Microcontrollers#

A microcontroller is a whole small computer on one chip: a CPU, its memory (flash for the program, RAM for data), and a set of peripherals, all on a single die. For most small systems it is the first and most common answer to the question this chapter asks — where did the logic go? It went into a program running on a cheap MCU.

Logic That Became a Program#

The peripherals around an MCU’s CPU are, quite literally, the functional building blocks of this book absorbed onto the die and made software-configurable. A timer/counter peripheral is a counter — a 7490 that moved inside the chip. A UART or SPI block is shift registers plus a small state machine. GPIO is latches and buffers. Where a 1975 design wired those functions together from 74xx parts, an MCU design configures them in firmware and wires them with lines of code.

That is the trade the microcontroller makes: instead of choosing and soldering a counter, a decoder, and a handful of gates, a designer writes a program that sets up the equivalent peripherals and sequences them. The logic is the same; the medium became software.

What It Is Good At, and What It Is Not#

The strength is flexibility and cost. One inexpensive part, reprogrammable at will, replaces a board of fixed logic — which is exactly why so many discrete parts landed in the obsolete column with “MCU timer” as their replacement.

The limit is that a CPU is sequential: it executes one instruction at a time, so timing is bounded by software and interrupt latency rather than by parallel hardware. When a task needs many things to happen truly at once, or with hard nanosecond determinism, the MCU hands off to actual parallel hardware — an FPGA or custom silicon. The firmware side of microcontrollers — clocks, peripherals, interrupts, and real-time behavior — is the subject of the Embedded Systems notebook; here the point is simply that the MCU is where the bulk of everyday logic quietly moved.