Editing Digital electronics (section)

===Trade-offs===

====Cost====

Since a digital system may use many logic gates, the overall cost of building a computer correlates strongly with the cost of a logic gate. In the 1930s, the earliest digital logic systems were constructed from telephone relays because these were inexpensive and relatively reliable.

The earliest integrated circuits were constructed to save weight and permit the [[Apollo Guidance Computer]] to control an [[inertial guidance system]] for a spacecraft. The first integrated circuit logic gates cost nearly US$50, which in {{Inflation/year|US}} would be equivalent to ${{Inflation|US|50|1960}}. Mass-produced gates on integrated circuits became the least-expensive method to construct digital logic.

With the rise of [[integrated circuit]]s, reducing the absolute number of chips used represented another way to save costs. The goal of a designer is not just to make the simplest circuit, but to keep the component count down. Sometimes this results in more complicated designs with respect to the underlying digital logic but nevertheless reduces the number of components, board size, and even power consumption.

====Reliability====

Another major motive for reducing component count on printed circuit boards is to reduce the manufacturing defect rate due to failed soldered connections and increase reliability. Defect and failure rates tend to increase along with the total number of component pins.

The failure of a single logic gate may cause a digital machine to fail. Where additional reliability is required, redundant logic can be provided. Redundancy adds cost and power consumption over a non-redundant system.

The ''reliability'' of a logic gate can be described by its [[mean time between failure]] (MTBF). Digital machines first became useful when the MTBF for a switch increased above a few hundred hours. Even so, many of these machines had complex, well-rehearsed repair procedures, and would be nonfunctional for hours because a tube burned-out, or a moth got stuck in a relay. Modern transistorized integrated circuit logic gates have MTBFs greater than 82 billion hours ({{val|8.2|e=10|u=hours}}).<ref>MIL-HDBK-217F notice 2, section 5.3, for 100,000 gate 0.8 micrometre CMOS commercial ICs at 40C; failure rates in 2010 are better, because line sizes have decreased to 0.045 micrometres, and fewer off-chip connections are needed per gate.</ref> This level of reliability is required because integrated circuits have so many logic gates.

====Fan-out====

[[Fan-out]] describes how many logic inputs can be controlled by a single logic output without exceeding the electrical current ratings of the gate outputs.<ref>Kleitz , William. (2002). Digital and Microprocessor Fundamentals: Theory and Application. 4th ed. Upper Saddler Reviver, NJ: Pearson/Prentice Hall</ref> The minimum practical fan-out is about five.{{citation needed|reason=No numbers cited in [[Fan-out]]. [[FO4]] makes it tempting to change this to 4.|date=November 2021}} Modern electronic logic gates using [[CMOS]] transistors for switches have higher fan-outs.

====Speed====

The ''switching speed'' describes how long it takes a logic output to change from true to false or vice versa. Faster logic can accomplish more operations in less time. Modern electronic digital logic routinely switches at {{val|5|ul=GHz}}, and some laboratory systems switch at more than {{val|1|ul=THz}}.{{Citation needed|date=June 2019}}.