Just a quick question, I was wondering if the EMX is USB 2.0? Also, I’m looking to use a USB hub chipset and was wondering if anyone had any recommendations to use with the EMX and any cautions re: noise for the design.
Thank you in advance.
Yes it is fully USB2.0 compliant, but note that it is Full Speed not high speed USB.
Noise is somewhat ambiguous. Do you mean radiated emissions? Or do you mean immunity of the USB bus to external radiation? THere are the obvious things such as coupling the USB traces to a solid ground plane and keeping sensitive analog signals away from them.
Then there’s also the USB shield which should have a capacitor-resistor pair connecting it to the circuit ground plane. This will give the shield a low imedance path to a “ground” at higher frequencies which are more likely to radiate and cause issues.
The engineer in charge of this ended up using a TI hub controller TUSB2036 chip (http://focus.ti.com/lit/ds/symlink/tusb2036.pdf) and followed the design spec on page 17 but left the USB ports shield floating and we’re having some wicked noise problems in some of our clients locations (mainly those with older buildings that have 2 pronged outlets causing our board to drop connections frequently. The hard part is this is only happening at some locations leaving me to believe its mainly due to the poor implementation of the hub design.
After review of the design, as mentioned above there was no ground at all let alone a on the usb connectors shield let alone a capacitor-resistor pair to ground nor did he use any ferrite beads on any of the data or V+/V- lines next to the ports. So, now I’m revisiting his design and trying to patch things up. Is there a default recommend value for the ferrite beads that fit “most” USB applications or are they very specific to each design? On a side note, the board this connects to also has a floating USB shield to prevent ground loops and has been a working proven design for the last 2+ years when it was been controlled by a OEM motherboard. Any tips and or advise would be great appreciated as I’m trying to correct this in a short amount of time and thank you very much for your time.
I don’t ever put ferrite beads on the USB signal lines as any frequencies the beads would filter out are frequencies needed to provide a good digital signal at te other end of the cable.
You definitely do not want to leave the USB shield floating as that will definitely cause numerous problems, including packet corruption if you’re near any kind of emissions source. As general principle, a cable shield should only be connected via a RC pair at one end. The other end floats, and this eliminates ground loops.
USB can suffer a lot of problems with external noise. Since the differential voltage is so low it does not take a lot of noise to wreak havoc. There are a few companies that make common mode chokes (Wurth, Coil Craft) for the data lines which can really help. You must make sure that the grounds on the two systems do not float relative to each other. AD also makes a nice USB isolator chip now that can help in difficult cases.
Thank you very much guys for the information guys and I’ve ordered up a common mode choke kit to test and see which one works for our application best for the data lines and hoping I don’t have to go to a USB isolator for cost and I also don’t have much real estate on this board left. This combined with properly grounding the USB connectors shield should get rid of this problem we’re having.
If anyone has any other suggestions while I’m working on my board, please feel free to chime in and again, thank you.
Is there a general rule of thumb for the RC pair between the USB shield and ground that’s recommended for the two parts? And also, should I ground the CPU to the chassis or leave that floating? Imnit sure if this makes a difference but these units only have a 2 pronged plug as they are often used in older buildings and we found that our clients kept on breaking the ground lug off so we just switched the cords we supply to them now. Thanks in advance