Single Button Multi Actions Driver

Single Button (aka SButton) is an kind of keyboard that uses only a single physical button (Switch) in the board. This kind of button is used with simple interfaces like those used on 3D Printers or other devices where all the user needs is to move to the next option and confirm the selection.

It could be done detecting if the button was pressed for a short period of time (i.e. less than 500ms) or long pressed. If it is a short press the driver will return TAB and if it is a long press the driver will return ENTER. Using it is possible to navigate on those kind of menu.

How does it work?. The driver uses a simple config data (this config data is equivalent to the platform data on Linux kernel) to map the pin from MCU will be used to register and detect the interrupt from this pin physically connected to the button.

It uses a kind of “polymorphism” in C to allow the driver to get access to the functions responsible to attach and enable the interrupt and to get the status of the pin. See include/nuttx/input/sbutton.h and boards/arm/stm32/common/src/stm32_sbutton.c to understand better how it works. But basically the board file (config data) creates a struct when the first field (variable) is the config struct used the but SButton driver (drivers/input/sbutton.c).

Every time the user presses or releases the key button an interrupt is generated. The ISR of this interrupt inside sbutton (sbutton_interrupt()) calls a workqueue to process it (because we cannot spend time inside the ISR processing data, it could degradate the performance of the RTOS). All that workqueue (sbutton_worker()) needs to do it measure the elapsed time (ticks) from the moment the key was pressed until the moment it was released to decide if it is a “KEY_1” (TAB) or a “KEY_2” (ENTER). Then is call the keyboard_event() from the keyboard upper to send this key stroke to the user application.