Hardware Reference
In-Depth Information
A digital circuit is one in which only two logical values are present. Typically,
a signal between 0 and 0.5 volt represents one value (e.g., binary 0) and a signal
between 1 and 1.5 volts represents the other value (e.g., binary 1). Voltages out-
side these two ranges are not permitted. Tiny electronic devices, called
gates
, can
compute various functions of these two-valued signals. These gates form the hard-
ware basis on which all digital computers are built.
The details of how gates work inside is beyond the scope of this topic, be-
longing to the
device level
, which is below our level 0. Nevertheless, we will now
digress ever so briefly to take a quick look at the basic idea, which is not difficult.
All modern digital logic ultimately rests on the fact that a transistor can be made to
operate as a very fast binary switch. In Fig. 3-1(a) we have shown a bipolar tran-
sistor (the circle) embedded in a simple circuit. This transistor has three con-
nections to the outside world: the
collector
, the
base
, and the
emitter
. When the
input voltage,
V
in
, is below a certain critical value, the transistor turns off and acts
like an infinite resistance. This causes the output of the circuit,
V
out
,totakeona
value close to
V
cc
, an externally regulated voltage, typically +1.5 volts for this type
of transistor. When
V
in
exceeds the critical value, the transistor switches on and
acts like a wire, causing
V
out
to be pulled down to ground (by convention, 0 volts).
+V
cc
+V
cc
+V
cc
V
out
V
1
Collector
V
out
V
out
V
in
V
2
V
1
V
2
Emitter
Base
(a)
(b)
(c)
Figure 3-1.
(a) A transistor inverter. (b) A
NAND
gate. (c) A
NOR
gate.
The important thing to notice is that when
V
in
is low,
V
out
is high, and vice
versa. This circuit is thus an inverter, converting a logical 0 to a logical 1, and a
logical 1 to a logical 0. The resistor (the jagged line) is needed to limit the amount
of current drawn by the transistor so it does not burn out. The time required to
switch from one state to the other is typically a nanosecond or less.
