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Digital Logic Basics

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Digital Logic - the stream of one's and zero's represented by high and low voltage signals that is the underlying language of all digital electronics. Digital signals can control the state of a device, transmit complex information, and be encoded in a number of formats that provide discrete advantages like error checking, fault tolerance, addressing, and flexible data handling.

Logic

Digital logic is an electronic implementation of Boolean logic, which deals with discrete states, on and off, high and low, one and zero, or yes and no for example. These states can be represented by discrete voltage levels and for the basis of digital logic systems. Using these states, Boolean algebra can be used such as the And, Or, and Not operations. Combined appropriately, Boolean algebra operations allow for the very complex digital logic operations that drive all modern digital electronics.

The most basic implementation of digital logic is in the form of digital logic gates. Each individual logic gate performs one Boolean algebraic operation. Combined with inverter gates, the logic gates allowed very complex systems to be designed by combining dozens, hundreds, or thousands of logic gates. Modern digital systems are built on the same type of digital logic gates, often with dedicated specialty modules and even sections that can be programmed to behave as fixed digital logic for very high speed operations.

Digital Logic Voltage Levels

Digital logic signals, high and low, are represented by voltage bands that allow for a noise and voltage tolerance in the signal. The voltage levels for high and low vary depending on the voltage logic level used in the system. Decades ago, most logic was 5 volt logic and the I/O and digital communication voltages usually matched. As 3.3 volt devices started to emerge they were generally tolerant of being fed 5 volt logic, but may not have been capable of outputting 5 volt compatible logic. Today common logic voltages include 5v, 3.3v, 2.5v, 1.8v, and 1.5v and low voltage devices are generally not tolerance of higher logic voltages. In order to communicate reliably between the logic voltage levels, level shifting techniques or ICs are often the best choice. Unfortunately, no level shifting technology is perfect and accommodating different communication standards, operational frequencies, and voltage levels always involves some tradeoffs.

Digital Logic Advantages and Limitations

Digital logic offers significant advantages over analog equivalents including:

  • Digital signals are much more noise tolerant than analog signals
  • Additional precision can be achieved with the same hardware, although processing speed will be slower. Analog hardware would require a redesign to achieve the same precision.
  • Software can be built on top of the physical digital logic of a digital system to adapt and change the function of the digital system as desired.
  • Information can be stored easier and longer without degradation compared with analog systems.
  • Information can be compressed
However, digital systems do suffer from several drawbacks which include:
  • Digital logic can require more power to achieve the same effect compared to specially designed analog circuits
  • Analog alternatives can be more affordable that digital electronics in small quantities
  • The translation of analog to digital is not perfect and quantization errors can occur
  • A missed digital bit can be a significant problem if error correcting and other protection schemes are not implemented

Challenges in Digital Logic

Digital logic does have a decent amount of fault tolerance to noise and voltage levels but the digital nature of the signals themselves can create significant challenges in digital logic systems. With digital logic, the failure of data to be read correctly or an errant pulse that is interpreted as a digital signal can have a large impact on the operation of the digital electronic system. The sharply defined transition between states can cause data to not be received if a system is operating near the logic threshold is the impedance of the system shifts (due to a change in temperature, operating environment conditions, power supply fluctuation, etc.). Glitches can be especially troublesome in digital electronics which can easily happen when noise from another part of the circuit couples in to a digital logic line and is interpreted as a digital signal.

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