Analog Output Example

In this example we will see how to write an analog value to a pin. The micro:bit doesn't have a real digital to analog converter, so the analog signal is actually a Pulse Width Modulation (PWM).

This is good enough to control the speed of a motor or the brightness of an LED.

There is a limit of three analog (PWM) signals on the micro:bit, if you try to write an analog value to more than three pins an exception will be raised.

Wiring Diagram

We use the same circuit as the Digital Out example.

Extra hardware:

A breadboard
An LED
A 47k ohm resistor

Wiring:

Code

To write an analog value to the IO pin we are going to use the procedure Write of the package MicroBit.IOs.

   procedure Write (Pin : Pin_Id; Value : Analog_Value)
     with Pre => Supports (Pin, Analog);

Arguments:

Pin : The id of the pin that we want to read as digital input
Value : The analog value for the pin, between 0 and 1023

Precondition:

The procedure Write has a precondition that the pin must support analog IO.

In the code, we are going to write an loop with a value that goes from 0 to 128 and set write this value to pin 0. We could go from 0 to 1023 but since the LED doesn't get brighter after 128, there is no need to go beyond that value.

We also use the procedure Delay_Ms of the package MicroBit.Time to stop the program for a short amount of time.

Here is the code:

with MicroBit.IOs;
with MicroBit.Time;

procedure Main is
begin

   --  Loop forever
   loop

      --  Loop for value between 0 and 128
      for Value in MicroBit.IOs.Analog_Value range 0 .. 128 loop

         --  Write the analog value of pin 0
         MicroBit.IOs.Write (0, Value);

         --  Wait 20 milliseconds
         MicroBit.Time.Delay_Ms (20);
      end loop;
   end loop;
end Main;