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Topic 4: Digital Circuits

Topic 4: Digital Circuits

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Topic 4: Digital Circuits

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  1. Topic 4: Digital Circuits (Integrated Circuits Technology)

  2. What is on the agenda? • Introduction • Basic Operational Characteristics and Parameters of Integrated Circuits • CMOS Technology • Bipolar Technology • Some Practical considerations

  3. 1. Introduction • So far we have analyzed the mathematical theory (combinational logic fundamentals) that presents some of the logic concepts commonly used in digital systems design. • We will be more concerned here about technology. It can be defined as means and physical implementations of real digital circuits whose behaviors are dictated by the digital laws previously studied. • To understand some of the issues related to the technology, a number of questions must be answered such as: • What type electronic basic element (passive or active) can be used to implement a simple logic gate such as an “inverter”? • How efficiently is an implementation in terms of power and speed? • What is the level of integration? • How to characterize a digital electronic device? • We are very fortunate that these questions were answered properly in the past by many physicists. We will present here their outcomes in terms of technology.

  4. 2. Basic Operational Characteristics and parameters • Digital components are called “Integrated circuits”. They are implemented using transistors. • In Digital electronics, transistors are always configured to work in switching modes. • The type of transistor being used defines the technology: • TTL (transistor-transistor-logic) for bipolar transistors • CMOS (complementary MOS) for MOSFET transistors. • MOSFET = metal-oxide semiconductor field-effect transistor • The following figure show an IC packages that contains “nand” gates.

  5. Figure 1: IC Package containing Nand Gates.

  6. 2.1. Logic Levels • The concept of logic levels is used to represent logic variables in digital electronic circuits. • There are four different logic-level specifications: • VIL (Voltage input Low) • VIH (Voltage input high) • VOL (Voltage output low) • VOH (Voltage output high) Figures 2 and 3 show the CMOS and TTL logic Levels respectively.

  7. Figure 2: Inputs and output logic levels for CMOS

  8. Figure 3: Input and output Logic levels for TTL Figures 2 and 3 show clearly that these two technology don’t support all the ranges of voltages. If and input falls into the unallowed region the behavior of the circuit is unpredicatble, therefore its output doesn’t represent a valuable information

  9. 2.2. Noise • Noise is unwanted voltage that is included in electrical circuits and can present a threat to a proper operation of the circuit. • Examples of noise: • Thermal noise • Electromagnetic noise • Power-line voltage fluctuation noise • In order not to be adversely affected by noise, a logic circuit must have a certain amount of noise immunity. • The following figure shows some of the consequences of noise on a logic gate.

  10. v(t) i(t) VDD • Noise – unwanted variations of voltages and currents at the logic nodes • from two wires placed side by side • capacitive coupling • voltage change on one wire can influence signal on the neighboring wire • cross talk • inductive coupling • current change on one wire can influence signal on the neighboring wire • from noise on the power and ground supply rails • can influence signal levels in the gate

  11. NMH = VOHmin - VIHmin Noise Margin High Noise Margin Low NML = VILmax - VOLmax • For robust circuits, want the “0” and “1” intervals to be a s large as possible VDD VDD VOHmin "1" VIHmin Undefined Region VILmax VOLmax "0" Gnd Gnd Gate Input Gate Output • Large noise margins are desirable, but not sufficient …

  12. 2.3 Noise Immunity • Noise margin expresses the ability of a circuit to overpower a noise source • noise sources: supply noise, cross talk, interference, offset • Absolute noise margin values are deceptive • a floating node is more easily disturbed than a node driven by a low impedance (in terms of voltage) • Noise immunity expresses the ability of the system to process and transmit information correctly in the presence of noise • For good noise immunity, the signal swing (i.e., the difference between VOH and VOL) and the noise margin have to be large enough to overpower the impact of fixed sources of noise

  13. V(x) V(y) VOH = ! (VOL) VOL = ! (VOH) 2.4 Static Gate Behavior • Steady-state parameters of a gate – static behavior – tell how robust a circuit is with respect to both variations in the manufacturing process and to noise disturbances. • Digital circuits perform operations on Boolean variables x {0,1} • A logical variable is associated with a nominal voltage level for each logic state 1  VOH and 0  VOL ! = complement • Difference between VOH and VOL is the logic or signal swing Vsw

