Analogue IO

Analogue-to-Digital Convertor (ADC)

The Board has two potentiometers (Analogue inputs). These can be read using the ADC (Analogue-Digital-Convertor).

The repository s3.1 illustrates how these can be read.

The output from the library is scaled into a 0…1 range.
We can recover the actual value read using a knowledge of the hardware that generated the signal.

1. The potentiometers are connected between 0V and 3.3V, the slider is the input to the ADC.
   • Thus the input voltage to the ADC will be between 0V and 3.3V.
2. The ADC reference voltage is 3.3V.
   • The ADC reads 0 for 0V and 1.0f for 3.3V
3. The ADC uses 16 bits for the conversion.

Analogue Output — PWM (Pulse Width Modulation)

One form of Analogue output is Pulse-Width-Modulation (PWM). This is often used for motor speed control or as a dimmer mode for controlling LEDs.

Repository s3.2 shows an example of PWM used to control an LED’s brightness.

The example uses a simple ramp

        for( duty=0 ; duty<1.0 ; duty+=0.01 ) {
            red.write(duty);
            wait_ms(100);
        }

A more complex algorithm uses:

        for( duty=0 ; duty<4.6 ; duty+=0.23 ) {
            float brightness = exp(duty)/100;
            red.write(brightness);
            wait_ms(100);
        }

What do you observe as different about the two effects [ hint ]?
(you may want to play about with the delays and values)

• What value do you need to write for the LED to be?
  • fully on?
  • off?
  • dim (about quarter brightness)
  • bright (about three-quarter brightness)

• Use the input from one of the Potentiometers to set the brightness of an LED.
• Use both potentiometers to mix RED and GREEN LED colours.
  What range of colours can be displayed?

Analogue Output — Frequency Output

We can use the PWM output in another way for another form of analogue output. Here we are going to use a fixed 50% duty cycle, equal times on and off. We will vary the period (total time of on and off).

Example s3.3 scans through a frequency range.

        for (f=20.0; f<20.0e3; f+=100) {
            speaker.period(1.0/f);
            ...
        }

• what period corresponds to 20Hz?
• what periof corresponds to 20kHz?

What are the lowest and highest frequencies you can hear?

Look at the table of piano note frequencies.

• How would you map a note to a frequency to apply to the speaker?
• What data structure would you use to handle a note’s tone and duration?
• How would you play Music on the Speaker?