USB host (was Re: What's the real problem with wireless on the Ben?)
Ron K. Jeffries
rjeffries at gmail.com
Tue Oct 4 12:45:39 EDT 2011
Werner's self-described USB host "hack" would significantly increase
sales of Ben Nanonote. When NN is used at home or office,
adding a USB hub would allow connecting a large, comfortable keyboard
and a mouse.
Naturally adding USB host via Ben's 8:10 port and UBB
offers a Hobbsian choice: do you want/need 802.15.4 radio,
VGA, or USB host.
Ron K. Jeffries
On Tue, Oct 4, 2011 at 09:03, Werner Almesberger <werner at almesberger.net>wrote:
> I wrote:
> > That's what I'd do, yes. UBB, a USB A receptacle, one pull-down
> > resistor, and a bit of ribbon cable should get you quite far.
> Like this:
> On the top is the standard configuration of low-speed host and
> device. The pull-up R1 indicates to the host that this is a
> low-speed device. D+ has no pull-up on the device side and is
> therefore pulled low by the host's R4.
> The pull-ups and pull-downs are not only used for device speed
> identification, but also for driving the signals to defined
> levels if neither side is driving them.
> R2 and R3 define the impedance of the drivers but can probably
> be omitted in our case, since the CPU's I/O pins already have
> an impedance roughly in that range.
> The UBB-USB hack is below. Inside the Ben, all signals but CLK
> are pulled up. This means that we can only used CLK with a weak
> pull-down (alternatively, we could try to add a strong
> pull-down, but that may have unwanted side-effects.)
> The pull-down would be between GND and CLK and be soldered
> directly to UBB. Then a ribbon cable would connect to a USB
> This setup assumes that the USB device is content with 3.3 V.
> If it isn't, an external 5 V supply could be used for
> development, e.g., from a lab power supply or by tapping the
> Ben's inbound VBUS.
> - Werner
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