  14. V(x) V(y) V(y)=V(x) Switching Threshold VM 2.5 DC Operation Voltage Transfer Characteristics (VTC) • Plot of output voltage as a function of the input voltage V(y) f VOH = f (VIL) VOL = f (VIH) VIL VIH V(x)

  15. VOH "1" VIH Undefined Region VIL VOL "0" 2.6. Mapping Logic Levels to the Voltage Domain • The regions of acceptable high and low voltages are delimited by VIH and VIL that represent the points on the VTC curve where the gain = -1 V(y) Slope = -1 VOH Slope = -1 VOL VIL VIH V(x)

  16. 2.7 Directivity • A gate must be undirectional: changes in an output level should not appear at any unchanging input of the same circuit • In real circuits full directivity is an illusion (e.g., due to capacitive coupling between inputs and outputs) • Key metrics: output impedance of the driver and input impedance of the receiver • ideally, the output impedance of the driver should be zero • input impedance of the receiver should be infinity

  17. Fan-out – number of load gates connected to the output of the driving gate • gates with large fan-out are slower N • Fan-in – the number of inputs to the gate • gates with large fan-in are bigger and slower M 2.8 Fan-In and Fan-Out

  18. g = -  2.9 The Ideal Inverter • The ideal gate should have • infinite gain in the transition region • a gate threshold located in the middle of the logic swing • high and low noise margins equal to half the swing • input and output impedances of infinity and zero, resp. Vout Ri =  Ro = 0 Fanout =  NMH = NML = VDD/2 Vin

  19. Propagation delay tp = (tpHL + tpLH)/2 50% tpHL tpLH 90% signal slopes 50% 10% tf tr Delay Definitions Vin Vout Vin input waveform t Vout output waveform t

  20. 2.10 Modeling Propagation Delay • Model circuit as first-order RC network vout (t) = (1 – e–t/)V where  = RC R vout C Time to reach 50% point is t = ln(2)  = 0.69  vin Time to reach 90% point is t = ln(9)  = 2.2  • Matches the delay of an inverter gate

  21. 2.11 Power and Energy Dissipation • Power consumption: how much energy is consumed per operation and how much heat the circuit dissipates • supply line sizing (determined by peak power) Ppeak = Vddipeak • battery lifetime (determined by average power dissipation) p(t) = v(t)i(t) = Vddi(t) Pavg= 1/T  p(t) dt = Vdd/T  idd(t) dt • packaging and cooling requirements • Two important components: static and dynamic

  22. Propagation delay and the power consumption of a gate are related • Propagation delay is (mostly) determined by the speed at which a given amount of energy can be stored on the gate capacitors • the faster the energy transfer (higher power dissipation) the faster the gate • For a given technology and gate topology, the product of the power consumption and the propagation delay is a constant • Power-delay product (PDP) – energy consumed by the gate per switching event • An ideal gate is one that is fast and consumes little energy, so the ultimate quality metric is • Energy-delay product (EDP) = power-delay 2

  23. 3. CMOS Technology • The basic building blocks in CMOS logic circuits are MOS transistors.

  24. 3.1 MOS Transistors

  25. Gate Source Drain Substrate (Body) (a) NMOS transistor V G V V S D (b) Simplified symbol for an NMOS transistor NMOS transistor

  26. Gate Drain Source V DD Substrate (Body) (a) PMOS transistor V G V V S D (b) Simplified symbol for an PMOS transistor PMOS transistor

  27. V DD R R + 5 V V V - f f V V x x (a) Circuit diagram (b) Simplified circuit diagram x f x f (c) Graphical symbols A NOT gate built using NMOS technology

  28. V DD V f V x 1 x x f 1 2 0 0 1 V x 0 1 1 2 1 0 1 1 1 0 (a) Circuit (b) Truth table x x 1 1 f f x x 2 2 (c) Graphical symbols NMOS realization of a NAND gate

  29. NMOS realization of a NOR gate

  30. V V DD DD V f A V x 1 x x f 1 2 0 0 0 V x 2 0 1 0 1 0 0 1 1 1 (b) Truth table (a) Circuit x x 1 1 f f x x 2 2 (c) Graphical symbols Figure 3.8 NMOS realization of an AND gate

  31. NMOS realization of an OR gate