Design Considerations in Adding USB Communications to Embedded Applications

Design Considerations in Adding USB Communications to Embedded Applications

Designing an application with universal serial bus (USB) communications enables a system to be able to communicate with a wide variety of USB host devices and provides a convenient power option through the USB connection. Today's printers, cell phones, digital cameras, media players, external hard drives and gaming systems all transfer data using the USB protocol. Having both power and data communication capabilities in one cable adds convenience and flexibility to applications.

In addition, the USB specification allows up to 127 devices to be attached to a single bus and supports a variety of device classes, including the human interface device (HID) class, which is natively supported by most operating systems and does not require driver installation. Host machines determine the type of connected USB device during the enumeration process. After plugging a USB device into a host, the USB device sends descriptors to the host that indicate the device type and which drivers to load.

Developers can upgrade legacy systems to add USB connectivity, or they can design new systems that include USB from the start. A USB MCU or a fixed-function USB bridge can address both scenarios. The following table shows options for adding USB communications to a system, as well as developer and end-user requirements.

Choosing a USB communication option depends on several factors including whether a developer is upgrading an existing system or creating a new system.Developers creating a new system have flexibility in choosing the best method of adding USB communications. The developer can design the system around a USB MCU or a fixed-function USB communication bridge and change aspects of the design to fit the USB solution.

Fixed-function USB communication bridges provide the easiest solution to add USB communications to a new design but provide the least amount of flexibility. They are available as an HID or non-HID fixed-function USB communication bridge, such as a USB-to-UART virtual COM port (VCP) bridge. When using these communication bridges, USB expertise is not necessary because USB firmware and driver development are not required. For non-HID class devices, manufacturers provide the necessary drivers for supported operating systems. Manufacturers also often provide dynamic-link libraries (DLLs), which aid in the development of USB host applications.

The HID device class is gaining acceptance as a general connection option for embedded systems because of the flexibility, overall throughput and lack of driver installation. Because the HID class is natively supported by most operating systems, driver development is not required, and end-users can plug in a device right out of the box and begin using it.

USB MCUs provide the most flexibility and control over the USB communication interface but require the most amount of design effort. Developers must generate all USB firmware, and if a non-HID class device is created, they must write device drivers. This requires some USB experience, as writing USB firmware and device drivers is not a trivial exercise.

Adding USB functionality to a system adds convenience and flexibility by enabling communications with a wide array of USB host devices as well as adding a power option that can provide up to 500 mA in a single cable. Silicon Labs offers a wide variety of USB MCUs as well as fixed-function USB communication bridges, including USB-to-UART, HID USB-to-UART, and HID USB-to-SMBus bridge devices. Small USB MCUs and fixed-function USB communication bridges provide a cost-effective way to add USB communications to new designs or legacy systems.

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