CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES INTRODUCTION The development of digital integrated circuits naturally parallels the maturation of the semiconductor industry. The discovery in 1947 of the point‐contact transistor by Bardeen and Britain was followed by Shockley’s discovery in 1948 of the bipolar junction transistor. These discoveries changed the world and together these three Bell Labs researchers shared the 1972 Nobel Prize in Physics. The MOSFET transistor was not fully realized until the successful experiments of Kahng and Atalla, another set of Bell Labs researchers, in 1959. The ten‐year maturation lead held by the BJT device ensured that BJT‐based digital logic circuits would dominate throughout most of the 1960s, and 1970s. However, the advantages of MOSFET switches in size and power made CMOS the dominant technology by the 1990s. Logic equations require the ability to form the complement of a signal. The design of a family of digital integrated circuits begins with the design of the inverter. Circuit theory equations are developed to achieve output performance goals for the inverter such as the output low voltage, power consumption, or propagation delay. These equations are solved to determine the required component values in the circuit. The results from this reference inverter are then scaled to n‐input logic gates. The inverter drives the output to two different logic values using the pull‐up circuit and the pull‐down circuit. This is diagrammed abstractly in Figure 1. Figure 1: The basic structure of a digital logic gate Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES Each circuit can include passive components such as resistors and capacitors, as well as semiconductor components such as diodes and transistors. The classic logic families shown in Figure 2 achieve their performance goals through different configurations of these devices. INTEGRATED CIRCUIT LOGIC FAMILY INVERTER CIRCUIT Resistor‐transistor logic (RTL) VCC Fairchild Semiconductor, 1961 Passive pullup resistor Active pulldown BJT RC Advantage: Y Simple design RB A NPN Disadvantages: Q1 Speed Resistor Size Static Power Input current must be sourced Diode‐transistor logic (DTL) VCC Signetics, 1962 Passive pullup resistor Active pulldown BJT RB RC Advantage: Y Better noise margin than RTL Disadvantages: A NPN Q1 D1 D2 Speed Resistor Size Static Power Current flows out of input Figure 2: Inverters in Various Logic Families Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES INTEGRATED CIRCUIT LOGIC FAMILY INVERTER CIRCUIT Transistor‐transistor logic (TTL) VCC Sylvania Universal, 1963 Passive pullup resistor Back‐to‐back N‐P‐P‐N input diodes of DTL replaced with NPN transistor RB1 RC2 Active pulldown BJTs Advantages: Y Only one semiconductor structure NPN Better speed than DTL A NPN Q2 Q1 Disadvantages: Resistor Size Static Power Current flows out of input MOS logic (PMOS) -VDD General Microelectronics, 1964 Passive pulldown resistor to ‐VDD Active pullup resistor to 0V Historical point: Intel 4004 was PMOS RD Advantage: Y MOSFET size smaller than BJT No input current Disadvantage: A Resistor Size Static Power Negative voltages Figure 2 Continued: Inverters in Various Logic Families Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES INTEGRATED CIRCUIT LOGIC FAMILY INVERTER CIRCUIT Complementary MOSFET (CMOS) VDD RCA, 1965‐1968 Active pullup PMOS Active pulldown NMOS PMOS Advantage: Speed Y Size No static power No input currents A NMOS Disadvantage: Two types of transistors Sensitive to electrostatic discharges Figure 2 Continued: Inverters in Various Logic Families Engineers optimize speed, size, power, and cost. These four areas are balanced in trade‐off decisions during design. Design decisions lead to advantages and disadvantages. Figure 2 lists some advantages and disadvantages of each family; and arguably there are more that could be included. Historically, engineers made modifications to the basic circuits in Figure 2 as they attempted to overcome disadvantages. These modifications formed family sub‐types by adjusting component values or by adding additional passive and active components in the pullup and pulldown networks. One example subtype is low power TTL (74LS) which used larger pullup resistances to reduce current and power consumption. The quest to optimize speed, size, and power led to continual development of new configurations. Today, the industry has settled on CMOS as the dominant circuit because the advantages far outweigh the disadvantages. MOSFET scaling followed Moore’s Law and continually reduced conduction distance. This allowed frequency scaling to high‐speed. Also, MOSFET devices are smaller than BJTs and more CMOS gates can be fabricated on a given surface area. Finally, the strongest advantage is no static power consumption. CMOS does not allow current to flow between the power supply and ground when the output is stable at either logic‐1 or logic‐0. Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES The electrical performance of an inverter is measured in the voltage and time domains. Voltage domain plots examine how the output changes as the input changes voltage. The resulting plot is called a voltage‐transfer characteristic (VTC). Figure 3 provides an example for a +5V inverter. Note the input voltage is plotted from 0 to 5V and the output responds as an inverter shape from 5V to 0V. 5.0V VOH VIL (569.620m,4.9739) 4.0V 3.0V 2.0V VM (743.514m,746.692m) 1.0V VIH (778.481m,136.750m) VOL 0V 0V 0.5V 1.0V 1.5V 2.0V 2.5V 3.0V 3.5V 4.0V 4.5V 5.0V v(4) VA Figure 3: An Inverter Voltage Transfer Characteristic There are five points of interest on the voltage‐transfer characteristic. They are: VOH: the output high voltage, VOL: the output low voltage, VIL: the last input considered low by the circuit (first point where dVY/dVA = ‐1), VIH: the first input considered high by the circuit (second point where dVY/dVA = ‐1), and VM: the point where the input and output voltages equal. These five points can be manipulated by changing the design of the pullup and pulldown networks or by adjusting the semiconductor parameters. The Figure 3 inverter has a very low input voltage noise margin. Consider the input signal held low at logic‐0. Any noise present on the signal will move the plot on the VA axis. There is only approximately 0.5V margin before this noise would be misinterpreted as a logic‐1 and inverted. The plot also shows a very wide high noise‐margin. The ideal noise margins would be balanced with a sharp output transition occurring at VDD/2. Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES A digital logic gate is design to be driven by the output of another logic gate from the same family. The input of any gate will transition from the previous gate VOL to the previous gate VOH. This gives these equations for the noise‐margin widths: logic‐low noise margin: NML = VIL – VOL logic‐high noise margin: NMH = VOH ‐ VIH Time domain plots examine input‐to‐output propagation delay, output signal rise time, and output signal fall time. Figure 4 shows one example measurement. 6.0V 4.0V 50% input (50.002n,2.5104) 2.0V 0V v(1) 6.0V TPHL = 50% output - 50% input 4.0V 50% output (50.026n,2.5104) 2.0V SEL>> 0V 48.0ns 48.5ns 49.0ns 49.5ns 50.0ns 50.5ns 51.0ns 51.5ns 52.0ns v(4) Time Figure 4: Measuring Output Time to Propagate High‐to‐Low in the Time Domain Time domain analysis has at least four values of interest: TPHL: the time for the output to propagate high to low, TPLH: the time for the output to propagate low to high, Tr: the time for the output to rise from low to high voltage, and Tf: the time for the output to fall from high to low voltage. Different manufacturers use different measurement techniques when stating time domain delays. The standard for propagation delays is to measure from 50% of the input change to 50% of the output change. Rise times and fall times typically use either 10% to 90% or 20% to 80%. The times are then quoted as 10‐90 rise‐time or 20‐80 rise time, for example. Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES LABORATORY OBJECTIVES Use SPICE to identify the voltage transfer characteristic of digital inverters. Use SPICE to characterize the static power of digital inverters. Use SPICE to determine the noise margins of digital inverters. Build and test multiple inverters in the laboratory. Document the laboratory work in a laboratory report. REQUIRED SOFTWARE PSPICE AD Lite Digilent Waveforms Word processor REQUIRED HARDWARE FROM EECS TECH SUPPORT Blue Wire Box Two (2) 3.9KΩ resistors One (1) 2n7000 NMOS transistor One (1) BS250 PMOS transistor Two (2) 2N3904 NPN transistors Two (2) 1N4148 signal diodes Analog Discovery USB instrumentation kits TEST CIRCUIT Function Generator Power Supply Oscilloscope V+ gnd V- gnd V+ gnd 1+ 1- 2+ 2- RC Y RB A NPN Q1 Dr. Russ Meier, Milwaukee School of Engineering, Last Update: April 9, 2018 CE3101: DIGITAL ELECTRONICS AND COMPUTER INTERFACING LABORATORY LW5: DIGITAL LOGIC FAMILIES PRE‐LABORATORY SIMULATION 1.