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Microelectronics – Systems and Devices
As a result of one of these factors, a logic gate may become set to the
wrong state and the system will fail.
Noise immunity
Fig. 2.12 shows the input and output levels of LSTTL and CMOS
gates. The voltage at a gate producing a high output is at least 2.7 V. A
gate receiving that output will accept it as a high input if it is at least
2.0 V. This means that the gate will still give the correct result if there
is a spike of, say −0.5 V on the output signal, taking it down to 2.2 V.
It still could just work if the spike is −0.7 V, but might fail
occasionally. These values show that the noise immunity of LSTTL is
0.7 V for logic high. For logic low the corresponding voltages are 0.5
V and 0.8V, giving a noise immunity of 0.3 V. Spikes as high as this
Electromagnetic interference
Any flow of electric current generates electromagnetic
radiation (radio waves), the larger the current the stronger
the radiation. When this radiation passes through a piece of
electronic equipment it generates voltages in the signal lines
and in the power lines. These may cause the circuit to
operate incorrectly. EMI may pass directly as radiation or
may be transmitted along the mains power lines.
There are many sources of EMI, one of the worst being
switching currents on or off. The switching of the clock in a
microelectronic system generates high frequency radio
waves which may interfere with the operation of the system
itself or of a neighbouring system. The switching of signals
in the data and address busses has the same effect.
EMI can also arise from outside sources such as domestic
equipment (switching the motors of refrigerators, washing
machines), in industrial plant, and in the ignition systems of
motor vehicles. In these examples the loads are inductive so
a high voltage is built up as the switch contacts open. This
causes arcing at the contacts, which is a powerful source
of EMI.
Another source of EMI is the electric mains, where the
50 Hz (or 60 Hz in USA) may appear as ripple on the power
lines of a microelectronic system. This can be eliminated
with good shielding and a properly designed power supply.
Natural sources of EMI include thunderstorms and the
effects of cosmic radiation from outer space.