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USB Data Lines Clamping

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I'm taking the leap to USB interfaces for my projects, but I'm having problems with voltage limiting on the USB data lines. I prefer to use a +5v supply because many of the great IC's out there need it to run stability, but that means using a couple of 3.6v zeners on the USB data lines with a 1K5 pull-up on D- (and maybe a 15K pull-down on D+???).

I know I need to use low capacitance zeners (most people say low wattage, but it usually amounts to the same thing in that respect), but I still have no luck! I've tried about a dozen different zeners, and I always end up with a "unknown device" error from the O.S. and this is with a circuit and code known to function (a very very simple SNES controller to USB adapter that I chose to learn USB from).

From my measurements of the most recent zener I used, the data lines don't exceed 3.32v, I made a rig to check capacitance in-circuit, and it doesn't exceed 290pF and that is consistent with the data sheet (which is better than the parts specified in working circuits). And yes, I know how to build circuits and program MPU's. I've made hundreds over the last 20 years, it's just a problem with USB interfacing, and I suspect the clamping zeners, because that is always a unreliable and unpredictable way of doing things where precision is needed.

Can the USB cable be an issue? I didn't use a cheap one, resistance and capacitance should be low. Any ideas?

Perhaps I need to request the help of a electro-exorcist?

Just a thought; Is there a nice little IC out there that I can put between the MCU and the USB port that runs from a +5v supply that can do the trick? If not, maybe I should talk to the awsome engineers at Maxim!
 
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Sceadwian

Banned
You're checking your high voltage, but what about your low voltage?
 

ericgibbs

Well-Known Member
Most Helpful Member
hi,
If a remote IC is designed for USB use, usually the USB connection lines will be enabled for 3.3V operation.??

The 18F USB series of PIC's have on board Vusb generators for the 3.3Vusb.

Do you have an example of an IC device you are trying to connect to the PC's USB.???
 

Sceadwian

Banned
All USB negotiating protocols by the standard start at the default low speed mode so yes they would start at the 3.3V voltage levels. After a high speed negotiation was finished it would then go to the alternate signaling levels.
 
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