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The Universal Serial Bus (USB) is by far the most widely used serial I/O interface today. It appears in most PCs and laptops plus their peripherals, external storage units, consumer electronics (e.g. cameras), and smartphone and tablet chargers. But in spite of its ubiquity and usefulness, USB has limitations.
Now, with the USB Implementers Forum (IF) new Type-C connector specification, this interface is about to become more popular. With its high data-rate capability, higher power-delivery specification, and physically smaller, more flexible connector, Type-C usage could make USB’s adoption even more widespread, fully living up to its “universal” designation. Here’s a summary of this superior new connection scheme.
A Quickie USB Primer
Launched in the mid-1990s, USB quickly became the primary serial I/O interface for PCs and peripherals. It replaced the RS-232 serial port, the parallel printer port, and a bunch of other lesser-used interfaces. Subsequently, its adoption grew broader into cellphones, tablets, and e-reader battery chargers, as well as the popular USB flash drive. Over the years, upgrades and enhancements to the standard made it even more useful. It continues to reign as the number one serial interface.
As mentioned above, USB is a standard of the USB Implementers Forum (USB-IF). The standard defines the serial transmission path, a dc power supply, a protocol, as well as cables and connectors. The USB interface includes a high-speed data path and dc power-delivery capability. The network topology is a multilevel star bus.
The standard defines one master host controller and up to 127 slave nodes; multiple hubs may be implemented. The host initiates all communications. Transmission is half duplex on most versions, but full duplex on the most recent versions. Data transmission, which is differential with NRZI line coding, is done in packets (along with control and status packets).
The data-transmission rate has increased over the years, from 1.5 Mb/s to 10 Gb/s today. Table 1 shows each version and the corresponding top data rate.
The dc power-delivery specs have also increased over the years. Table 2 provides a summary by version.
Moreover, the USB standard defines the cable and connectors. The data cable is shielded twisted pair. Multiple connector types are used—Type-A is the most common and familiar, while Type-B is the common peripheral connector. Types A and B have four pins, two for the differential pair and two for dc power.
Smaller connector types are also defined for smartphones, tablets, e-readers, and other miniature devices. These include the Mini-A, Mini-B, Micro-A, and Micro-B, all of which have five pins. The fifth pin is used to indicate whether the device is a host or slave. In addition, a special USB 3.0 Micro-B connector has been defined for high-speed devices. It uses 10 pins and defines three differential pairs for data as well as dc power. The newest connector is the Type-C, the focus of this article.
As for cables, the wire is usually #26 with shielding on the data lines. Maximum cable length is five meters for USB 2.0 and two to three meters for USB 3.0/3.1.
USB Type-C
Type-C mainly defines a new connector, and provides guidelines for implementing the new higher data rates, dc power capabilities, and other features of USB 3.1. Another purpose of the new connector is to consolidate and simplify connector choices. The ultimate goal is one lasting connector that can handle larger computers and peripherals as well as On the Go (OTG) battery-operated devices like smartphones and tablets.
Figure 1 shows the connector. The small, 24-pin device can be used in both thin mobile devices as well as larger equipment. One connector will serve both host and slave devices. A key feature of Type-C is that it’s “flippable,” meaning that it can be plugged into in either direction like Apple’s Lightening connector.
Pin connection details are illustrated in Figure 2:
• Four dc power pairs (VBUS/GND)
• Four differential pairs for USB 3.1 full-duplex mode (RX1/TX1, RX2/TX2)
• USB 2.0 differential pair (D+/D-)
• Two sideband pins for use in Alternate Mode (SBU1/SBU2)
• Two cable configuration pins for setting the Power Delivery mode (CC1/CC2)
A key feature of Type-C and USB 3.1 is the Alternative Mode. It allows the cable and connector to carry other interface signals. For example, the connector can be set up to handle DisplayPort 1.4 video, Thunderbolt 3.0, or Mobile High-Definition Link (MHL) eC bus 3.3. It could conceivably carry PCI Express or Ethernet as well. These connections utilize the four high-speed lanes and the sideband pins for messages.
The CC pins are used to determine the selection of the Power Delivery (PD) mode. These pins are initially detected when the connector is first plugged in to signal the connector orientation and the power delivery is selected. Multiple configurations are supported, as is shown in Table 3.
Finally, the Type-C cable can be set up in two ways. The first configuration is a passive pass-through cable. A second form is an active cable containing a chip with ID functions that dedicate the cable to some vendor-specific operation or power level via the CC pins.
Design Considerations
It’s expected that Type-C will be adopted quickly, which has triggered considerable design activity. Type-C has so much to offer, but the many new benefits come at a price—greater complexity. For instance, all new designs require additional switches, MOSFETs, multiplexers, a PD controller, and other auxiliary circuits. Multiple vendors have chips and devices to fill that need, and reference designs are available.
Another consideration is testing and certification to meet USB-IF guidelines. Such tests are tricky, so check with major oscilloscope vendors for appropriate equipment and software.