Know where your data goes before you make USB go fast

Know where your data goes before you make USB go fast
USB provides peripherals with both power and data interfaces. This means that customers only have to plug in one connector to power most peripherals, reducing the forest of cables behind most PCs. This free external power source reduces the BOM cost for most peripherals, since they can ship external peripherals without having to provide a separate power supply. USB is hot pluggable, so new peripherals can be added to the bus at any time.

There are three speeds available to the device designer including: low-speed with a raw signaling rate of 1.5Mbps; full speed with a raw signaling rate of 12Mbps; and high speed with a raw signaling rate of 480Mbps. These numbers are interesting, but actual throughput numbers are what are really important to system engineers. Table 1 illustrates that low-speed can carry 8,000 Bytes per second; full speed can carry 1.2 Mbytes per second; and high speed can carry 53 Mbytes per second. There is a 5000x performance variation from low-to-high speed. Yet the difference in power and cost between low speed and high speed are usually less than 10x.

Table 1. USB Bandwidth (Note: K = 1000).

The first step towards selecting the proper speed is to look at the performance needs of the peripheral. Having extra data transfer capacity is not advantageous if the peripheral does not utilize this additional capacity. As a rule of thumb, low speed USB is the lowest cost and lowest power option - similar in speed to RS-232. At 12-Mbits/s, the full speed transfer rate is similar to 10-Mbit Ethernet without the protocol overhead and collisions of Ethernet. At 480-Mbits/s, high speed approximates the performance of the PCI bus, but with just 4 wires and end-user hot plug capability. Comparing the needs of various peripherals to these three standards (RS-232, 10 Mbit Ethernet, and 33 MHz PCI) will help the peripheral designer gain an intuitive feel for the capability.

Low speed USB is similar to RS-232. Naturally, the initial peripherals that migrated to low-speed were those that were previously hooked up via a serial or PS/2 port -- mice, keyboards and joysticks. Off the shelf silicon is readily available at less than $1 to provide an easy interface between low-speed USB and a peripheral migrating from RS-232. The timing accuracy required for low-speed devices is loose enough that a crystal oscillator is not required. Some parts, such as the CY7C637xx series, do not have external clock pins! To prevent low-speed devices from using the bus inefficiently, the spec only permits low-speed devices to use a small portion of the bus bandwidth. This leaves plenty left over for full and high speed devices.

Table 2. USB transfer types.

Until 2001, the only alternative to low speed was full speed. Since the speed is similar to 10-Mbit Ethernet, full speed is a good match for broadband connections like cable and DSL modems. In addition, full speed provides isochronous capability, which provides reserved bus bandwidth and guaranteed delivery of time sensitive data. This makes full speed a good match for audio delivery and compressed, low-resolution video web cams. Full speed is the standard interface for digital cameras, transferring a compressed 1600 x 1200 color image (~500k bytes) in under a second.

In 2001, high-speed USB arrived providing PCI-like data transfer rates outside of the box. In fact, the performance of high-speed USB, at 53 Mbytes/s allows external hard disk drives to match the performance of internal hard disks. The initial explosion of high-speed peripherals has tapped this potential -- hard disk drives, DVD drives, CD RW drives and removable media drives have proliferated in the short time that USB 2.0 has been available. But the high-speed transfer rates allow far more than just external mass storage. High-speed allows a digital camera with 100 images to be transferred to the PC in seconds. That same transfer would take over a minute using full-speed USB. High-speed can carry isochronous data like full speed, but at much higher rates. At rates as high as 24-Mbytes/sec, full-motion video can be carried across the 4-wire, hot-pluggable interface. (These applications are compared in figure 1.)

Figure 1. Applications require differing bandwidths

Although high-speed is new to USB, it is important to keep in mind that it is 100% backward compatible with USB devices introduced before USB 2.0. Most high-speed devices can operate like full-speed devices when they are attached to a full speed hub or host. A good example of this is a USB disk drive, which will slow its data transfer rate. All low and full speed devices can operate as well as--or better--a high-speed host controller.

High speed may not appear to be the best choice for power-sensitive devices, but that is not always the case. Although high-speed USB controllers consume more power, the increase in current is typically 2-5x with a throughput increase of 40x. In a digital camera that has strict power consumption requirements, switching to high-speed USB will transfer the pictures in 1/40th of the time and will also reduce power consumption by an order of magnitude because the USB interface is only active for a short time.

The availability of high-speed bandwidth outside of the box gives engineers more freedom to use the host CPU and memory instead of adding additional memory or CPU cost to peripheral. Imagine a printer that can use the host CPU and memory instead of its own. Consider a cable modem that uses the host CPU as a DSP. How about an optical mouse that sends images to the host? PCI created new possibilities for internal peripherals; now high-speed USB does the same for external peripherals.

The first step in designing a USB peripheral is to select the speed -- low, full or high. Low speed will always yield the lowest cost, but it can hardly approach the speed of 56K modem. If time sensitive data will be transferred, either full- or high-speed will be required. Beyond that, the selection between full or high-speed should be made based on the speed requirements of the device. With fully forward and backward compatibility between full-speed/high-speed hosts and peripherals, any designer should take a serious looking at jumping straight to high-speed. That will provide the highest possible performance, with a minimal cost increase, and - quite possibly - power savings. Selling a USB product requires a USB vendor ID and product ID, available from the USB Implementers Forum (www.usb.org). The USB I/F also runs compliance testing, which is required to sell products with the USB logo. The USB I/F website contains copies of all of the USB specifications as well. #