To USB or Not to USB


USB 3.0 Performance Test

Tom’s Hardware ran a series of performance tests using a Samsung USB 3.0 Hard Drive and an NEC USB 3.0 Host card (or a card using the NEC Host chip).    On average the system ran about 40% faster in 3.0 mode over 2.0 mode.   To summarize, the portable hard drive at 640GB sells for less than $100, and the speeds for USB 3.0 are consistently faster.


I’ve noticed in the results that the speeds never go above ~190 Megabytes per second.


I think the reason is that the PCIe interface limits the speed.


The NEC Host card seen here, is plugged into the PCIe x1 slot.

Images from Tom’s Hardware

  This shows that the slot is a PCIe x1 slot (although it’s mentioned in the article) this is an important point. 

This means that the PCIe Gen 1 throughput is limited to something like 2.5 Gigabits per second (250Megabytes per second).  USB 3.0’s maximum throughput is actually 4 Gbps (400Megabytes per second0 (after removing 1 Gbps for 8b10b encoding).   This means that the throughput of the NEC USB 3.0 Host will be limited to 2.5 Gigabits per second or 250 Gigabytes per second.  Our tests indicate at in a PCIe x1 plugged into a laptop achieves speeds in the range of 100 Megabytes per second. 


Shameless plug for Synopsys USB 3.0 IP:  We can achieve full USB 3.0 speeds with just the Synopsys Host with a  PCIe Gen 2 x1 connection on our Host.


The article concludes that the performance for USB 3.0 is greater, and the cost is now pretty comparable to USB 2.0.   I encourage you to click through the Tom’s Hardware article quickly here.

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