# CHAPTER 16 Memory Circuits

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1 CHAPTER 16 Memory Circuits

2 Introduction! The 2 major logic classifications are! Combinational circuits: Their output depends only on the present value of the input. These circuits do not have memory.! Sequential circuits: Logic circuits that incorporate memory are called sequential circuits. Their output depends not only on the present value of the input but also on the previous value of the input.! 2 approaches to provide memory to a digital circuits:! Static sequential circuits: They employ positive feedback to provide a circuit with 2 stable states (bistable).! Dynamic sequential circuits: They utilize a capacitor to store the charge.

3 16.1 Latches and Flip-Flops The Latch! The basic memory element, the latch, is shown below.! Two inverters Positive feedback! Bistable circuit with two complementary outputs Figure 16.1 Basic latch.

4 The Latch Figure 16.1 (a) Basic latch. (b) The latch with the feedback loop opened. (c) Determining the operating point(s) of the latch.

5 " Point B is an unstable operating point. By considering the noise that causes positive (negative) small increment at W, the regenerative process will shift the point B upward (downward) to point C (A). Figure 16.2

6 The SR Flip-Flop NOR The latch together with the triggering circuitry forms a flip-flop. NOR A B Y Figure 16.3 (a) The set/reset (SR) flip-flop and (b) its truth table. _ " When S/R are raised to 1 simultaneously, Q/Q become 0. However, when S/R return to reset state (S=R=0), the state of flip-flop will be undefined.

7 CMOS Implementation of SR Flip-Flops! The clocked version of an SR flip-flop has the 2 cross-coupled inverters as the heart and only NMOS transistors are used in the set-reset circuitry (no conducting path between V DD and ground). CMOS implementation of a clocked SR flip flop. Figure The clock signal is denoted by φ

8 CMOS Implementation of SR Flip-Flops! The clocked version of an SR flip-flop has the 2 cross-coupled inverters as the heart and only NMOS transistors are used in the set-reset circuitry (no conducting path between V DD and ground). " If the flip-flop stores 0 (Q=0 and Q=1) initially, we wish to set it. # Arrange V DD on S input while R is held at 0V. # φ goes high " Q5/Q6 conduct and pull the V Q down. \$ If V Q goes below the V TH of Q3/Q4 inverter, it will change state " V Q rises. % The increase of V Q is fed to Q1/Q2 inverter, causing the V Q further down

9 CMOS Implementation of SR Flip-Flops! The normal operation of the flip-flop is based on 2 important assumptions. # Transistors Q5/Q6 supply sufficient current to pull the node Q down to a voltage at least slightly below the V TH of Q3/Q4 inverter. & The set signal remains high for an interval long enough to cause regeneration to take over the switching process..

10 A simpler CMOS Implementation of the clocked SR Flip-Flop Pass-transistor logic is employed to implement the clocked set-reset functions The clock signal is denoted by φ Figure 16.7 A simpler CMOS implementation of the clocked SR flip-flop. This circuit is popular as the basic memory cell in the design of static random-access memory (SRAM) chips.

11 MULTIVIBRATOR CIRCUITS The flip-flop has two stable states and is called a bistable multivibrator There are two other types of multivibrator: monostable and astable. The monostable multivibrator has one stable state in which it can remain indefinitely. It has another quasi-stable state to which it can be triggered. pulse standardizer Figure 11.9 The monostable multivibrator (one-shot) as a functional block, shown to be triggered by a positive pulse Microelectronic Circuits - Fifth Edition Sedra/Smith

12 A CMOS Monostable Circuit With Ideal NOR gates (and Vth=VDD/2) NOR A B Y Figure A monostable circuit using CMOS NOR gates. Signal source v I supplies positive trigger pulses. Microelectronic Circuits - Fifth Edition Sedra/Smith

13 A CMOS Monostable Circuit With real NOR gates Microelectronic Circuits - Fifth Edition Sedra/Smith

14 EFFECT OF: Diodes at each input of a two-input CMOS gate Figure (a) Diodes at each input of a two-input CMOS gate. (b) Equivalent diode circuit when the two inputs of the gate are joined together. Note that the diodes are intended to protect the device gates from potentially destructive overvoltages due to static charge accumulation.

15 # DDE_04.html#NOR

16 Output resistance 2input CMOS NOR gate A B Y Figure Output equivalent circuit of CMOS gate when the output is (a) low and (b) high.

17 Figure Timing diagram for the monostable circuit in Fig τ>(t P1 +t P2 ) A B Y Stable state Vo2=0 Microelectronic Circuits - Fifth Edition Sedra/Smith

18 Figure Timing diagram for the monostable circuit in Fig Figure Circuit that applies during the discharge of C (recovery time) (at the end of the monostable pulse interval T). Microelectronic Circuits - Fifth Edition Sedra/Smith

19 Soglia di commutazione V t =V th 2

20 20

21 Τ Τ Τ

22 An Astable Multivibrator Circuit Figure (a) A simple astable multivibrator circuit using CMOS gates. Animazione senza diodi Microelectronic Circuits - Fifth Edition Sedra/Smith

23 An Astable Circuit Neglecting the finite output resistance of the CMOS gate and assuming that the clamping diodes are ideal (thus have zero voltage drop when conducting). 1 (b) Waveforms for the astable circuit. The diodes at the gate input are assumed to be ideal and thus to limit the voltage v I1 to 0 and V DD. Microelectronic Circuits - Fifth Edition Sedra/Smith

24 An Astable Circuit with ideal diodes 1 1 I 1 Microelectronic Circuits - Fifth Edition Sedra/Smith

25 An Astable Circuit with ideal diodes 2 2 I 2 Microelectronic Circuits - Fifth Edition Sedra/Smith

26 An Astable Circuit with ideal diodes Neglecting the finite output resistance of the CMOS gate and assuming that the clamping diodes are ideal (thus have zero voltage drop when conducting). 1 (b) Waveforms for the astable circuit. The diodes at the gate input are assumed to be ideal and thus to limit the voltage v I1 to 0 and V DD. Microelectronic Circuits - Fifth Edition Sedra/Smith

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