Digital Signals
Being an electronic circuit, the operation of a computer is about voltages and current flow. Understanding the basic principles of voltages and current flow within the computer is mandatory if you’re going to produce a working system. Common operating voltages inside a computer are normally either 5V or 3.3V. For some low-power or exceptionally fast computers, voltages may be as small as 1.8V or even lower.
An output pin of a digital device can be in one of three states. It
can be high
(logic 1), low
(logic 0), or
tristate
(high
impedance
, also known as
floating
).
A logic high is defined as the output voltage at the pin being higher
than a given threshold. When a device’s pin is
outputting a high, it is said to be sourcing
current
to that connection. Similarly, a logic
low is when the output voltage is below a given threshold, and the
device’s pin is said to be sinking
current
. Typically,
components can sink more current than they can source.
A tristate pin is outputting neither a high nor a low. Instead, it becomes high impedance (high resistance) so that current flow in or out of the pin is negligible. It is, in effect, invisible to other components to which it is connected. For example, within a computer system may be several memory devices connected to the data bus. When a particular device is being read, its data outputs will be either high or low (corresponding to the bit pattern being read back). All other memory devices in the system, because they are ...
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