The microcontroller (PIC): architecture, input/output ports, memory, the program cycle, and the advantages of programmable control over discrete logic.

A microcontroller (for example a PIC, made by Microchip) is a complete computer on a single chip: it contains a processor (CPU), memory and input/output ports, and runs a stored program to control a system. The user writes a program that reads inputs, makes decisions and sets outputs.

Its main parts:

• Processor (CPU): fetches and executes the program instructions and makes decisions.
• Memory: program memory (often flash, holding the program) and RAM (holding data/variables while running); some have EEPROM for non-volatile data.
• Input/output (I/O) ports: pins configured as inputs (read switches, sensors via the ADC) or outputs (drive LEDs, transistors, displays).
• A clock/oscillator sets the speed, and built-in timers and an ADC are common.

Three advantages over discrete logic: (1) fewer components and a smaller, cheaper circuit (one chip replaces many gates/timers); (2) flexibility - behaviour is changed by reprogramming, not rewiring, so the same hardware serves many products and is easy to modify/iterate; (3) complex functions (timing, counting, sequences, decisions) are easy in software, and reliability is higher with fewer connections.

Markers reward the computer-on-a-chip definition, the CPU/memory/I-O parts, and three valid advantages (fewer parts, reprogrammable/flexible, complex behaviour in software).

Each I/O pin can be configured in software as either an input or an output by setting a data-direction register (in PIC terms, the TRIS register): setting a pin as an input lets the program read the logic level applied to it (from a switch or sensor); setting it as an output lets the program drive the pin high or low to control something.

A transistor (or MOSFET) is often needed on an output pin because a microcontroller output can only supply a small current at low voltage (typically a few milliamps), which is not enough to drive a motor, lamp, relay or solenoid directly. The pin switches a transistor, which in turn switches the larger load current (with a flyback diode for inductive loads).

Markers want the software-configurable direction (read vs drive) and the reason for the transistor (the pin cannot supply enough current/voltage for a power load).