Moving Data on Digital Logic Notebook

Shift Registers

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

Shift Registers#

A shift register is a row of flip-flops wired so that each one’s output feeds the next one’s input. On every clock edge the whole row shifts its contents one position along. Where a plain register holds a word still, a shift register makes it move — which is the basis for nearly every way hardware moves data on a small number of wires.

Serial and Parallel, in Four Combinations#

The value of a shift register is converting between serial and parallel form, and parts are named for how data enters and leaves:

Bus Transceivers

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

Bus Transceivers#

A bus transceiver is a bidirectional buffer with direction control — the part that lets a device drive a shared bus, receive from it, and get out of the way when it is not its turn. It exists because a bus is a set of wires many devices share, and sharing wires safely is harder than it looks.

The Problem of a Shared Bus#

On a bus, several devices connect to the same lines, but only one may drive them at any instant. If two devices drive the same wire to opposite levels, they fight — a low-impedance path from supply to ground that corrupts the data and can damage the drivers. This is bus contention, and avoiding it is the whole discipline of bus design: every device must present a three-state output that can go high-impedance (electrically disconnected) whenever it is not the active driver.

Buffers

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

Buffers#

A buffer is a gate that computes nothing — its output simply equals its input — yet it is one of the most useful parts in the toolbox. What a buffer provides is not logic but electrical help: drive, isolation, and, in its three-state form, the ability to disconnect.

The Gate That Does Nothing, Usefully#

A non-inverting buffer is, at heart, two inverters in series — the double inversion cancels, leaving the logic value unchanged but the signal freshly restored and able to drive a much heavier load than the original could. That buys three things: