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CISC-340 Digital Systems Everything You Need to Know About Electronics - and more!! Electric Energy Free Electrons! Let's start with the basics. Matter is composed of molecules. Molecules are composed of atoms. Atoms are composed of a centre mass called the nucleus, containing positively charged particles called protons and neutral particles called neutrons. Orbiting around the nucleus at different but well defined energy levels (often called "rings" or "bands") are negatively charged particles called electrons. There are only about 120 different elements know to mankind. The number of electrons orbiting the nucleus (and not coincidently, the number of protons in the atom) determine the "atomic weight" of the element. Each element in the "periodic Table of the Elements" differs in its atomic weight. The outermost (highest energy) band of electrons orbiting the nucleus is called the "valence band". It is the valence band that is primarily responsible for the element's electrical (and also chemical) properties. An element with 8 electrons in its outer band is very stable. It is chemically inert and electrically non-conductive (an "insulator"). Atoms with 1,2, or 3 atoms in their outer band are good conductors. For example, copper and gold each have 1 electron in their valence band. Silicon, the material on which integrated circuits are built, has 4 electrons in its outer band. It is therefore somewhere between a good conductor and a good insulator. Consequently, it is called a "semi-conductor" material. There are things that can be done to make silicon either a good conductor or a good insulator. Even at room temperature, electrons in the outer band of atoms have enough energy to occasionally break free from their atomic structure and randomly move from atom to atom. This leaves the atom from which the electron left, positively charged. When it comes to charged particles, opposites attract. A positively charged atom with attract a negatively charged free electron from a neighboring atom that has broken free of its atomic bounds and is looking for a place to go. In the absence of any other forces, this migration of electrons from atom to atom is random in direction and the net (directional) flow of electrons through the material is zero. If we were to apply an electrical force to cause the net migration of electrons to be in one direction, we have an electrical "current". Current flow through a conductor such as a metal wire is the rate of flow of electrons past a specific point in the wire. One ampere of current flows when 6.25 x 10**18 electrons pass a circuit point per second. (The symbol used for current flow is I) The electrical force that can cause electrons to flow in one direction is called "voltage" . (V) You can think of voltage as a form of potential energy; electrical "pressure" if you like. (Current on the other hand is a form of kinetic energy). There are many ways to generate voltage including electromagnetic methods (hydro generating stations) and chemical methods (batteries). Resistance and Ohm's Law The resistance (R) of a material to current flow is measurable and its units are "ohms". A resistance of 1 ohm exists when the current of 1 amp(ere) flows through a material with an applied voltage of 1 volt. Not surprisingly, there is a common electronic circuit component called a "resistor" that is used to control the current flowing in a circuit. (It's just a small cylinder of carbon based material with connecting wires at each end). Resistors can be purchased from your local electronic store in various resistances ranging from a few ohms to millions of ohms. Most dial type volume controls such as that found on your portable CD player are variable resistors. This relationship of resistance, current and voltage is called "Ohm's Law" (after George Simon Ohm 1787 - 1854). V = I * R where V = voltage in volts, I = current in amps, R = resistance in ohms Also, from this we can generate the following equations I = V/R and R = V/I Electrical Power Electrical Power is the rate at which electrical work is done, or the rate at which electricity is consumed. Power (P) = V * I , or I**2 R Power is measured in watts, kilowatts, megawatts. For example, an incandescent light bulb may consume 60 watts, a hair dryer typically consumes 1500 watts. Your local power company charges you for total consumption = power * time. If you pay for your own electricity, and used your hair dryer for an hour, Ontario Hydro would charge you for 1500 watt-hours or 1.5 kilowatt-hours (currently about 7 cents ) Electronic circuit analysis. Consider the following circuit of a voltage source V and two resistors R1, and R2. (A jagged line is the schematic symbol for a resistor) The current in the circuit I = V/Rtotal = Vbattery/ R1+R2 (The total resistance of resistors connected end-to-end as in the drawing is equal to the sum of the individual resistances. Furthermore, since there is only one path for current to flow in this circuit, the current is the same in all parts of this circuit) Vout = Voltage across R2 =( I) * R2 = (Vbattery/R1+R2) * R2 = Vbattery*R2/R1+R2 Now let's simplify the drawing by assuming that the positive terminal of the battery or source voltage is connected to a point in the circuit called Vdd, and the negative terminal of the battery or source voltage is connected to "ground" (the common reference point of all voltage measurements). We then have the following simplified drawing: (The voltage Vout is the voltage measured between the point labelled Vout and Ground. This is also the voltage measured between the two ends of R2, or the voltage "across" R2.) We won't actually analyze this circuit, although we now have the background necessary to do so. We will see similar circuits and we will analyze those. First, let's learn about another electronic component - the transistor. Transistors as Switches Current microprocessor technology uses a type of transistor called a Metal Oxide Semiconductor Field Effect Transistor (MOSFET for short). Transistors and integrated circuits are built on a foundation of silicon. Remember that silicon atoms have 4 electrons in their outer band. This makes them semiconductors. We can add minute quantities of a second element to control silicon's electrical properties. If we add an element that has 5 electrons in its outer band (such as arsenic), 4 of the electrons will tend to bind to neighboring silicon atoms (effectively forming bands in each atom of 8 electrons - four from the silicon atom and 4 from the arsenic atom. This sharing between atoms is called a "covalent bond") and the fifth electron will easily escape the bounds of the arsenic atom and willingly take part in current flow. This material would be called N-type silicon as it would have an excess of Negative charges to participate in current flow. It doesn't take many arsenic atoms to significantly change the conductive properties of pure silicon. One arsenic atom for every few hundred MILLION silicon atoms is plenty! A MOSFET transistor with source and drain areas made of Ntype silicon is called an NMOS transistor. Instead of adding arsenic, we could add a second element that had only three electrons in its outer ring, such as gallium. Now the "covalent bond" between silicon and gallium will only have 7, not 8, electrons. This absence of the one electron to make a stable covalent bond is called a "hole". This atom pair will willingly accept any free electrons that happen to be passing by to fill the bond and create a stable shared ring of 8 electrons. Transistors made with source and drain areas made of P-type material are called PMOS transistors. Gate Source Gate Source Drain N N P P-type semiconductor substrate Silicon Dioxide (insulating layer) NMOS transistor P N N-type semiconductor substrate P PMOS transistor D G Drain D substrate S G substrate S Switch analogy the "transfer function" of a MOS transistor is such that a small change in gate voltage will cause a large change in source - to-drain resistance. In digital electronics, we operate the gate at the two limits of input voltage (0 volts, or the supply voltage Vdd). The result is two possible values of source-to-drain resistance - no resistance or an infinite resistance. This is similar to the resistance of an electrical switch. the substrate is usually connected internally or externally to the source terminal. the following describes NMOS and PMOS enhancement mode transistors. When a sufficient voltage of a specific polarity is applied to the gate, the source to drain resistance is reduced to a very low value. NMOS (N-type Metal Oxide Semiconductor) transistor • When the gate to substrate voltage is positive, the source-to-drain resistance is reduced from a very high value to almost zero, like the contacts of a switch that is closed. PMOS (P-type Metal Oxide Semiconductor) transistor • When the gate to substrate voltage is negative (substrate is more positive than the gate) the source-to-drain resistance is reduced from a very high value to almost zero, like the contacts of a switch that is closed. • In both NMOS and PMOS transistor, the gate terminal is electronically insulated from the rest of the transistor. Thus no current can flow through the gate terminal. This is an important feature as under static conditions, no power is consumed by current flowing into or out of the gate terminal. Simple logic gates can be constructed using NMOS transistors and resistors. An important characteristic to note is that with an NMOS gate, when the output level is zero, a current path exists from the negative power supply terminal (ground), through one or more NMOS transistors, through the resistor, and back to the power supply positive terminal (Vdd). Thus power is consumed during the entire time that the output is at a logic 0. An improvement is with CMOS logic. CMOS circuits use both NMOS and PMOS transistors. The circuits are similar to the NMOS inverter circuit except that the resistor has been replaced with a PMOS transistor. When Complementary MOS transistors are used, at any instant in time, EITHER the PMOS or the NMOS transistor is conducting but NOT both. Therefore there is no longer a path for current to flow from the positive terminal of the power supply to the negative (ground) terminal. The circuit generates an output of 0 or 1 (0 volts or +5 volts) depending on the input voltage, but seemingly, no power is consumed! Capacitance and Capacitors - The hidden electronic component! Whenever two parallel conductors are separated by insulator material, there exists a "capacitance". The effect of capacitance is to oppose any change in circuit voltage. The simplest implementation of a "capacitor" would be two parallel metal plates separated by air (an insulator) metal plates air Capacitor contruction schematic symbol The "capacitance" of the diagram above can be measured. The unit of capacitance is called a "farad". The value of capacitance is dependent on the area of the metal plates, the distance separating the places, and the type of insulating material separating the plates (called the dielectric). A "farad" is a very large capacitor. Typical capacitors used in the electronics industry would be measured in microfarads (10**-6 farads) or picofarads (10**-9 farads). Unfortunately, it's so easy to make a capacitor, often one is created whether we like it or not. Think about two wires in an electrical cable, such as your network cable, printer cable, cable TV cable, phone cable, and so on. Here we have two (or more) parallel conductors separated by an insulator (the vinyl insulation around the wires). A capacitor therefore exists and is prepared to resist signal voltage levels. Also consider the construction of a MOS transistor. The gate is a metal or conductive plate. Parallel to it is the N-type or P-type substrate on which the transistor is build. The two are separated by an insulating layer (silicon dioxide). A capacitor therefore exists between the gate and the substrate. This "gate capacitance" is prepared to resist signal changes and is responsible for delays within digital logic components. Capacitor action. Consider the following circuit. The circuit consists of a battery generating a voltage V, switches S1 and S2 , a resistor R, and a capacitor C. If switch S1 closes, there still is no completed conductive path for current to flow because of the insulated space between the metallic plate of capacitor C. When the switch closes however, the voltage on the top plate of the capacitor is equal to +V. The top plate of the capacitor is connected to the positive terminal of the supply voltage V through the resistor and the closed switch. Electrons tend to accumulate on the lower plate of the capacitor, attracted by the positive voltage on the top plate. At the same time, as electrons accumulate on the bottom plate (causing the bottom plate to become negatively charged), electrons are repelled from the top plate as they feel the influence of the negative charge on the bottom plate and also the positive voltage from the top battery terminal. Thus there is a net flow of electrons from the top plate of the capacitor back to the battery, and from the lower battery terminal to the lower plate of the capacitor. The top plate of the capacitor accumulates a positive charge (lack of electrons) and the bottom plate of the capacitor accumulates a negative charge (surplus of electrons). This difference in charges on the top and bottom plates constitute a voltage across capacitor C. The speed at which the voltage rises is dependant on the size of the capacitance C and of the resistance R, according to the following equation: -t/RC ) where e = 2.7182 (a constant commonly used in exponential and Vc = V * (1 - e hyperbolic equations where ln e = 1) Vc = voltage that exists across capacitor C t = elapsed time in seconds since switch S1 is closed R = resistance of R in ohms C = capacitance of C in farads. The value RC is called the "time constant" of the circuit. Vc will reach about 63% of V in 1 time constant. After a period of time, the voltage across the capacitor will equal the supply voltage. At this point there will be no further flow of electrons. If switch S1 is then opened, and switch S2 closed, the capacitor will lose its charge. This is because the lower plate of the capacitor is now connected, through the resistor, to the positive voltage that has accumulated on the upper plate. Electrons that had accumulated on the lower plate of the capacitor are now attracted back to the upper plate of the capacitor and there is a path for them to reach it - through switch S2, through R and on to the upper plate of C. As electrons arrive back at the upper plate, they reduce the charge and thus the voltage on the upper plate. The rate at which the voltage falls, is still dependant on the value of R and C as before. At some point the voltage across C will decrease ("discharge") to 0 volts, at which point current flow will stop again. During both charging and discharging cycles, there is current flow in the circuit, but no electrons actually flow through the capacitor. The apparent current through the capacitor is a result of "charge displacement" rather than electrons flowing through the capacitor Switch S1 opened, S2 closed V Vc Switch S1 closed time Graph of voltage across capacitor The Dynamic Operation of CMOS gates. Consider the operation of one logic gate driving the input to another logic gate. We'll