Improving USB 3.0 with better I/O management

June 14, 2011 //By Sangram Keshari Maharana and Avineet Singh
Improving USB 3.0 with better I/O management
Sangram Keshari Maharana and Avineet Singh of Cypress Semiconductor explores how to improve USB 3.0 performance by using better I/O management techniques.

USB has been popular in the market for its simplicity, maturity and plug-and-play features. However, the 480 Mbps speed of USB 2.0 was not sufficient to support new generation storage and video. Therefore, the time was ripe for migration to a faster standard; this has led to the development of the new USB 3.0 protocol. The challenge that arises for developers is how to leverage USB 3.0’s full potential.

This article will explore the impact on hardware and software design to implement USB 3.0 with particular focus on handheld products. First, we will compare the capabilities of USB 2.0 and USB 3.0 and the impact of the transition on the components that interact with the USB 3.0 module.

In a common scenario, on the device side, the processor is connected to USB, storage, and peripherals directly.

Data rate comparisons
The basic difference between USB 2.0 and USB 3.0 is bandwidth. The theoretical bandwidth provided by USB2.0 is 480Mbps. In reality, the maximum throughput received is about 320Mbps (40MBps), which is roughly two third of the theoretical value. With USB3.0, the raw throughput is 4.8Gbps.

If we use the same proportion rate, then the expected data speed is 3.2Gbps (400MBps). However, many developers expect to be able to provide even higher throughput. Figure 1 below shows the data rate difference between USB 2.0 and USB 3.0 for a Buffalo external storage disk for different transfer sizes. It should be noted that the USB 3.0 data rate is restricted by the storage device; otherwise a data rate of 400 Mbps can easily be achieved.

 

(Click on image to enlarge)

 

Figure 1: USB 2.0 and USB 3.0 data rate differences

 
It can be seen that as the transfer size of a single request increases, the data rate increases in tandem. That is because as the transfer size increases per request, the number of requests and hence interrupts that the MSC

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