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Luke Gorrie

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USB [Apr. 7th, 2009|06:39 pm]
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Here's what I've discovered about some kinds of USB device:
  • Keyboard and mouse: they support a simplified bootstrap mode that's very simple to deal with. Here's a driver for the common parts of bootstrap-mode keyboard and mouse: 
    USBHID : USBDevice ()

USBHID init: devicename
[
  super init: (OFW open: devicename).
  self setConfiguration: 1.
  self setBootstrapProtocol.
]

USBHID setBootstrapProtocol
[
  self controlSetIndex: 0 value: 0
       requestType: (DR_HIDD bitOr: DR_OUT) request: SET_PROTOCOL
]

USBHID setIdle: ms
[
  self controlSetIndex: 0 value: (ms // 4 << 8)
       requestType: (DR_HIDD bitOr: DR_OUT) request: SET_IDLE
]

USBHID selftest
[
  self init.
  1000 timesRepeat: [ self readReport ifNotNilDo: [ :e | e println]. ]
]
    and here's a subclass that knows how to turn the reports into mouse events: 
    USBMouse : USBHID ()

USBMouse init         [ self init: '/mouse' ]
USBMouse init: device [ super init: device. self setIdle: 0 ]

USBMouse readReport   [ ^MouseEvent fromUSB: (self intrIn: 1) ]

MouseEvent fromUSB: report
[
  self := self new.
  left  := (report first bitAnd: 1) ~~ 0.
  right := (report first bitAnd: 2) ~~ 0.
  dx := report second.
  dy := report third.
  "Treat bit 8 as sign."
  (dx bitAnd: 0x80) ~~ 0 ifTrue: [ dx := dx - 256 ].
  (dy bitAnd: 0x80) ~~ 0 ifTrue: [ dy := dy - 256 ].
]
    The keyboard is similar: it reports the set of keys (identified by scancode) that are currently pressed, and uses a fixed mapping from scancodes onto key identifiers.

    I regret that I haven't come back to look at the full-blown USB Human Input Device protocol yet!

  • Ethernet: In theory the USB Communications Device Class specifies the high-level interface for ethernet-like network adapters, but in practice most USB-ethernet dongles seem to be built from an ASIX USB-ethernet chip. For example, the Apple USB-ethernet dongle is internally an ASIX 88772A.

    ASIX chips aren't compatible with the USB CDC standard, they use their own custom protocol instead. The protocol is simple but it's lower-level than CDC: you're sometimes peeking and poking registers instead of exchanging messages.

  • Display: My previous posts (first, second) describe the USB-VGA adapter I found. I'm unhappy with this device: taking a notoriously closed graphics chip and blindly bridging its PCI bus onto USB is really kludgy. The components don't even seem to be particularly cheap (Octopart suggests $15 for the USB-PCI bridge chip) so I suppose the intention was to save development money by not having to program the dongle. I'd be happier if they'd spent the same money on an ARM- or FPGA- based dongle and programmed it to do the job gracefully, but that's me being a naive software guy.
I'm finding driver writing frustratingly slow going. I'm used to a very experimental and exploratory programming style, but that's not very effective because when you make a mistake the devices won't tell you where! This reminds me of talking to SIM cards and web services. I'm looking for a smarter working style.
LinkReply

Comments:
[User Picture]From: [info]kragen
2009-05-20 12:28 am (UTC)

(Link)

Pretty cool stuff. Any idea what the simplified bootstrap interface is called?
From: (Anonymous)
2009-05-20 09:12 am (UTC)

(Link)

The details are in the main USB HID standard that's online in PDF -- it's called something like "bootstrap protocol" and intended for BIOSes. I'd chase up the link but I'm on a klunky airport internet terminal that only allows one non-tabbed Internet Explorer window to be open on the machine.. my god, what year is this?