TrueTask® USB is MCCI's portable embedded USB host stack and USB platform. It supports USB 3.1, USB 3.0 and USB 2.0 host controllers, as well as USB 3.1, 3.0, 2.0 and 1.1 hubs. It has been tested at the USB-IF PIL as part of xHCI certification in multiple designs, and has shipped to over 50 million PC and embedded systems customers.
Most recently, TrueTask USB was used by Microsoft as the USB stack for Windows 10 IoT Core for Raspberry Pi 2.
The notion of a software platform is familiar: it's a design environment that allows code to be written that is subsequently unaffected by changes to the operating system, CPU, compiler, and so forth. In the embedded system world, Linux is a good example of a platform for application programs.
For reasons of efficiency, USB system code (the host and device stacks, the class and protocol drivers, and the management code) normally resides in the operating system kernel. Unfortunately, most operating systems that serve as application platforms do not serve as good system software platforms. In Linux, for example, the kernel APIs are not stable, and may change from kernel version to kernel version (184.108.40.206 may be different than 220.127.116.11, for example). Products based on embedded systems have similar differences between major versions. For a system vendor, this is very inconvenient, because multiple source versions must be maintained; bug fixes and system-specific adaptations cannot be automatically applied to any given environment, but must be scrutinized for correctness.
This is particularly a problem when it comes to USB host stacks. By its nature, USB is an open standard. No matter how well tested and supported a stack is, patches may be required to support a new and popular device that deviates from USB standards (but just happens to work with Windows or OS X). This presents problems. If the stack is from a commercial RTOS vendor, often you must update to the latest version of the RTOS to get all the corrections. However, if the stack is not designed to be a platform, you will have to modify your application code; and you may have to re-port the stack onto your platform. This may not be practical, because extensive testing is generally required.
If working with Linux, the problem can be even more difficult. Because of changes in kernel APIs, it is usually not practical to port the entire USB stack back to an earlier kernel version. Instead bug fixes must be identified, and then back ported. However, fixes may depend on other corrections made to the stack; so they may not work by themselves. Furthermore, fixes in the enumeration code may cause other devices to stop working.
TrueTask USB offers all the basic features one expects from an embedded USB host stack: extensive SoC support, small memory footprint, efficient run-time operation, and a variety of class drivers. TrueTask USB is different than a traditional embedded USB host stack, in that it's based on a foundation of stable APIs. Stable APIs allow seamless reuse of your investment in code based on TrueTask USB. These APIs are designed portably, so that the C code using them is portable across CPUs, operating systems, and host/device controller register models. They are also designed to be stable over time, so that code written with one version of TrueTask USB will be reusable with subsequent versions. Furthermore, they're internally stable as well. This means that a change, for example, in the host controller driver will not affect TrueTask USB code in any other layer; only minimal retesting is needed.
MCCI's commitment to stable APIs protects our customers' investments in USB software, and enables our customers to continue reusing their applications and class drivers from previous projects. For example, customers who upgrade a product using TrueTask USB to a new SoC will be able to reuse their legacy USB software applications and USB class drivers seamlessly. This allows customers to maximize the return on their investment of development time, testing, and field experience with prior products.
MCCI has been creating stable APIs for USB for twenty years. Several distinctive design practices have resulted.
MCCI is able to evolve APIs as the USB standards evolve; by maintaining the older, stable APIs, we isolate our customer's legacy code from changes in the stack.
Customers switching from existing USB stack implementations to TrueTask USB can still reuse their existing software investment. TrueTask USB includes local operation system USB host stack emulations. These emulations are fast and highly compatible, allowing TrueTask USB to support "in-box" class drivers shipped with the local operating system or developed by our customers. Whatever USB applications customers have running on the existing USB host stack implementation will seamlessly work over the TrueTask USB host, and will be able to access the additional features provided by TrueTask USB: SuperSpeed support, composite device support, role switching, and so forth.
TrueTask USB includes a production-quality Windows stack emulation. This allows direct test and validation of new USB controller designs on Windows, with a minimum of effort, and allowing test with an enormous variety of class drivers and device applications. The TrueTask USB host controller driver (HCD) API is a platform API, and allows direct test of non-EHCI/xHCI architectures such as the Synopsys DesignWare OTG USB 2.0 core, using the full variety of Windows devices.
When developing an embedded system, you therefore can first test the system components on Windows. This exercises the HCD, the hub drivers, the enumeration logic. Customers can also code to our generic APIs, and test large portions of their applications on Windows. When the software is moved to the embedded system, any variations are systematic and easy to track down.
Other embedded USB host stacks implement class drivers according to the USB-IF specifications. Why does it so often happen that these embedded host stacks fail to support certain USB devices, despite the user’s claim “but it works with Windows”?
The answer is found in the gap between the class driver specifications of USB-IF and what real-world device manufacturers produce.
Simply put, other embedded USB host stacks implement class drivers by their interpretation of the USB-IF specification - while USB device makers test and verify against the Windows USB host stack. The result is a mismatch between device and host implementation.
It is no secret that consumer device makers with USB connectivity mostly target the PC market, and use standards only for reference. Product test consists of interop test with Windows and OS X. As a result, these devices work perfectly with Windows or OS X; but there is no real guarantee that they are in fact standard compliant, so there's no guarantee that they will work with an embedded USB stack.
TrueTask USB is not just compatible with Windows; it is shipped in mass production as the USB host stack with multiple PC and laptop brands as the sole USB host stack on Windows. The USB enumeration logic, the handling of suspend/resume, the timing and sequencing of USB operations have all been tested and proven by customer experience to be compatible with what CE device makers test against. All this "USB business logic" is embedded in the portable portion of the TrueTask USB stack, so all users benefit from this experience.
It is increasingly common to multiplex a single physical USB connector to operate it either as a host or device. To support this, TrueTask USB has complete support for dual-role USB Type C, dual-role USB 3.1/3.0/2.0, and USB 2.0 On The Go, based on the MCCI USB DataPump. The following block diagram illustrates the configuration for a dual-role USB 2.0 or USB 3.x host/device.
TrueTask USB includes role-switching support for root hub and Microchip role-switching hubs, in support of Apple CarPlay.
Kernel mode: USBD and the HCD run as part of the local operating system kernel, and access hardware directly.
User mode: USBD and the HCD run as unprivileged applications, using low-level proxy services provided by the kernel to access hardware and program DMA. Depending on the strength of the local operating system's abstraction mechanisms, this can isolate the rest of the system from failures in the USB host stack and class drivers.
Hub, Composite, Local OS bridge, Ethernet Control Model (ECM), Ethernet Emulation Model (EEM), Network Control Model (NCM), Mass Storage (BOT), Human Interface Device (HID) keyboard and mouse, ASIX Ethernet over USB adapters, MediaTek (RaLink) RT3530-based USB Wi-Fi adapters, MediaTek (RaLink) RT5730 USB Wi-Fi adapters, generic class driver, custom class drivers.
Support for local OS USB class driver API emulation to enable any pre-existing class driver.
For additional class drivers, please contact email@example.com.