However, there are several significant disadvantages. This enables unidirectional digital logic signals to be isolated with standalone, general-purpose isolators. It can go between the USB transceiver and Serial Interface Engine (SIE), or between SIE and USB controller. Instead of bisecting the USB cable with isolation, these solutions place isolation inside the hardware of either the host or peripheral. There are other issues described later.įor now, let’s shift focus and consider non-transparent alternatives like those discussed in. Many optocouplers cannot run at 12 Mbps or above, and have long propagation delays and timing errors that don’t meet USB 2.0 timing requirements. For example, standalone optocouplers or digital isolators generally don’t provide USB-compatible drive characteristics, or support bidirectional half-duplex communication. This approach is very appealing provided the concept can really be implemented, but there are a number of challenges to overcome. ![]() Hosts and peripherals originally designed for non-isolated applications easily connect to the USB isolator, and exchange standard USB signals without needing significant modification.įigure 2a: Isolation splitting the cable (concept)įigure 2b: Isolation splitting the cable, showing extra resistors The transparent USB isolator component is simply inserted between one of the transceivers and the USB cable, along with an isolated power supply. In this ‘transparent’ concept, communication between host and peripheral works very similarly to the non-isolated connection of figure 1. Therefore some extra resistors are needed as shown in figure 2b, to mimic the connection of their counterparts across the isolation. Unfortunately, the isolation prevents the host from ‘seeing’ the downstream pullup resistor, and the peripheral can’t ‘see’ the upstream pulldown resistors. GND1 (the upstream side’s ground reference) is now a separate node from GND2 (the downstream side’s ground reference). Information about the state of D+ and D- can cross the barrier, but current does not. One isolation possibility is shown in figure 2a, where the dashed line shows isolation that conceptually splits the USB cable. The complete bill of materials can become long, and it may be difficult to find discretes that fully conform to signaling requirements. This complicates implementations that are built from discrete components. In all cases, multiple signals must be isolated, and the signals may run at fast speeds or bi-directionally, depending on where the isolation is located. As noted in, there are several options for placement of the isolation barrier. Now imagine electrically isolating the host and peripheral. ![]() Methods of isolating a USB host and peripheral They also indicate to the host when the peripheral connects or disconnects, and the peripheral’s desired communication speed (1.5Mbps, 12Mbps, 480Mbps).įigure 1: Full-speed (12 Mbps) USB connection (non-isolated) The idle states help initialize the bus between one packet and the next. Sometimes the bus is idle, meaning that neither transmitter is active, and at these times, resistors attached to the ends of the cable establish ‘idle’ bus states at D+ and D. Data is organized into packets, with special signal sequences indicating start-of-packet and end-of-packet. During communication, the USB transmitters drive differential or single-ended states onto D+ and D. The signaling is bi-directional half-duplex, meaning that data can flow in either direction along the cable, but at any particular time, at most one transmitter actively drives the cable. The VBUS and GND wires provide 5V power and ground, while D+ and D- carry data. Figure 1 shows a standard USB connection. One reason for USB’s popularity is its simple 4-wire interface that provides power to a peripheral and a serial data link between peripheral and a PC.
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