Microcontrollers, flowcharts & programming

Microcontrollers

A microcontroller is a small programmable computer on a chip, with inputs and outputs, used in washing machines, microwaves, toys, traffic lights, car systems and industrial machines.

• Advantages over hard-wired circuits: reprogrammed without changing the circuit, so products can be changed or improved quickly; fewer components, so smaller, cheaper, quicker to assemble and more reliable (fewer parts to fail); one chip replaces many parts, so less stock and fewer resources used; can be reused, so less waste; can carry out complex sequences, counting and timing.
• Disadvantages: needs a computer, software and programming skills; software can be expensive; the chip can cost more than simple ICs; a fault in the program can stop the whole system; it may need driver circuits for high-current outputs.

Flowchart symbols

• Start / stop: rounded box (terminator).
• Output: parallelogram, e.g. "pin 7 on", "pin 7 off".
• Process / delay: rectangle, e.g. "wait 0.5 s". Always give a unit.
• Decision: diamond with Y and N exits, e.g. "pin 1 on?" or "repeated 6 times?".

Checking an input is a decision, not an output: the 2025 report saw input checks drawn as parallelograms. Give pin numbers and delay units, and make every loop arrow return to the right place.

Loops

• Continuous (infinite) loop: the program returns to the start forever.
• Fixed (finite) loop: repeats a set number of times, e.g. "for b0 = 1 to 15 … next b0", then carries on. The 2025 N5 report: most didn't name a fixed loop.
• A decision that loops back to itself waits for an input.

Program commands (PBASIC style)

main:
  if pin1 = 1 then flash   ' wait for the switch
  goto main
flash:
  for b0 = 1 to 6           ' fixed loop: 6 times
    high 7                  ' LED on
    pause 500               ' 0.5 s (pause is in ms)
    low 7                   ' LED off
    pause 500
  next b0
  goto main                 ' continuous loop

Other languages are fine (e.g. Arduino digitalWrite, delay). You won't be asked to write code from scratch, but you may read, complete or correct it, and flowchart answers must be detailed.

Design challenge: pedestrian crossing

The brief. Design the control for a crossing outside an Inverness primary school: traffic lights stay green until a button is pressed, then change to let people cross.

1. Draw a flowchart for one complete cycle.
2. Use a loop so the system keeps running.
3. State suitable time delays for each stage.
4. Write the program in PBASIC-style commands.

Higher programming HIGHER

• PWM (pulse-width modulation): switching a motor on and off rapidly; the average power depends on the mark-to-space ratio (duty cycle). A longer "mark" (on time) gives a faster motor. Draw PWM signals with all marks at the same voltage and at least three marks.
• Analogue inputs: read a sensor value (e.g. 0–255) and compare it with a threshold to make a decision.
• Up to four inputs and four outputs; logic (AND/OR) and arithmetic (counting, adding) in decisions.
• Microcontrollers controlling solenoid valves give electro-pneumatic systems.

Sub-procedures, counting and analogue inputs HIGHER

main:
  let b1 = 5
  gosub flash        ' call the sub-procedure
  pause 2000
  end                ' stops it running into the sub-procedure
flash:
  for b2 = 1 to b1   ' b1 sets the number of flashes
    high 7
    pause 100
    low 7
    pause 100
  next b2
  return             ' back to the line after gosub

• A sub-procedure is a section of code that can be called from different places with gosub and ends with return. It saves writing the same code twice.
• Arithmetic: count events with a variable, e.g. let b0 = b0 + 1 each time a sensor is triggered, then if b0 = 10 then … to act on the total (stock counters, car parks, lifts).
• Analogue inputs: an ADC converts a voltage into a number from 0 to 255, e.g. readadc 0, b1. Reading = V_in ÷ V_ref × 255, so 2 V with a 5 V reference gives 102. Compare the reading with a threshold to decide what to do.
• Signal conditioning: if a sensor's voltage doesn't fit the ADC range, an op-amp scales it: gain = required output ÷ input signal.
• PWM program: high motor, pause mark, low motor, pause space, loop. A longer mark gives a faster motor. Relays can't be used for PWM because the rapid switching would wear out their contacts: use a MOSFET.

Key terms

Quick check