USB host (was Re: What's the real problem with wireless on the Ben?)

Werner Almesberger werner at almesberger.net
Tue Oct 4 12:03:12 EDT 2011


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:
http://downloads.qi-hardware.com/people/werner/ubb/usb/concept.pdf

Source:
http://projects.qi-hardware.com/index.php/p/ben-blinkenlights/source/tree/master/ubb-usb/

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
receptacle.

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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