Characteristics of digital electronic devices
History of Logic The history of logic is a broad subject that has vastly progressed since the early 18th century; however many of the designs shown are variations of similar principles but each has their own benefit. Much of the logic exists from algebra and Boolean algebra, however earlier ideas were presented with regards to considering inputs and outputs as logic or numbers or ‘0’ or ‘1’. Pre-dating these eras were the basis of logical decisions based on valid inference; this is based on reasoning from logical universal truths. However this is a digressive subject and not concerned with electronic devises; although much of the underlying principles are based on Boolean algebra or abstract algebra. Mechanical types – This is the most basic type and one of the original forms of logic and would consist of mechanical means of producing an output. For example the activation of water gates in a water system, depending on the requirements (such as weighted floats) would determine if the water would pass at different times. A more related example and one of the forefronts of computer logic and programming was the Analytical engine; a description is given below.
The analytical engine The design and implementation of such a machine extends far beyond the purpose of this research and description. The machine is very complicated and the design functions are too vast for description; however a brief explanation is included. The analytical engine was the very first design of the modern day computers and provided a mechanical means of producing calculated results using logical functions and calculations that are pre-built into the design of the engine. It would allow for variables to be entered into the device as per the operators request, and based on the calculations would perform a set of functions until another variable value is to be entered (such as the logarithm of a number); eventually this will produce an answer to the calculation. The card design is similar to that used in the ‘Jacquard Loom’; in which functions are performed based on whether the card in use has a hole or not at different points. The analytical engine differs slightly from the difference engine in one important principle. The analytical engine was to be able to jump to any function and calculation based on the input from the operator. The difference engine is in most parts a calculator in which once the calculation has begun it must be completed or ended. Once the analytical engine had been designed the difference engine was improved and made simpler Electromagnetic types such as relays and switches This is one of the most common types of control that was employed prior to logic gates and logic programming. The principle of the relay logic is to control a sequence through the activation of various relays at designated points within a programming sequence. In some instances the relays can be controlled sequentially by the output of the previous step, however the ‘electro’ aspect of the logic would most often be performed by a controller in order to provide the necessary control power
to operate the relays. This is the predecessor to the more prevalent logic control from the P.L.C systems, which also utilises switching devices, which are however fixed switches in the controller as opposed to the moving switches of the relays. The relay logic was more cumbersome as each individual relay required mounting and wiring as opposed to wiring in and out of one unit. A variation for these types is in the form of solenoid valves; these are used for pneumatic control in which air is employed, or hydraulics in which oil is employed. The principle is still the same in that they are controlled from a main processor. Vacuum tubes This were another slight advancement on the relay logic but was still before the P.L.C logic era. It was designed for non-moving parts to eliminate one of the problems of the relay logic. Furthermore they are more compact than the relay equivalents and with the advancements made they had significantly reduced in size. These vacuum tubes are the principle behind the diode devices and certain amplifiers. They utilise an anode (positive) and cathode (negative); the cathode is heated until the process of thermoionic emission occurs in which the electrons permeate the vacuum tube. If the anode or the plate is more positive than the filament then current will flow. When the heat is removed the electrons gradually return to the cathode, however over time the conductivity is reduced from a lack of electrons and causes the device to fail. Transistor logic The types of devices are described below in the relevant section; the transistor logic is one of the most common types of logic that dominates the logic methods at the current time. It utilises various combinations of transistors and other devices to produce a logical output based on the functional design of the transistor and the current condition of the circuit. Electronic types (i.e. Boolean logic) Effectively the Boolean logic is a principle that practically implements the theoretical Boolean logic through the use of electronic logic gates, such as the transistor types or the older types. The method can be written down theoretically using the Boolean symbols for the relevant gating (AND, OR, NOT etc.); however in order to implement that theory the semi-conducting methods are usually implemented, controlled by a clock timer to ‘refresh’ the logic. The control clock function in most logic systems is done via a ‘P.L.C’, the computer based programme is compatible with electronic ‘cards’ containing different semi-conductors and fixed switches. Types of devices that are available, operation and their uses BJTs – These are ‘Bipolar junction transistors’ and are commonly used in electronic circuits as switches or as amplifiers. The principle is such that the introduction of a positive voltage to the base of the transistor will cause current to flow through the collector, base and emitter to ground. The magnitude of such current is based on an ‘ohms’ law calculation using the supply voltage and any resistive element in the circuit. The transistor will therefore reach saturation at the point where the voltage difference between the collector and emitter is almost equal to zero. If the device is used as an inverter, the principle follows that it is used similar to a resistor in a voltage divider. As the voltage to the base of the transistor is increased, the output of the leg above the transistor will decrease as more voltage is let to ground. The two most common types are the PNP and NPN type
