[PATCH v3 3/4] dt-bindings: input: add GPIO charlieplex keypad

[Geert Uytterhoeven]

Hi Hugo,

On Wed, 25 Feb 2026 at 16:54, Hugo Villeneuve <hugo at hugovil.com> wrote:
> From: Hugo Villeneuve <hvilleneuve at dimonoff.com>
>
> Add DT bindings for GPIO charlieplex keypad.
>
> Signed-off-by: Hugo Villeneuve <hvilleneuve at dimonoff.com>

Thanks for your patch!

> --- /dev/null
> +++ b/Documentation/devicetree/bindings/input/gpio-charlieplex-keypad.yaml
> @@ -0,0 +1,106 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +
> +$id: http://devicetree.org/schemas/input/gpio-charlieplex-keypad.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: GPIO charlieplex keypad
> +
> +maintainers:
> +  - Hugo Villeneuve <hvilleneuve at dimonoff.com>
> +
> +description: |
> +  The charlieplex keypad supports N^2)-N different key combinations (where N is
> +  the number of lines). Key presses and releases are detected by configuring
> +  only one line as output at a time, and reading other line states. This process
> +  is repeated for each line. Diodes are required to ensure current flows in only
> +  one direction between any pair of pins.
> +  This mechanism doesn't allow to detect simultaneous key presses.

Indeed, e.g. pressing S1 and S2 simultaneously will show a ghost
S5 keypress.

> +
> +  Wiring example for 3 lines keyboard with 6 switches and 3 diodes:
> +
> +  L0  --+---------------------+----------------------+
> +        |                     |                      |
> +  L1  -------+-----------+---------------------+     |
> +        |    |           |    |                |     |
> +  L2  -------------+----------------+-----+    |     |
> +        |    |     |     |    |     |     |    |     |
> +        |    |     |     |    |     |     |    |     |
> +        |  S1 \  S2 \    |  S3 \  S4 \    |  S5 \  S6 \
> +        |    |     |     |    |     |     |    |     |
> +        |    +--+--+     |    +--+--+     |    +--+--+
> +        |       |        |       |        |       |
> +        |    D1 v        |    D2 v        |    D3 v
> +        |       - (k)    |       - (k)    |       - (k)
> +        |       |        |       |        |       |
> +        +-------+        +-------+        +-------+

Don't you need pull-down resistors on L[0-2], and/or a way to specify
in DT to enable internal poll-down on GPIO controllers that support it?
Some controllers may support internal pull-up only, but I guess that
can be handled using GPIO_ACTIVE_LOW?

> +
> +  L: GPIO line
> +  S: switch
> +  D: diode (k indicates cathode)
> +
> +allOf:
> +  - $ref: input.yaml#
> +  - $ref: /schemas/input/matrix-keymap.yaml#
> +
> +properties:
> +  compatible:
> +    const: gpio-charlieplex-keypad
> +
> +  autorepeat: true
> +
> +  debounce-delay-ms:
> +    default: 5
> +
> +  line-gpios:
> +    description:
> +      List of GPIOs used as lines. The gpio specifier for this property
> +      depends on the gpio controller to which these lines are connected.
> +
> +  linux,keymap: true
> +
> +  poll-interval: true
> +
> +  settling-time-us: true
> +
> +  wakeup-source: true
> +
> +required:
> +  - compatible
> +  - line-gpios
> +  - linux,keymap
> +  - poll-interval
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    #include <dt-bindings/gpio/gpio.h>
> +    #include <dt-bindings/input/input.h>
> +
> +    charlieplex-keypad {

"keyboard", as per Devicetree Specification Generic Names
Recommendation.

> +        compatible = "gpio-charlieplex-keypad";
> +        debounce-delay-ms = <20>;
> +        poll-interval = <5>;
> +        settling-time-us = <2>;
> +
> +        line-gpios = <&gpio2 25 GPIO_ACTIVE_HIGH
> +                      &gpio2 26 GPIO_ACTIVE_HIGH
> +                      &gpio2 27 GPIO_ACTIVE_HIGH>;
> +
> +        /* MATRIX_KEY(output, input, key-code) */
> +        linux,keymap = <
> +            /*
> +             * According to wiring diagram above, if L1 is configured as
> +             * output and HIGH, and we detect a HIGH level on input L0,
> +             * then it means S1 is pressed: MATRIX_KEY(L1, L0, KEY...)
> +             */
> +            MATRIX_KEY(1, 0, KEY_F1) /* S1 */
> +            MATRIX_KEY(2, 0, KEY_F2) /* S2 */
> +            MATRIX_KEY(0, 1, KEY_F3) /* S3 */
> +            MATRIX_KEY(2, 1, KEY_F4) /* S4 */
> +            MATRIX_KEY(1, 2, KEY_F5) /* S5 */
> +            MATRIX_KEY(0, 2, KEY_F6) /* S6 */
> +        >;
> +    };

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds