To ground or not to ground...

Lord Steve

I'm making a board with the FT232 chip.· I noticed that the Propeller Demo Board schematic (rev D/E/F) has the USB Mini B connector's pin 5 grounded.· Is that the connector's housing?· Should that pin be grounded?· How does one know when to ground it?

Thanks!

Post Edited (Lord Steve) : 12/27/2009 7:32:36 PM GMT

Leon

According to the USB spec, the shell on the USB peripheral device shouldn't be grounded. I've grounded it, though, with a PIC board of mine, and it worked OK.

Leon

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Post Edited (Leon) : 12/27/2009 9:04:42 PM GMT

Phil Pilgrim (PhiPi)

Assuming the shell is connected to the cable shield, and assuming the shield is grounded at the host end (both valid assumptions, I believe), the shell should not be connected to Vss at the client end.

-Phil

whicker

My own take on this, is that you generally don't want return current flowing down your shield. In the instance of USB, you've got a 5V (VCC) and a 0V (GND). Whatever flows down the 5V line flows back on the 0V line. Any voltage difference generated on the shield should stay on the shield, not be added to the 0V.

Similar to any other kind of serial bus, for EMI or ESD reasons, you might have to capactively couple 0V of the cable to the shield of the USB cable to a metal case of the device. But hobbyists really don't have a need to delve into this, and there's no bulletproof way to do this across the board.

I really don't think it starts to matter one way or the other until you start putting in electric motors (rumble feature, hard-drive motors) or audio related things.

Rayman

If that's the shield, then you leave it disconnected. Unless your device has a outer metal shell, in which case, I'd connect it to that...

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My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm

Cluso99

I connected it on my TriBlade without problems. However, it should NOT have been connected.

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Links to other interesting threads:

· Home of the MultiBladeProps: TriBlade,·RamBlade,·SixBlade, website
· Single Board Computer:·3 Propeller ICs·and a·TriBladeProp board (ZiCog Z80 Emulator)
· Prop Tools under Development or Completed (Index)
· Emulators: CPUs Z80 etc; Micros Altair etc;· Terminals·VT100 etc; (Index) ZiCog (Z80) , MoCog (6809)
· Search the Propeller forums·(uses advanced Google search)
My cruising website is: ·www.bluemagic.biz·· MultiBladeProp is: www.bluemagic.biz/cluso.htm

Toby Seckshund

The mini USB conectors sometimes have 5 wired pins and the shield.

I think, but do not know for sure, that the 5th pin is ground and the 4th is N/C (as shown on the DemoBoard CCT ). I would "ground" the shield, at the prop end via a 100K or 1M resistor in the hope that these touch first and would get rid of static before the delicates touch.

A good 10% of the busted, firewire conected, video stuff we get is static damage. Straight up the chip, for a warentied repair a new card, £500 thankyou very much.

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TonyWaite

Lord Steve,

An empirical solution that I have used successfully: tie the Screen at the 'receiving' end to USB ground via a 1 Megohm resistor in parallel with a 4n7 capacitor.

This was in an application where my board connected to an automotive PC host, over which I had absolutely no control. The kit was installed in buses or trams by unskilled labourers to no particular specification unfortunately.

In an ideal application, there's an EMC debate to be had; but in this instance I preferred to have some control over the Screen rather than none.

Regards,

T o n y

Toby Seckshund

The dreaded earth loop could be a problen on long runs or in a hostile enviroment but over the Metre or so of a USB lead there shouldn't be any problems, but if the system were to be vetted somebody might get picky, and generate a drama over it. A large value resistor and a small value capacitor is the "norm" that few comercial suppliers follow.

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Ariba

The 5th pin has nothing to do with the shell. It is an ID pin for the USB-OTG standard.
Grounded means the Connector is a type A (= Host), not connected means it is a type B (=Device).
So you should not connect it for an FT232 chip.

The attached picture comes from Wikipedia. As you see, Pin5 is Ground and Pin4 is the ID.

Andy

mctrivia

Shields should always be grounded at the side with the best earth ground and not connected on the other side. With generic wiring(such as USB) this is impossible to know so you take the assumption that the computer will have the better earth ground and leave all the devices disconnected from the shield. the reason is if the shield is only connected at 1 side it will act as a feriday cage and stop emi interference from entering the cable. If both sides are connected current can travel through the shield causing earth loops making emi worse then if you had no shield.

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James Long

I view this as a master slave situation.

The shield should be connected on one end only (to prevent the ground loop problem, but to remove noise off the shield).

The Master should have the shield connection. Any slaves would let if float.

I pretty much agree with mctrivia.

James L

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James L
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Cluso99

mctrivia is absolutely correct. We had such problems in the 70's with video terminals connected to the minicomputer over long runs. The terminals ultimately had their screens attached to ground via large caps (cannot recall the value). What was happening in our case was that operators would discharge static electricity to the terminal and that would cause a spike up the shield and cause the mini to crash that program.

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Links to other interesting threads:

· Home of the MultiBladeProps: TriBlade,·RamBlade,·SixBlade, website
· Single Board Computer:·3 Propeller ICs·and a·TriBladeProp board (ZiCog Z80 Emulator)
· Prop Tools under Development or Completed (Index)
· Emulators: CPUs Z80 etc; Micros Altair etc;· Terminals·VT100 etc; (Index) ZiCog (Z80) , MoCog (6809)
· Search the Propeller forums·(uses advanced Google search)
My cruising website is: ·www.bluemagic.biz·· MultiBladeProp is: www.bluemagic.biz/cluso.htm

Toby Seckshund

Ariba

I am a bit confused (easy ) I assumed that the fifth pin was the pin5, and so as the demo board cct shows I would conect that to ground at he prop end. The shield should be loose bonded for discharge reasons via a high value resistor and possibly a small cap. The other pin (pin4) was a mystery to me until you showed the diagram, but the demo board shows that unconected. So hopefully the question was about the shield and not the Pin4. Perhaps the shield should have been named "Pin6" to prevent this confusion.

The normal non-mini has only the four conns so that will only have 5 points of contact, the fifth being the shield, perhaps that was the question.

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Post Edited (Toby Seckshund) : 12/29/2009 10:37:11 PM GMT