Sequencers#

A sequencer is a state machine in its most restricted, most common form: one that steps through a fixed sequence of states, mostly marching forward rather than branching on inputs. Where a general FSM asks “given where I am and what just happened, where do I go?”, a sequencer mostly just asks “what is the next step?” That narrower job has simple, efficient realizations.

How They’re Built#

• Counter plus decoder — a synchronous counter advances through binary states and a decoder turns the current count into one active “step” line. Compact, and the count is also directly available.
• Ring or Johnson counter — a ring counter is the simplest sequencer of all: the single circulating one-hot bit is the current step, already decoded and glitch-free, with no separate decoder needed.
• A small FSM — when a few input-dependent decisions are needed (wait here until ready, skip a step under some condition), a general state machine handles the branches the pure counter forms cannot.

Where They’re Used#

Sequencers run the parts of a system that must happen in a fixed order: multi-phase clock generation, memory and bus access cycles, stepper-motor phase drive, display and keypad scanning, and pattern or waveform generation. Their most consequential use is the control unit of a processor — a microsequencer that steps an address counter through a ROM whose contents are the control signals for each step (microcode) and the address of the next step. That is how a great many CPUs turn an instruction into an ordered series of register-transfer operations: a sequencer, a memory, and the datapath registers it steers. The counting family’s pattern circulators and the state-machine family’s controllers meet exactly here.