Pericom switches routinely boast -33dB but can be as “clean” as -41dB at 2.5 GHz. Minimal crosstalk: Crosstalk describes the extent that signals affect each other through the switch through capacitive, inductive, or conductive coupling. For example, Pericom’s PI3USB302-A USB 3.0 2:1 switch has a return loss of -23.3 dB at 2.5 GHz. By convention, the reciprocal is used: the reflected power over incident power (in dB). Alternatively, this is the ratio of the power into the switch versus reflected power (in dB) where a higher absolute number is better. Lowest return loss: A measure of the impedance mismatch between signal and switch, with a design goal to maximize power transfer.
Greater than ~-0.8 dB attenuation will lower signal peaks and slow rise/fall times all of these degrade signals and could result in violating the USB 3.0 spec. Lowest insertion loss: It’s desirable to minimize the power loss caused by the switch at operating frequency. Vendors like Pericom Semiconductor boast up to 10.6 GHz for their best switches. A good target for a USB 3.0 switch should be >2.5 Gbps. This is trivial for USB 2.0 at 480 Mbps, but significant for USB 3.0’s 5 Gbps. Maximize 3dB bandwidth: A common industry convention is to measure the max signal frequency of a channel at the 3dB loss point. Besides the fanout of the switch, there are a number of key parameters that characterize them.įigure 1: A charge pump-enhanced NMOS transistor is at the root of USB signal switches. Adding a charge pump widens the rail-to-rail output voltage and allows a higher Vdd operating range while still maintaining moderate ~250μA power consumption (Figure 1). The basis for a USB switch is an NMOS transistor that’s suitable for hot-swap/plug applications and fast switching. Let’s keep it simple for now in this short primer. Protocol switches also exist-and our sponsor Pericom Semiconductor makes a ton of them-but they deal with additional layers of the stack and can interface to more than just USB for example, they can also switch PCI Express and USB. The former are concerned mostly with the electrical physical layer (PHY) of the OSI model. We’re confining our discussion to signal switches as opposed to protocol switches.
Switches used for USB 3.0 are backward-compatible with USB 2.0 signals, but not vice versa. The four backwardly compatible USB 2.0 pins are cleverly integrated into the upgraded seven pin 3.0 Standard A connector. The common, large USB Standard A connector is the end inserted into a USB hub, looks similar for USB 2.0 and 3.0, but is labeled “SS” (Super Speed) for 3.0 and is backwards compatible into USB 2.0 slots. USB 2.0 has four pins, while USB has nine (Tables 2 and 3).
Table 1: USB switches are used extensively for USB 2.0/3.0 channels, and designers should be concerned about signal integrity (SI) with 3.0’s speed. While USB 1.1 and 2.0 are relatively low speed, the 5 GHz frequency of USB 3.0 presents some signal integrity challenges, as we’ll see later (Table1). The most common switch configuration is a 2:1, although other configurations are possible such as Pericom Semiconductor’s PI2USB4122 4:1. We’ll stick to USB 2.0 and 3.0 since they make up the bulk of the market.Ī USB switch is basically a MUX/De-MUX that bi-directionally moves USB signals between multiple ports and maintains adherence to USB-IF specifications. I’m going to tell you three things you need to know about USB switches-the digital MUXes and crossbars that fanout USB signals as part of most embedded designs. And every one of Apple’s recently announced ultra-slim MacBook will sport the very latest-and wickedly flexible- Enhanced SuperSpeed USB 3.1 Type-C connector. It’s tough to forecast how many USB 2.0 (480 Mbps) or USB 3.0 (5 Gbps) channels there are in the world because USB is (nearly) as ubiquitous as LED lights.īut looking at some ultra-mobile devices that contain USB gives some idea just how popular USB is: 317million PCs, 321 million tablets, and 2 billion mobile phones (source: Gartner Device Shipments, July 2014). If you’re designing with USB 2.0 or 3.0, you need to know these three things about moving USB signals around your system.Ĭhances are you are now-or soon will be-designing USB into your embedded system. Ciufo, Editor-in-Chief, Embedded Intel Solutions magazine
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