Connectors & cables
capability has increased to match a conventional power adapter. Also, USB-C and USB 3.0 introduced a range of different voltage outputs, adding 9V, 12V, 15V, and 20V. Since the beginning, USB has strived to continue its initial goals. Cable lengths are kept relatively short, the intention being to connect devices in the same physical location as the host rather than stray into networking use cases. Also, the tree connection topology requires all communication to pass through the host controller. Devices cannot directly talk to each other.
Figure 2 - USB evolution from 12 Mbits/s to 20 Gbits/s (Source: Mouser) USB Evolution
The first universal serial bus (USB) specification, USB 1.0, was released in 1996 by the USB Implementers Forum (USB-IF). Established by leading hardware vendors of that era, Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel, USB grew out of the need to establish a standard method of connecting peripherals to computers. The explosive growth of desktop computers and peripheral accessories, everything from external hard disks, keyboards, mice, printers, webcams, and portable media players, led to many different, and sometimes proprietary, ways of connecting them. No connection standard also limited market growth for the many independent peripheral manufacturers. In the mid-1990s, the usual methods of connecting peripherals included the popular RS232 serial interface and the parallel Centronics interface - neither of which provided any power delivery capability and required technical knowledge to configure. Other connector types included the IBM PS/2, DIN, and SCSI. From the outset, the USB specification required a consumer-friendly interface. That meant it needed to be self-configuring and hot- swappable, permitting devices to be plugged in and removed without interrupting the host computer's operation. Almost all the intended attached peripherals required power to operate, so the USB 1.0 specification established the 5VDC / 0.5A / 2.5W capability. Powering a peripheral also removed the need for additional power supplies and cables. USB also specified a small number of host and peripheral connector types. The electrical specification couldn't have been more straightforward; it uses a four-wire cable, two wires for power, and a twisted pair for data. The first decade of USB adoption saw multiple types proliferate, although many of these have now become deprecated. Sometimes the connector specification, e.g., Type A, becomes confused with the bus specification, for example, USB 3.1 - See Figure 1. Over the years, the USB specification has continued to evolve in data signaling transfer rates and power delivery capabilities. The initial USB
Instrumentation Monthly September 2023
1.0 specification established two differential signaling rates of 12Mbits/s and 1.5Mbits/s. To keep technical terminology to a minimum for consumer audiences, each speed specification had a non-technical name. The 12Mbit/s standard became known as USB Full Speed and the 1.5Mbits/s as USB Low Speed. The USB-IF followed this approach with USB 2.0 in 2000, with a 480Mbit/s standard known as High Speed, and in 2008, USB 3.0, capable of up to 5Gbit/s, known as SuperSpeed USB - see Figure 2. In 2013, USB 3.1 SuperSpeed+ delivered up to 10Gbit/s, and USB 3.2 in 2017 introduced a dual-lane approach doubling the data rate to 20Gbit/s. In just over a decade, the USB transfer rates increased by 1666-fold, and USB adoption expanded rapidly. USB also survived several competing technologies during the period, such as Apple's faster but more complex to implement, Firewire.
The USB 3.0 specification saw several data rate iterations and, more significantly, the announcement of the USB-C connector type. The USB-C connector is notable for the broader range of devices it serves and that it doesn't matter which way it is inserted. USB-C has quickly established itself as a primary method of charging and powering portable devices, from smartphones to laptops. The power delivery
USB ARCHITECTURE AND CONCEPTS The functional architecture of USB functions on a tiered-star host approach, with the root host “upstream” controller typically being internal to a desktop PC or laptop. A maximum of 127 devices can attach to a single controller through several external hubs. The connected peripherals are categorised according to class types (HMI, media streaming, etc.). Each device, a keyboard, for example, is uniquely identified with an address and, typically, three logical endpoint channels. Each endpoint has a specific function, and the specification accommodates up to 32 endpoints in a single device - see Figure 4. Communication between the host controller and device takes place via bidirectional pipes, either control or data. The pipe functions depend on the device class, which usually defines the data transfer types. There are four types of data transfer: control, interrupt, bulk, and isochronous. Figure 5 illustrates the attributes of each data transfer type and lists an example use case for each.
The device initialisation (enumeration) process is instigated by connecting a USB device to a host controller. This step involves the host issuing a reset signal to the device and requesting the device's parameters to establish the device class and transfer speed. Once received, the host controller allocates a unique 7-bit address to the device, after which data transfer can commence. For a more in-depth technical explanation of the USB architecture and operation, the reader will find helpful resources on the USB-IF website. The Infineon (was Cypress) application note AN57294 also gives a detailed explanation of USB 2.0 operation.
Figure 3 - the increase of USB power delivery capability from 2.5W to 100W permits powering of many more device types (Source: USB-IF)
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