On Sat, Mar 28, 2015 at 7:03 AM, Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > On 28/03/15 00:43, Andrew Bresticker wrote: >> Add a binding document for a generic ADC keypad. Buttons on an ADC >> keypad are connected in a resistor ladder to an ADC. The binding >> describes the mapping of ADC channel and voltage ranges to buttons. >> >> Signed-off-by: Andrew Bresticker <abrestic@xxxxxxxxxxxx> >> Cc: Rob Herring <robh+dt@xxxxxxxxxx> >> Cc: Pawel Moll <pawel.moll@xxxxxxx> >> Cc: Mark Rutland <mark.rutland@xxxxxxx> >> Cc: Ian Campbell <ijc+devicetree@xxxxxxxxxxxxxx> >> Cc: Kumar Gala <galak@xxxxxxxxxxxxxx> > > Just thinking about this, what options do we have for how such a keypad might > be wired. The interesting cases are the more than one key at a time ones. > > If that happens, then we end up with a voltage that should allow us to > work out which combination of keys is pressed. > > http://en.wikipedia.org/wiki/Resistor_ladder > > The binding as it stands only works for single keys being pressed at a time > (I think!) Right. The binding was meant for the simplest resistor ladder case, something like http://linksprite.com/wiki/images/d/d2/Lcd-button-ladder.png where only a single button press can be detected. It will also work with the more complex resistor ladders, but will only be able to detect if a single button is pressed. > Do we want to make it flexible enough to cope with multiple keys? > I guess we'd need to model the common resistor ladder forms and provide > a way of specifying the different setups in the device tree. Given that the hardware I'm dealing with doesn't support this, I'd prefer to leave it as a possible future extension :). That said, one way to do this would be to specify a table mapping the possible combinations to voltages in the DT, though that table would grow exponentially. Another way would be to describe the circuit (R-2R, logarithmic, etc.) with resistor values in the DT and then have the driver do the math. Thanks, Andrew -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
