USB host systems to coexist in close proximity so that peripheral sharing becomes compelling or even unavoidable. Product designers pondering the new possibilities brought forth by the versatile USB system bus have previously had to curb their enthusiasm because of the complexity associated with a multi-host environment. They now have a simple solution that makes peripheral connectivity in the emerging multi-host ecosystem more manageable. While USB technology of yesterday and today was about connectivity, USB of the future is about sharing.

Fig. 6. Sharing USB peripherals with a second PC via media drive-bay.