libwebsockets/lib/drivers/button

LWS GPIO Button class drivers

Lws provides an GPIO button controller class, this centralizes handling a set of up to 31 buttons for resource efficiency. Each controller has two OS timers, one for interrupt to bottom-half event triggering and another that runs at 5ms intervals only when one or more button is down.

Each button has its own active level control and sophisticated state tracking; each button can apply its own classification regime, to allow for different physical button characteristics, if not overridden a default one is provided.

Both the controller and individual buttons specify names that are used in the JSON events produced when the buttons perform actions.

Button electronic to logical event processing

Buttons are monitored using GPIO interrupts since this is very cheap in the usual case no interaction is ongoing. There is assumed to be one interrupt per GPIO, but they are pointed at the same ISR, with an opaque pointer to an internal struct passed per-interrupt to differentiate them and bind them to a particular button.

The interrupt is set for notification of the active-going edge, usually if the button is pulled-up, that's the downgoing edge only. This avoids any ambiguity about the interrupt meaning, although oscillation is common around the transition region when the signal is becoming inactive too.

An OS timer is used to schedule a bottom-half handler outside of interrupt context.

To combat commonly-seen partial charging of the actual and parasitic network around the button causing drift and oscillation, the bottom-half briefly drives the button signal to the active level, forcing a more deterministic charge level if it reached the point the interrupt was triggered. This removes much of the unpredictable behaviour in the us range. It would be better done in the ISR but many OS apis cannot perform GPIO operations in interrupt context.

The bottom-half makes sure a monitoring timer is enabled, by refcount. This is the engine of the rest of the classification while any button is down. The monitoring timer happens per OS tick or 5ms, whichever is longer.

Declaring button controllers

An array of button map elements if provided first mapping at least GPIOs to button names, and also optionally the classification regime for that button.

Then the button controller definition which points back to the button map.

static const lws_button_map_t bcm[] = {
	{
		.gpio			= GPIO_NUM_0,
		.smd_interaction_name	= "user"
	},
};

static const lws_button_controller_t bc = {
	.smd_bc_name			= "bc",
	.gpio_ops			= &lws_gpio_plat,
	.button_map			= &bcm[0],
	.active_state_bitmap		= 0,
	.count_buttons			= LWS_ARRAY_SIZE(bcm),
};

	struct lws_button_state *bcs;

	bcs = lws_button_controller_create(context, &bc);
	if (!bcs) {
		lwsl_err("%s: could not create buttons\n", __func__);
		goto spin;
	}

That is all that is needed for init, button events will be issued on lws_smd when buttons are pressed.

Regime settings

The classification regime is designed to reflect both the user interaction style and the characteristics of a particular type of button.

Member	Default	Meaning
ms_min_down	20ms	Down events shorter than this are ignored
ms_min_down_longpress	300ms	Down events longer than this are reported as a long-click
ms_up_settle	20ms	After the first indication a button is no longer down, the button is ignored for this interval
ms_doubleclick_grace	120ms	The time allowed after a click to see if a second, double-click, is forthcoming
ms_repeat_down	0 / disabled	If held down, interval at which to issue stilldown events
flags	LWSBTNRGMFLAG_CLASSIFY_DOUBLECLICK	Control which classifications can apply

lws_smd System Message Distribution Events

The button controller emits system messages of class LWSSMDCL_INTERACTION, using a JSON formatted payload

{
	"type":  "button",
	"src":   "controller-name/button-name",
	"event": "event-name"
}

For example, {"type":"button","src":"bc/user","event":"doubleclick"}

JSON is used because it is maintainable, extensible, self-documenting and does not require a central, fragile-against-versioning specification of mappings. Using button names allows the same code to adapt to different hardware or button mappings. Button events may be synthesized for test or other purposes cleanly and clearly.

All the events are somewhat filtered, too short glitches from EMI or whatever are not reported. "up" and "down" events are reported for the buttons in case the intention is the duration of the press is meaningful to the user code, but more typically the user code wants to consume a higher-level classification of the interaction, eg, that it can be understood as a single "double-click" event.

Event name	Meaning
down	The button passes a filter for being down, useful for duration-based response
stilldown	The regime can be configured to issue "repeat" notifications at intervals
up	The button has come up, useful for duration-based response
click	The button activity resulted in a classification as a single-click
longclick	The button activity resulted in a classification as a long-click
doubleclick	The button activity resulted in a classification as a double-click

Since double-click detection requires delaying click reporting until it becomes clear a second click isn't coming, it is enabled as a possible classification in the regime structure and the regime structure chosen per-button.

Typically user code is interested in, eg, a high level classification of what the button is doing, eg, a "click" event on a specific button. Rather than perform a JSON parse, these events can be processed as strings cheaply using lws_json_simple_strcmp(), it's dumb enough to be cheap but smart enough to understand enough JSON semantics to be accurate, while retaining the ability to change and extend the JSON, eg

	if (!lws_json_simple_strcmp(buf, len, "\"src\":", "bc/user")) {
		if (!lws_json_simple_strcmp(buf, len, "\"event\":", "click")) {
			...
		}
		...
	}

Relationship between up / down and classification

Classification	Sequencing
click	down-up-click (it's classified when it went up and cannot be a longclick)
longclick	down-longclick-up (it's classified while still down)
doubleclick	down-up-down-doubleclick-up (classified as soon as second click down long enough)

If the regime is configured for it, any "down" may be followed by one or more "stilldown" at intervals if the button is down long enough