digital_in

Digital Input Example

In this example we will see how to read the digital state of a pin. This means reading if the pin is at 0 volts (low) or 3.3 volts (high).

Wiring Diagram

We start from the same circuit as the Digital Out example, and we add a push button.

Extra hardware:

• A breadboard
• An LED
• A 470 ohm resistor
• A push button

Wiring:

Code

To read the state of the IO pin we are going to use the function Set of the package MicroBit.IOs.

   function Set (Pin : Pin_Id) return Boolean
     with Pre => Supports (Pin, Digital);

Arguments:

• Pin : The id of the pin that we want to read as digital input

Precondition:

The procedure Set has a precondition that the pin must support digital IO.

As you can see, the function Set to read the pin has the same name as the procedure set that we used to control the pin in the Digital Out example. It is called overloading, two subprograms with the same name that provide different services.

In the code, we are going to write an infinite loop that reads the state of pin 1. If it is high, it means the button is not pressed so we turn off the LED on pin 0. It if it is low, it means the button is pressed so we turn on the LED on pin 0.

Here is the code:

with MicroBit.IOs;

procedure Main is
begin

   --  Loop forever
   loop

      --  Check if pin 1 is high
      if MicroBit.IOs.Set (1) then

         --  Turn off the LED connected to pin 0
         MicroBit.IOs.Set (0, False);
      else

         --  Turn on the LED connected to pin 0
         MicroBit.IOs.Set (0, True);
      end if;
   end loop;
end Main;