[Arm-netbook] How best to get touch working on EOMA?

luke.leighton luke.leighton at gmail.com
Mon Dec 9 19:56:15 GMT 2013

On Mon, Dec 9, 2013 at 2:52 PM, Aaron J. Seigo <aseigo at kde.org> wrote:
> On Monday, December 9, 2013 14:23:38 joem wrote:
>> I'm thinking may be a better bet is to have a mouse
>> like generic USB gadget built. Then you would connect
>> the resistive or capacitive panel to it,
>> and it would convert the signals to standard
>> mouse signals.
> Usually the real trick is the multi-touch events and proper screen calibration
> with X. Having a generic USB gadget for this would be fantastic ...

 on the flying squirrel i considered doing the conversion by having
the I2C capacitive devices connected to the STM32F (and then to the
ATSAM4SB on the rev3 board) but to be honest it's a lot of hassle.  in
the end, so as not to have to write any special linux kernel drivers,
i went with just a plain I2C IC (built-in to the panel in order to
reduce stray capacitance SNR leakage).

 for those capacitive converter ICs that have USB, they already have
linux kernel drivers: no work is needed there either.

 for resistive ICs, the standards are either SPI or I2C - not USB.
extra work, both in hardware and software, would be needed.

>> EOMA doesn't have the touch pins brought out,
>> so having such a modular unit could benefit EOMAs and
>> other similar boards.

 ok for capacitive "keys", this is already done (in some form or other):


 for capacitive touchpanels you *really* do not want to be getting
individual ICs: the stray capacitance of having the IC far away from
the actual panel is so far above the capacitive panel's capacitance
that you get wildly inaccurate readings.  therefore you should be
looking at an FPC breakout board and just wiring directly up on the
EOMA68 I2C bus.  problem solved.

 for resistive i'm having a little more difficulty tracking anything
down, notably because most of sparkfun's stuff is designed for
arduinos etc. etc. all of which have at least two ADC pins.  so, you'd
need to e.g. follow this:


and then get an I2C ADC Converter IC e.g. this:


 (ok get 2 of those) plus a break-out board for a SOT-23 (or 2) and
you're pretty much done.

the alternative is of course to use something like an arduino PIC, or
one of those Freescale $0.40 embedded controllers.  then, aaron, yes,
absolutely: you'd be programming it (unless the source is already
available like it is for many arduino-style project) to put it into
USB HID mode.

 but, if you're doing it as I2C then you'd need a linux driver kernel
module on whatever EOMA-68 CPU Card is being used, reading the I2C bus
but written as a mouse hid device.  straightforward enough.


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