use the example of an inverter driving another inverter but the information presented applies to any type of CMOS logical component. A Vx a) an inverter gate driving another inverter Vdd Vdd A C b) the capacitive load at node A The physical gate/oxide/substrate structure of NMOS and PMOS transistors form capacitors. The effect of this is to add a capacitance at the input of the second inverter. This is because the input to the second inverter is actually connected to the gates of both the PMOS and the NMOS transistors implementing this inverter. The equivalent circuit showing this capacitance C is shown above. This capacitance has a negative effect on the time it takes the second inverter to change states as a result of a change in the output of the first inverter. The voltage across a capacitor cannot change instantaneously. The time it takes node A to switch to Vdd or to 0 volts will depend on the size of C and the on resistances of the transistors in the first inverter. (We have been assuming that the on resistance is 0, but actually its a few ohms). Typically, the magnitude of C may be around 50 fF. (50 femtoFarads or 50 * 10**-15 Farads). Since C depends on the surface area of the gate area, it ultimately depends on the level of integration of the integrated circuit. Gate capacitance within a microprocessor will be much, much smaller than the gate capacitance of a CMOS inverter chip because the size of the transistors will be much smaller. If the output of a gate is connected to the inputs of multiple logic gates, the total capacitance at the output is equal to the sum of capacitances of any connected gates. The delay that this capacitance adds to the response time of a logic gate is called the propagation delay. It can be seen in voltage waveforms of the input and output waveforms as shown below. The input to the first inverter in our example is the waveform for Vx. The output of the first inverter is VA. Vdd Gnd Vx 50% 50% Vdd Gnd VA 50% 50% propagation delay propagation delay Propagation delay : the time between a change of input and a resulting change in output. Propagation delay is measured at the 50% points of the signals. A typical propagation delay of a CMOS inverter is 10 nanoseconds (ns). Propagation delays increase as more loads are connected to a gate's output. Capacitance has another negative effect on the dynamic operation of logic gates. It was stated earlier that under static conditions, a CMOS logic gate consumes no power because no current flows through the gate terminal of a MOS transistor. When the input signal changes however, the gate capacitance must charge (or discharge). This does consume power, as charge or discharge current flows through the transistors of the previous gate that is generating the input voltage. The bottom line here is that CMOS logic gates only consume power when the output switches states. At this time it must charge or discharge the gate capacitance of any logic gates connected to the output. The power consumed by an individual transistor may be very small, but in an integrated circuit that is as complex as a microprocessor, the total power required can be a major problem. When current flows through any resistance, some of the energy consumed is converted to heat. Increasing the operating switching speed of a circuit will also increase its power consumption (and the generated heat) because more transistors will be switching per unit time. Here's a practical example: Let's say that a gate's typical capacitance is 50 fF and we have a circuit that operates at 1000 MHz. (10**9 switching or clock cycles per second).Let's say that the dynamic power consumed by each gate at this speed is 1.75 microwatts. Let's also say that we have a microprocessor that has 10 million gates and that on average, 20% of them switch states per clock cycle. How much power does the microprocessor require? Power required = 107 (gates) * 0.20 *1.75 * 10-6 = 35 watts As a real, practical example, the power consumption of a current Pentium 4 operating at 2.4 GHz is about 75 watts. Small wonder that core operating temperatures rise above 40 degrees C even with a heat sink and fan on top of the microprocessor. Summary - Points to Remember Ohm's Law: V = I / R where V = voltage in volts, I = current in amperes, R = resistance in ohms Power = V * I current digital integrated circuit use CMOS technology to implement complex logic circuits. CMOS only consumes power when transistors switch states (on-to-off, or off-toon) CMOS consumes less power than any other current technology. Power consumption generates heat. Steps may have to be taken to remove heat from an integrated circuit. (fan cooling, liquid cooling) Power consumption is proportional to the level of integration of the chip (the number of transistors) and operating frequency. Capacitance exists whenever 2 parallel conductors are separated by an insulator. (MOS transistor gate/oxide/substrate structure, wires within a cable, parallel traces of copper foil on a printed circuit board, etc) The effect of capacitance is to oppose voltage changes and this causes the propagation delay associated with digital logic components.