Anyone has experience in connecting a 20X4 parallel LCD with 3X4
Keypad directly, displaying the depressed key + sending the info BX-
thanks in advance
--- In basicx@basi..., "Don Kinzer" <dkinzer@...> wrote:
> --- In basicx@basi..., "arhodes19044" <spamiam@c...> wrote:
> > I have not tested the limit of the number of potential
> > keys, but 3 is no problem.
> I believe that 3 is a problem in one particular case. Consider
> keys that reside at the corners of any rectangle
in the matrix.
> you press any three of them, the fourth will
appear to be pressed
> whether it is or not. This is known as a "phantom key" and is
> by there being a conductive path from a row line,
> closed keyswitches to the column line of the key
that is not
> thus making it appear that a key is closed at that
> intersection. The common way of preventing this is to use diodes
> series with the keys to prevent the circuitous
> > This does use 7 pins, which is significant on the BX platform.
> > reference to a multiplexing technique is well
> It occurred to me that you might be able to connect a 4x4 keypad
> only 2 I/O lines and only one external component -
a PCF8574 I/O
> expander. If you already have other I2C devices connected, the
> incremental I/O line usage would be zero since you could use the
> SDA/SCL lines.
> Although I haven't prototyped this circuit, I believe that it
> work to connect the row/column lines directly to
> lines with no resistors. The expander's output
stage is designed
> be tolerant of being shorted to ground when in the
logic 1 state.
> fact, the way that you prepare a pin for input is
to write a 1 to
> bit. The PCF8574 and PCF8574A are about $2 at
> difference between the two parts is the I2C base address.
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