Does USB supply a "constant" 5V or "up-to" 5V?

Question

I am trying to reverse engineer a digitizer* inside an old laptop. I am fairly certain it is a USB device because I noticed a twisted pair of cables next to a ground wire. Just to make sure it is USB I probed the pin to the other side of (what I suspect to be) D-. When I did this I saw different voltages flashing very quickly but nothing close to 5V. I believe the max I saw was 1.2V and min being somewhere around .3V.

I don't want to wire this up to a USB cable out of fear of destroying the digitizer if it can't in fact handle 5V.

This would make sense to be USB VCC if it does offer up to 5V but I couldn't find anything definitive that I could understand at least.

* It's a Wacom digitizer in a Fujitsu T901 laptop from about the year 2011.

Classic USB had 4 wires. Power, Ground, data+ and data-. Power will be 5V when present (with some tolerance). Ground is defined as zero Volts. Data+ and Data- are often 3.3V signals, but they are not required to be. — user57037, Jan 30 '19 at 06:41
Apparently there is a mix-up between VBUS power over USB, with signaling levels over D+/D- data lines. I am just curious, what kind of USB "digitizer" could be in "an old laptop"? — Ale..chenski, Jan 30 '19 at 06:52
From what I understand about power supplies, the voltage level is not ever *constant*; it fluctuates within a range (4.75V to 5.25V for USB, as pointed out in Jasen's answer). — Agi Hammerthief, Jan 30 '19 at 14:14
If the laptop still operates, look at the USB device tree in your OS to determine if its a USB device. — sstobbe, Jan 30 '19 at 16:13
If you have access to a 'scope (eg at work/school), use it and show us what it looks like. It's pretty easy to tell if it's USB signaling. — yhyrcanus, Jan 30 '19 at 18:24

score 19 · Answer 1 · edited Mar 16 '22 at 15:50

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TL;DR just because it's connected to the USB does not mean 5V or even that it is powered from USB.

For versions 1 of USB The spec says 5V +/- 5%, so you should design to accept 4.75V to 5.25V.

For USB 2.0 the spec says: • The voltage supplied by high-powered hub ports is 4.75 V to 5.50 V. • The voltage supplied by low-powered hub ports is 4.4 V to 5.50 V.

USB Version 3.something introduces the availability of increased voltages (up to 20V). but a device has to explicitly request increased voltage, else they just get the same Approximately 5V.

When I plug my phone into its charger via a power meter I see the voltage jump to 9V

My laptop's charger uses a USB C connector and can provide 5,9,15, or 20V (according to the label)

Now, your laptop's internal device may be designed for some voltage other than 5V. The USB data lines D+ and D- signal at only 3.3V. So, 3.3V could instead be the supply to the (touch-screen) digitizer.

In general the USB host can't see the device's supply voltage, it only sees the data signals. Some USB devices (eg: laser printers) do not use the USB VCC line for power.

edited Mar 16 '22 at 15:50

Richie Ellingham

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answered Jan 30 '19 at 08:13

Jasen Слава Україні

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*"Some USB devices (eg: laser printers) do not use the USB VCC line at all."* - this would render the USB device quite uncertifiable... which would be strange for such reputable and expensive devices as a printer. – Ale..chenski Jan 31 '19 at 04:20
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What's the minimum VCC current draw for a compliant device? – Jasen Слава Україні Jan 31 '19 at 06:06
There is no minimum, but there is a maximum of 100 mA a compliant device can use before being configured. Why do you ask? – Ale..chenski Jan 31 '19 at 07:06
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my reasoning is that because 0mA ia acceptable, not using the VCC line is legal. – Jasen Слава Україні Jan 31 '19 at 07:16
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"not using the VCC line is legal" - no it is not. A compliant USB device must use VBUS (your "VCC") to start connection (pull one of D line up, depending on device speed). And it must not start connect signaling until VBUS is up. See my answer here, https://electronics.stackexchange.com/a/323551/117785 – Ale..chenski Jan 31 '19 at 17:40
ah, so it must at minimum sense acceptable voltage on VBUS before beginning signalling, `s/at all/for power/` – Jasen Слава Україні Feb 01 '19 at 00:48

score 8 · Answer 2 · answered Jan 30 '19 at 06:44

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USB Vcc is 5V - constant.

It should only drop if you draw too much current, in which case a PTC (automatically resetting fuse) will open and shut off the current. In that case the voltage will drop to near zero.

The voltage on the data lines is a different story. D+ and D- transmit data at high speed. Your multimeter can't make any sense of what is there - it changes way too fast for the meter.

answered Jan 30 '19 at 06:44

JRE

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Also note that the D+ and D- signals communicate through a differential-mode driver. More information here: https://electronics.stackexchange.com/a/303474/175287 – Paul Belanger Jan 30 '19 at 14:54

score 6 · Answer 3 · answered Jan 30 '19 at 12:12

If there isn't a clear sign that it is USB, consider that it could be SPI, I2C/SMBus or some other form of serial connection, particularly for an internal device. These are (and were) commonly used for internal data connections at fairly low data rates.

What did you probe the signal with? If a voltmeter, the readings of 0.3 to 1.2V could be (or almost certainly are) misleading on a data line. If an oscilloscope, can you post a picture of the signal?

It would also help if you post a picture (or details of markings) of any ICs on the digitizer, and any silkscreen markings on the PCB where the suspected USB line is attached.

it's common for a laptop's digitiser to present as a USB HID pointer device, but yeah it's also possible that that the interface to the digitiser is not USB and some intermediate device provides that translation. — Jasen Слава Україні, Apr 09 '21 at 00:42

score 0 · Answer 4 · answered Jan 30 '19 at 06:41

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USB will power 5v constant. If your digitizer has unkonwn input voltage requirement, it is better to do more research or ask the manufacturer of the device or laptop. If you directly power it for experiment, it may blow some component or IC which maybe very difficult to find in the open market.

answered Jan 30 '19 at 06:41

Rajesh Kakkad

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No, just as any other voltage source, a USB port has an internal impedance. The USB spec is actually explicit about the *voltage range* permitted for USB supplies und specified load. – Marcus Müller Mar 01 '19 at 11:33

score 0 · Answer 5 · answered Jan 30 '19 at 15:54

I recently bought two cheap measurent devices, one for Micro USB and another one for USB-C to verify the wall-charger/cable/QI-charger chain for different devices. Nowadays USB seems to constantly haggle about voltage and wattage! Some of my chargers would provide up to 12 V through USB (far off the original specification) but some devices would even consume it under varying circumstances.

To cut a short story shorter: You will not get around to measure your specific case. The specifications are interpreted in enough ways to ake this necessary.

score 0 · Answer 6 · answered Jan 30 '19 at 16:25

It sounds like you probed a data line.

USB supplies 5V for power. However, signalling is at lower voltages. USB 1.1 used 3.3V signalling, and USB 2.0 High Speed uses about 400mV. If you could see the line changing then it was likely a data line.

That is if it was USB at all of course. It could be PS2 or something else.

Does USB supply a "constant" 5V or "up-to" 5V?

6 Answers6