USB device detection voltage needed?

Phishfry

Phishfry

When I connect a USB cable to my flight controller it powers the device.

That can be undesirable in certain situations so I made a custom cable with only D+ and D- attached.
Sort of a reversed USB Power Cable. A data only cable.

Problem is when I plug it in to my laptop the USB device is not detected. No power means no recognition.

My question is what is the minimum voltage required for the system to pick up a USB device attached to a USB-A type jack.
Is it 5V that is standard on USB1/2?

What about USB3 ports? I know they have wide power range.
 
As far as I know it's always 5V except some chargers that use pulsating voltage to increase charging speed and battery lifetime of some devices.

The controller doesn't have some internal startup procedure that delays the moment it starts listening?
 
Phishfry said:
When I connect a USB cable to my flight controller it powers the device.

That can be undesirable in certain situations so I made a custom cable with only D+ and D- attached.
Sort of a reversed USB Power Cable. A data only cable.

Problem is when I plug it in to my laptop the USB device is not detected. No power means no recognition.

My question is what is the minimum voltage required for the system to pick up a USB device attached to a USB-A type jack.
Is it 5V that is standard on USB1/2?

What about USB3 ports? I know they have wide power range.
Click to expand...
USB A ports max out at 5v. From what I can tell all USB 1, 2 and 3 ports put out 5v. USB C ports can deliver considerably more and you'd want a USB PD board to handle that.

USB 1 and 2 ports put out 2.5 watts and 3 goes up to 4.5 watts. But at the same voltage.
 
Phishfry said:
When I connect a USB cable to my flight controller it powers the device.

That can be undesirable in certain situations so I made a custom cable with only D+ and D- attached.
Sort of a reversed USB Power Cable. A data only cable.

Problem is when I plug it in to my laptop the USB device is not detected. No power means no recognition.

My question is what is the minimum voltage required for the system to pick up a USB device attached to a USB-A type jack.
Is it 5V that is standard on USB1/2?

What about USB3 ports? I know they have wide power range.
Click to expand...

Did you connect ground as well? You need to connect ground for this to work.
 
Phishfry said:
That can be undesirable in certain situations
Click to expand...
Which ones? The equipment must be powered, whether from one side or the other makes no difference, the equivalent internal resistance of the voltage sources being much higher than the wire which connects them, the power dissipated by slightly different voltage sources will be low.
 
teekay said:
You need to connect ground for this to work.
Click to expand...
That sounds reasonable. I will try that.

Stupid related question: With UART you can operate with only one signal line attached.
For instance a GPS is transmit only. So only TX line needs attaching to connector..

Is the same true for USB? Can you use transmit only with USB signaling scheme?
D+ and D- do not seem the same as RX/TX.
 
doul said:
Which ones?
Click to expand...
OK here is the situation in full.

Flight Controller is powered by batteries. I have a switch for powering device.
But plug in USB and it powers up. Regardless of switch. I dislike that.
Why:
Well lets say I go to reboot Flight Controller. Issue software command and it reboots.
But with "shore power" onboard it does not really seem to truly reboot.
Most sensors are not reinitialized. More like a soft boot.

Many times I would just issue a reboot command to FC and flick the power switch.
Just to ensure cold boot. With USB also providing power that becomes messy.

I really should have considered the Ground wire.
That was an oversight on my behalf. I will try that soon.
More of a creature comfort to prevent wear and tear on USB jacks.

The reason I ask about voltage is I have 3.3VDC source available but unsure if it is proper.
 
I feel that any configuration that violates the USB layer 1 standard is a bad idea. IIRC, there is no (D+/D- only) configuration in USB. Where is your flight controller switch located? What is the unit behaviour when no batteries are installed? Provided that the unit does not come online when batteries are removed, I'd suggest your switch remove battery power only and you use a conforming USB cable. Addition of a ground might "seem" to fix the problem but it is still a violation of the standard.

This thread reminds me very much of that historical situation where some PC COMM chips would trigger on TTL signals, instead of requiring conforming RS232 signal levels, which led to a period where where many custom hardware folks didn't think they needed to invest in true RS232 driver chips.
 
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