Digital Design Fundamentals·Signals, Bits & Logic
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Last lesson we said digital design uses only two values, 0 and 1. On real hardware those values are voltages on a wire:
Click the button in the circuit above and watch the lamp follow it. One wire, two states, nothing in between. A whole computer is billions of transistors playing this same game at once.
Two reference voltages, the power rails, fix what 1 and 0 mean physically:
Figure: A simple CMOS inverter between VDD and GND
Every wire on a chip connects through tiny transistor switches to one rail or the other. Tied to VDD it reads 1, tied to GND it reads 0.
Real voltages do not jump instantly from 0V to the supply. For a brief moment while a wire switches, it passes through the values in between. So what is a wire sitting at, say, half the supply voltage?
Figure: On the left, a vertical voltage bar from 0V to VDD split into high, forbidden and low bands
A chip splits the voltage range into three bands:
A signal should pass through the middle band quickly on its way between valid levels, never rest there.
Why leave a whole band unused in the middle? Safety.
Wires pick up interference from their busy neighbours, so the bands are drawn with a gap between what a sender promises and what a receiver accepts. A sender pushes a 1 well up into the high band, and a receiver counts anything even close to that as a 1. The spare gap between them is the noise margin.
As long as noise stays smaller than that margin, a 1 stays a 1 and a 0 stays a 0. This built in slack is the whole reason digital hardware is so dependable.
From here on we stop thinking about volts. Once a chip guarantees that every wire reads as a clean 0 or 1, we can forget the electrical detail and work with pure bits.
That jump, from a messy analogue voltage to a reliable digital bit, is the digital abstraction. It is the foundation the rest of the course stands on. Next we start putting bits to work by counting with them.
