![]() ![]() This is the most promising place to do isolation. Furthermore, this interface is rarely accessible in modern USB designs that incorporate the SIE and transceiver in the same chip. VPO, VMO, RCV, VPI and VMI all operate at the 12MHz rate, and would need to be carefully matched for delay and skew. However, this site has the same problem as site 1: too high a data rate, and indeed more signals running at 12MHz to isolate. These unidirectional signals could, therefore, be considered for optical isolation. Moreover, in a peripheral with an integrated transceiver, the OE signal (which indicates direction) is not accessible.Ī USB peripheral that uses an external transceiver exposes the transceiver interface. The situation is further complicated because the bus is bidirectional, but isolators are unidirectional.The D+ and D- signals must be carefully matched for propagation delay and skew, a match that is difficult to achieve with optical isolators.The signaling rate is 12MHz, too high for cost-effective isolators.There are several reasons why optical isolation is not practical on the USB bus wires, as seen in Figure 2: D+ and D- are bidirectional, further complicating the isolation. The 83ns bit time and close matching of rise/fall times make it difficult to maintain signal fidelity through optical isolators. This scope trace shows the USB D+ and D- signals (site 1 in Figure 1) at the beginning of a packet. The gray rectangles, marked 1, 2, and 3, show three possible places to place optocouplers to electrically isolate a USB device from the host computer.įigure 2. The leftmost block is the application circuitry, which might be a microprocessor, an ASIC, or a Digital Signal Processor (DSP). The middle block, a USB Serial Interface Engine (SIE) translates the bus signals (as seen and sent by the transceiver) into data bytes and USB signals for use by the application that implements the USB peripheral. Considering this figure from right to left, a USB transceiver connects to the D+ and D- lines and either drives or receives data under the control of an OE (Output Enable) control pin. USB peripherals are built using the block diagram shown in Figure 1. In most current designs the SIE and transceiver are integrated, making interface (2) inaccessible. There are three possible interfaces where a USB peripheral could be isolated: site 1, the USB bus itself site 2, the transceiver interface and site 3, the application interface. The D+ and D- signals are bidirectional, operating at a signaling rate of 12Mbps (83ns per bit cell). The V BUS wire provides 5V of power up to 500mA. The USB connector contains four wires: two to supply power (V BUS and GND) and two to move the USB data (D+ and D-). A 12Mbps device operates with enough bandwidth for useful data transfers, and employs a data rate that is manageable for designs that use inexpensive optocouplers. This article discusses optical isolation of a full-speed (12Mbps) USB connection. Two obvious isolation applications are medical, where PC-based instruments are attached to patients, and industrial, where large supply rail offsets can occur. If you need to electrically isolate a device that is to be connected to a PC, USB is a natural connection interface because of its extensive industry support. The Universal Serial Bus (USB) has become the standard way to connect peripherals to personal computers. The SPI interface can run at any speed and consists of simple, unidirectional signals. A USB controller that attaches to your embedded system using the SPI interface is easy to electrically isolate. USB's extensive industry support and simple structure (only four wires in a USB cable) make it a popular PC interface. ![]() If you must isolate a device that also connects to a PC, the USB interface is a natural choice. ![]()
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