Wednesday, July 8, 2009

USB Design

Today saw the design of the USB circuits for the nControl^2. The goal of the design is to provide 8 USB 2.0 Host ports all capable of providing 500mA power to peripheral devices. To maxamize the bandwidth available to the end devices, I am using 2 High Speed interfaces on the OMAP processor and then using a 4 port USB Hub chip to provide the four powered ports from each of the OMAPs host ports.

The 500mA at 5V required by each port means that in total all 8 ports will require 4A at 5V, or 20W. This will be the major factor in the specification and design of the Power Supply for the nControl^2.

On each of the ports, an common mode EMI filter, has been added to the D-/D+ lines. ESD protection is also added to these lines and Ferrite Beads have been used to try and prevent any EMI issues with the power lines on the ports.

I found on the last nControl project that EMI certification was an issue, and we were only able to achieve FCC Class A certification as there was some intermodulation products from the 25MHz oscillator for the Ethernet PHYs. I am hoping by going to these lengths I will avoid conducted emmissions, and also minimise rediated emmissions to make for an easier certification track down the road.

If anyone has had experience with FCC / CE Certification of USB hubs, I would be keen to hear if there is anything else that I need to look out for on this one.

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