transistors; which individually include three regions of n-doped (N) or p-doped (P) regions. The basis for the operation is that a new current supplied to the base region causes a disturbance in the equilibrium between the thermally generated carriers and the electric field. The two types of carriers are the electrons and holes; the latter being the remnant positively charged atom that has been left due to the valence electron being free to move. FETs and other members of the group – The term ‘FET’ refers to a ‘field-effect transistor’; similar to the BJTs but simpler in design. The principle of such is that there are three main components; these being the source, drain and ‘gate’ or ‘gate electrode’. The electrodes of each component are separated from the main body from a heavily resistive dielectric material (around 1GΩ); however a small gap is present underneath the source and drain electrodes, the space under which are heavily doped n-type. The surrounding region, especially underneath the gate is heavily doped p-type. When a positive voltage is supplied to the gate, an inversion layer is created the top of the semiconductor device that is now populated with free electrons. This induced layer created is n-type in the way the source and gate are also. This forms a channel in the FET conducts and allows the current to flow. The ‘MOSFET’ (metal-oxide field-effect transistor) is a common example of this group in which a metal oxide is used in the design. CMOS – The acronym stands for ‘Complementary metal-oxide-semiconductor’ or ‘Complementary symmetrical metal-oxide-semiconductor (COS-MOS). The design of which is that it utilises pairs of ptype and n-type ‘MOSFETS’ (see above). The benefit of using such a design is the high noise immunity and low power consumption of the device; the only point in which a power spike is present is during the switching stage. When a high voltage is placed on the circuit it will cause the ‘nMOSFET’ to conduct and the ‘pMOSFET’ to not conduct; this is not a major issue unless higher frequencies are used. RTL – Referring to the ‘Resistor-transistor Logic’; this is one of the more basic and original designs and used resistors and transistors in the logic function. There are resistors placed on both before the base and the collector. The base resistor is used to expand the negligible transistor input voltage to a logic ‘1’ level by converting the input voltage to current. The collector resistor is used to convert the current into voltage. DTL – Termed ‘Diode-transistor Logic’ for the fact it uses diodes to perform the logic function and the transistors to perform the amplification function. TTL – This refers to ‘Transistor-transistor logic’ and is based on the fact both the logic function and the amplifying functions are completed by transistors. In many instances there will be a multiemitter transistors that is used within the Comparison between the types of devices available In theory it should be possible to interchange different logic devices; however due to manufacturer designs, practical implications and programming device compatibility this is not possible without considerations. The practical implications with different devices are that the input tolerances and environmental limitations mean that different components may not be used in place of others. One of the deciding factors for selecting ‘CMOS’ or ‘TTL’ is the power consumption of the device;
even at rest the ‘TTL’ device consumes significantly more power than the ‘CMOS’ device. As stated previously the ‘CMOS’ draws power mainly when it switches state due to the construction, however this would mean that the issue would only arise when higher frequencies are used. Another deciding factor is the sensitivity to ESD of the device. The ‘CMOS’ device is extremely delicate and is susceptible to very minute values of electrostatic discharge. Meaning that in environments and situations where the device may come into contact with potentially damaging signals, the ‘TTL’ would be better to choose. Propagation delay related to the time taken for the output to respond to a change in input. The delay is commonly less in ‘TTL’ devices than in ‘CMOS’ devices due to the ‘CMOS’ having a high output impedance value. Noise immunity is the ability of the circuit to avert logic level changes on signal lines when noise distortion is present. Generally the ‘CMOS’ devices have a better noise immunity than the ‘TTL’ family due to the margins created from the transition points of HIGH and LOW state changes. Another consideration would be for the output state depending on ‘floating inputs’, in the event of an input to the device not being connected to an input, the output of the device may change. In ‘CMOS’ devices, a floating input can often cause an increased susceptibility to noise and create excessive power dissipation. The input should then be connected to VDD or VSS respectively. The same problems can occur with the ‘TTL’ family and it is advisable to tie all unused inputs to a high or low voltage. If the input is connected as a convenient high it will help in reducing power dissipation and prevents oscillation of the gate output which would increase current spiking and power drawn.
Useful links http://en.wikipedia.org/wiki/Logic_gate#History_and_development http://www.coe.pku.edu.cn/tpic/201031320403199.pdf
Charles babbage – analytical engine http://www.computerhistory.org/babbage/engines/
History http://www.coe.pku.edu.cn/tpic/201031320403199.pdf - Logic Gates
The different types http://www.westfloridacomponents.com/blog/transistors-what-is-the-difference-between-bjt-fetand-mosfet/ - A description of the different active semi-conductor devices
http://staff.ui.ac.id/system/files/users/marta/material/kbab9martarizal.pdf - Different Logic families http://www.slideshare.net/Ajlaaa/logic-families-16246507 - Mainly CMOS and TTL subfamilies
Comparison between devices http://people.virginia.edu/~ag7rq/663/Fall10/MOS-BJT_Comparison.pdf - BJT vs MOSFET http://www.allaboutcircuits.com/vol_4/chpt_3/10.html - CMOS vs TTL http://www.differencebetween.net/technology/difference-between-cmos-and-ttl/ - CMOS vs TTL http://digital.ni.com/public.nsf/allkb/2D038D3AE1C35011862565A8005C5C63 - CMOS vs TTL http://www.electronics-tutorials.ws/logic/logic_1.html -0 - Introduction to Logic gates http://www.lns.cornell.edu/~ib38/teaching/p360/lectures/wk09/l26/EE2301Exp3F10.pdf - CMOS and TTL characteristics
Compatibility http://www.bustedgear.com/faq_transistor_substitution.html - BJTs and FETs http://www.ti.com/lit/an/szza036b/szza036b.pdf - Different logic devices from Texas Instruments