4 ms·

What actually happens when you plug in a USB device? (2007)

92 points by mafuyu 12y ago

pje 12y ago
I wonder how much of this applies to Darwin systems.
- duskwuff 12y ago
  Basically none of it, beyond the information in the "USB overview" section and a few scattered sentences elsewhere. This article is talking about the Linux USB stack in particular, not USB in general.
swetland 12y ago
Oddly (for how detailed it is in other places) this skips the actual wire protocol side of enumeration -- reset device, read descriptors, assign address, etc. Enumeration is an interesting process -- newly reset devices accept transfers to address zero, until the host assigns a non-zero addres. Then the next device (if their are additional devices detected on a hub, or if the just-enumerated device is a hub with devices below it) can be enumerated (initially responding at 0 after reset), walking the entire tree of hubs and devices (and revisiting it as necessary when hubs report connections and disconnections).
- delsarto 12y ago
  (original author here) Yeah, it was done after I was asked to run a short course on writing drivers, so the focus was on understanding that sort of layer. But you want to know how old this is? I actually used the word "palmtop" :)
porsupah 12y ago
On a related note: back in a previous, embedded incarnation, I was once tasked with writing a USB driver. Not the kind that might first come to mind - this was for the /device/, for which documentation is rather less easily found. With the aid of this resource (no, no spam), I was able to make our gadget come up as a fully compliant composite HID/microphone device. http://www.beyondlogic.org/usbnutshell/usb1.shtml http://www.beyondlogic.org/usbnutshell/usb1.shtml
- tcas 12y ago
  That is a great link. Also USB Made Simple is good addition to that. It's pretty amazing that the protocol works in the first place. It's a multi-driver, differential bus with 3 valid states (not including bus arbitration). It does all sorts of dirty things, like in order to identify a device as a USB1.0 or USB1.1 device, a resistor is either applied to D- or D+. In order to determine if the host/device should switch to USB2.0 mode, the host computer will drive both lines to low (SE0) and the device will send high current "chirps" back to the host. Essentially having multiple hosts driving the line at the same time. http://www.usbmadesimple.co.uk/ http://www.usbmadesimple.co.uk/
kabdib 12y ago
... and then, on Windows, you get to the Plug-and-Play stack, and horrible, eldritch things happen, many of which involve registry entries. Also, when I found that "SetupApi" was The Way you got to USB devices on Windows, I wanted to strangle somebody.
- derefr 12y ago
  > many of which involve registry entries You say this like it's a Windows-specific thing. In Linux, a lot of the USB support is exposed through /sys—pretty much the same idea.
  - gioele 12y ago
    Aren't, in this case, the window registry entries involved are more similar to udev rules?
Gorkys 12y ago
Ok, I', not that young but WTF is a "palmtop" and when did such a thing exist?
- hayksaakian 12y ago
  like "lap top" but it goes on top of the palm of your hand (at least that's my revisionist version)
- deleted 12y ago
  [deleted]
- delsarto 12y ago
  When I wrote this, I'm not sure if the iPhone had been released; certainly at that point if you were carrying around something computer-like in your (large :) pocket (more powerful than a PalmPilot) it was something like a Jornada http://en.wikipedia.org/wiki/Jornada_(PDA) http://en.wikipedia.org/wiki/Jornada_(PDA) This sort of form-factor came directly out of things calld "palmtop pc's" like http://en.wikipedia.org/wiki/HP_95LX http://en.wikipedia.org/wiki/HP_95LX
  - tesseract 12y ago
    To me "palmtop" strongly connotes something handheld, with a clamshell or a slide-out keyboard, and running a desktop-style OS. That HP 95LX is a good example; the OQO was a more recent one (2005 ish?). A device with a similar form factor but running a mobile OS would have been called a "MID" (mobile internet device) or possibly a tablet - https://en.wikipedia.org/wiki/Nokia_770_Internet_Tablet https://en.wikipedia.org/wiki/Nokia_770_Internet_Tablet for example.
- esrauch 12y ago
  Palmtop is typically used interchangeably with PDA.
- dTal 12y ago
  How about an IBM PC-110? http://www.basterfield.com/pc110/pc110idx.htm http://www.basterfield.com/pc110/pc110idx.htm
kimonos 12y ago
Interesting! Thanks for the info!
deciplex 12y ago
The first thing that happens when you plug in a USB device is you sigh in relief that the thing finally went in, on the third attempt. Then all this other stuff happens.
- roevhat 12y ago
  So true :-)
- teh_klev 12y ago
  No you have to observe the plug first, then it'll go in: http://imgur.com/ZG4L1nc http://imgur.com/ZG4L1nc
officialjunk 12y ago
Related is this talk on reverse engineering USB devices from 28c3: http://media.ccc.de/browse/congress/2011/28c3-4847-en-reverse_engineering_usb_devices.html http://media.ccc.de/browse/congress/2011/28c3-4847-en-revers...

