Re: Is a multiplexer using latches a gpio controller?

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Mon, Jul 18, 2022 at 10:22 PM Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:

> we have a customer board here that uses two 8 port latches to drive
> LEDs. The setup looks as follows:

Very nice drawing!

> So to change output 2 of latch #1 you have to apply the changed level on
> IN2, apply the previous level on the other inputs (to keep the other
> outputs constant) and toggle CLK1 once.
>
> This way you can drive 16 LEDs (or in general outputs) using only 10
> GPIOs. (And with a higher number of latches the proportion becomes a bit
> more useful.)
>
> Actually this construct is a general GPO (no input :-) controller, and I
> wonder if you would accept a driver that models it as a gpio controller.

In my opinion, yes.

> The dt binding could look as follows:
>
> latch-gpo {
>         compatible = "latch-gpo";
>         pinctrl-0 = <...>;
>         pinctrl-names = <...>;
>
>         clk-gpios = /* CLK0 */ <...>, /* CLK1 */ <...>;
>         data-gpios = /* IN0 */ <...>, /* IN1 */ <...>, ...;
> };
>
> What do you think?

I would use the actual 74xx numbers for the TTL circuits in the compatible.
I have actually merged this placeholder (without bindings nor driver):

        /*
         * 74HC4094 which is used as a rudimentary GPIO expander
         * FIXME:
         * - Create device tree bindings for this as GPIO expander
         * - Write a pure DT GPIO driver using these bindings
         * - Support cascading in the style of gpio-74x164.c (cannot
be reused, very different)
         */
        gpio_74: gpio-74hc4094 {
                compatible = "nxp,74hc4094";
                cp-gpios = <&gpio0 0 GPIO_ACTIVE_HIGH>;
                d-gpios = <&gpio0 1 GPIO_ACTIVE_HIGH>;
                str-gpios = <&gpio0 2 GPIO_ACTIVE_HIGH>;
                /* oe-gpios is optional */
                gpio-controller;
                #gpio-cells = <2>;
                /* We are not cascaded */
                registers-number = <1>;
                gpio-line-names = "CONTROL_HSS0_CLK_INT",
"CONTROL_HSS1_CLK_INT", "CONTROL_HSS0_DTR_N",
                                "CONTROL_HSS1_DTR_N", "CONTROL_EXT",
"CONTROL_AUTO_RESET",
                                "CONTROL_PCI_RESET_N", "CONTROL_EEPROM_WC_N";
        };

What's nice with 74HC4094 is that it has a strobe (str) signal so you can send
out all the values and then strobe them out in one go. But your construction
makes sense too!

Yours,
Linus Walleij