🚀 The Modern World on Digital Logic Notebook

Microcontrollers

Mon, 01 Jan 0001 00:00:00 +0000

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.

PLDs & CPLDs

Mon, 01 Jan 0001 00:00:00 +0000

PLDs & CPLDs#

Programmable logic devices are the step between fixed-function chips and the FPGA: user-defined logic without custom silicon. They exist to answer a specific frustration — a board covered in 74xx glue that has to be redesigned every time the logic changes — by putting that glue into a single part whose function is programmed, not wired.

From PAL to GAL to CPLD#

The original PLDs implemented logic as a programmable sum-of-products array: a grid of AND terms feeding OR terms, with fuses (PAL) or reprogrammable cells (GAL) selecting which connections are made. Because any Boolean function can be written in sum-of-products form, that one structure can implement a wide range of combinational logic — and with a flip-flop on each output, small state machines too. A single GAL could swallow a fistful of gates, decoders, and latches.

FPGAs

Mon, 01 Jan 0001 00:00:00 +0000

FPGAs#

A Field-Programmable Gate Array is the building blocks of this book made configurable: a fabric of logic and flip-flops whose function is defined after manufacture by loading a configuration. Where a microcontroller turns logic into a sequential program, an FPGA turns it into actual parallel hardware — every part of the design runs at once, the way discrete logic always did.

A Fabric of Lookup Tables and Flip-Flops#

The fabric is made of many small configurable blocks, each holding a lookup table (LUT) and a flip-flop, tied together by a mesh of programmable interconnect and surrounded by configurable I/O.

HDL Concepts

Mon, 01 Jan 0001 00:00:00 +0000

HDL Concepts#

A hardware description language — Verilog or VHDL — is how logic is actually specified once there is too much of it to draw. Nobody schematics a million gates; instead they describe the intended behavior in an HDL, and a synthesis tool turns that description into real gates and flip-flops, whether for an FPGA’s fabric or an ASIC’s cells.

It Describes Hardware, It Does Not Run#

The single most important idea — and the one that trips up newcomers from software — is that an HDL describes hardware that all exists at once; it is not a program that executes line by line. When a Verilog file says one signal is the AND of two others and, elsewhere, that a third is the OR of two more, both pieces of logic are built and both operate concurrently, forever. There is no “first this, then that.” Order of appearance is irrelevant; everything is simultaneous, because it is describing wires and gates, not instructions.

ASICs

Mon, 01 Jan 0001 00:00:00 +0000

ASICs#

An Application-Specific Integrated Circuit is custom silicon: the logic of a design fixed permanently into a chip made for one purpose. It is the building blocks of this book laid out in silicon at scale — millions or billions of gates and flip-flops, placed once and never changed.

The Primitives, Fixed in Silicon#

Most ASICs are built from a standard-cell library: a catalog of pre-designed, pre-characterized layouts for each primitive — a NAND cell, a NOR cell, a D flip-flop cell, a full-adder cell, and so on. That library is nothing more than the gates of Part II rendered as physical geometry. A designer writes the logic in an HDL, synthesis maps it onto those cells, and place-and-route tools arrange and wire millions of them across the die. (Gate arrays and full-custom design are the other, less common points on the spectrum, trading flexibility for effort.)

SoCs

Mon, 01 Jan 0001 00:00:00 +0000

SoCs#

A System on a Chip is integration taken to its conclusion: not just logic, and not just a processor, but a whole system on one die. Where a microcontroller put a CPU, memory, and simple peripherals together, an SoC adds everything else a product needs — multiple CPU cores, a GPU, a neural accelerator, memory controllers, high-speed I/O, radios, image and signal processors — onto a single piece of silicon.