92 points by mafuyu 12y ago

pje 12y ago

I wonder how much of this applies to Darwin systems.

duskwuff 12y ago
Basically none of it, beyond the information in the "USB overview" section and a few scattered sentences elsewhere. This article is talking about the Linux USB stack in particular, not USB in general.

swetland 12y ago

Oddly (for how detailed it is in other places) this skips the actual wire protocol side of enumeration -- reset device, read descriptors, assign address, etc. Enumeration is an interesting process -- newly reset devices accept transfers to address zero, until the host assigns a non-zero addres. Then the next device (if their are additional devices detected on a hub, or if the just-enumerated device is a hub with devices below it) can be enumerated (initially responding at 0 after reset), walking the entire tree of hubs and devices (and revisiting it as necessary when hubs report connections and disconnections).

delsarto 12y ago
(original author here) Yeah, it was done after I was asked to run a short course on writing drivers, so the focus was on understanding that sort of layer. But you want to know how old this is? I actually used the word "palmtop" :)

porsupah 12y ago

On a related note: back in a previous, embedded incarnation, I was once tasked with writing a USB driver. Not the kind that might first come to mind - this was for the /device/, for which documentation is rather less easily found. With the aid of this resource (no, no spam), I was able to make our gadget come up as a fully compliant composite HID/microphone device. http://www.beyondlogic.org/usbnutshell/usb1.shtml http://www.beyondlogic.org/usbnutshell/usb1.shtml

tcas 12y ago
That is a great link. Also USB Made Simple is good addition to that. It's pretty amazing that the protocol works in the first place. It's a multi-driver, differential bus with 3 valid states (not including bus arbitration). It does all sorts of dirty things, like in order to identify a device as a USB1.0 or USB1.1 device, a resistor is either applied to D- or D+. In order to determine if the host/device should switch to USB2.0 mode, the host computer will drive both lines to low (SE0) and the device will send high current "chirps" back to the host. Essentially having multiple hosts driving the line at the same time. http://www.usbmadesimple.co.uk/ http://www.usbmadesimple.co.uk/

kabdib 12y ago

... and then, on Windows, you get to the Plug-and-Play stack, and horrible, eldritch things happen, many of which involve registry entries. Also, when I found that "SetupApi" was The Way you got to USB devices on Windows, I wanted to strangle somebody.

derefr 12y ago
> many of which involve registry entries You say this like it's a Windows-specific thing. In Linux, a lot of the USB support is exposed through /sys—pretty much the same idea.
- gioele 12y ago
  Aren't, in this case, the window registry entries involved are more similar to udev rules?

Gorkys 12y ago

Ok, I', not that young but WTF is a "palmtop" and when did such a thing exist?

hayksaakian 12y ago
like "lap top" but it goes on top of the palm of your hand (at least that's my revisionist version)
deleted 12y ago
[deleted]
delsarto 12y ago
When I wrote this, I'm not sure if the iPhone had been released; certainly at that point if you were carrying around something computer-like in your (large :) pocket (more powerful than a PalmPilot) it was something like a Jornada http://en.wikipedia.org/wiki/Jornada_(PDA) http://en.wikipedia.org/wiki/Jornada_(PDA) This sort of form-factor came directly out of things calld "palmtop pc's" like http://en.wikipedia.org/wiki/HP_95LX http://en.wikipedia.org/wiki/HP_95LX
- tesseract 12y ago
  To me "palmtop" strongly connotes something handheld, with a clamshell or a slide-out keyboard, and running a desktop-style OS. That HP 95LX is a good example; the OQO was a more recent one (2005 ish?). A device with a similar form factor but running a mobile OS would have been called a "MID" (mobile internet device) or possibly a tablet - https://en.wikipedia.org/wiki/Nokia_770_Internet_Tablet https://en.wikipedia.org/wiki/Nokia_770_Internet_Tablet for example.
esrauch 12y ago
Palmtop is typically used interchangeably with PDA.
dTal 12y ago
How about an IBM PC-110? http://www.basterfield.com/pc110/pc110idx.htm http://www.basterfield.com/pc110/pc110idx.htm

kimonos 12y ago

Interesting! Thanks for the info!

deciplex 12y ago

The first thing that happens when you plug in a USB device is you sigh in relief that the thing finally went in, on the third attempt. Then all this other stuff happens.

roevhat 12y ago
So true :-)
teh_klev 12y ago
No you have to observe the plug first, then it'll go in: http://imgur.com/ZG4L1nc http://imgur.com/ZG4L1nc

officialjunk 12y ago

Related is this talk on reverse engineering USB devices from 28c3: http://media.ccc.de/browse/congress/2011/28c3-4847-en-reverse_engineering_usb_devices.html http://media.ccc.de/browse/congress/2011/28c3-4847-en-revers...