USB port, what kind of signal does it provide ? - Page 1
Forum section: electronics / beginners

author: wexeteme
author_rank: Contributor
timestamp: March 09, 2015, 05:09:39 pm
content: Hello,As far as I know the USB cable (using Vcc and Gnd wire) provides DC signal 5 volts, but today some guy mention that it produces square signal with a frequency of 1Mhz, so what is really happening ?Environment =>the cable is just plugged in to the Device, and its using its output for something, aint really matter what is it ...Thanks!

author: Monkeh
author_rank: Super Contributor
timestamp: March 09, 2015, 05:17:45 pm
content: The USB VBus pin indeed provides a simple +5VDC supply.Whoever told you it's a square wave is an idiot.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 09, 2015, 05:19:12 pm
content: Well, you can look up the standards, but that could be a bit of a data dump. Bottom line is the USB cable has Vcc (+5V), Ground, Data+ and Data-. I'd guess that the 1MHz square wave he was talking about would be the data.http://en.wikipedia.org/wiki/USB

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 09, 2015, 05:24:40 pm
content: The original USB (1.x, 2.x) interface has 4 pins. As you said, two are ground and power, and the other two are the (single) data path, but they are a balanced, differential PAIR.The data path is "half-duplex", that is, it sends data in both directions, but only in one direction at a time.The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devices. For USB 3.0, typical write speed is 70-90 MB/s, while read speed is 90-110 MB/s. (http://en.wikipedia.org/wiki/USB)A "square signal with a frequency of 1Mhz" sounds extraordinarily unlikely. I suspect that "guy" doesn't know what he is talking about.

author: wexeteme
author_rank: Contributor
timestamp: March 09, 2015, 05:25:06 pm
content: alright, thanks

author: Jeroen3
author_rank: Super Contributor
timestamp: March 09, 2015, 05:35:20 pm
content: USB power is 4.75 to 5.25 V. And can have significant noise depending on the quality of the host. Remember a maximum of 10 uF is allowed as slave-capacitance from VBus to GND.Read the specs yourself, they're free.http://www.usb.org/developers/docs/http://www.usb.org/developers/docs/usb20_docs/

author: Monkeh
author_rank: Super Contributor
timestamp: March 09, 2015, 05:38:34 pm
content: Quote from: Jeroen3 on March 09, 2015, 05:35:20 pm5.5V.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 09, 2015, 06:42:34 pm
content: USB Chargers are typically 5.5 Volts to compensate cable loss. This should not harm the electronics.The 3.1 spec says:

author: Monkeh
author_rank: Super Contributor
timestamp: March 09, 2015, 06:51:14 pm
content: Quote from: Jeroen3 on March 09, 2015, 06:42:34 pmPlease read the entire spec, not the unaltered original document. It comes in a big zip for a reason.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 09, 2015, 06:59:51 pm
content: I see. A zip file with a ton of pdf's can be ambiguous.Quote

author: mariush
author_rank: Super Contributor
timestamp: March 09, 2015, 07:20:42 pm
content: A lot of cheap chargers use a 5.6v zener diode to limit the voltage at low currents. So it can be a bit more than 5.5v, even though it shouldn't be more than 5.25vWith the thin wires they use, when the camera or phone or whatever you use draws a few hundred mA, the voltage drops close or below 5v.