Sequential Logic. References:

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1 Sequential Logic Reerences: Adapted rom: Digital Integrated Circuits: A Design Perspective, J. Rabaey UCB Principles o CMOS VLSI Design: A Systems Perspective, 2nd Ed., N. H. E. Weste and K. Eshraghian Pro. Kaushik Roy

2 Clocked Systems: Finite State Machines Inputs Combinational Logic Outputs Out = (In, State) Current state bits Next state bits Q D Registers Clock or clocks Registers serve as storage element to store past history Pro. Kaushik Roy

3 Clocked Systems: Pipelined Systems Inputs D Q D Q D Q Logic Logic Outputs Clock Registers Registers serve as storage element to capture the output o each processing stage Pro. Kaushik Roy

4 Storage Mechanisms Positive eedback Connect one or more output signals back to the input Regenerative, signal can be held indeinitely, static Charge-based Use charge storage to store signal value Need rereshing to overcome charge leakage, dynamic Pro. Kaushik Roy

5 Positive Feedback: Two Cascaded Inverters V i1 V o1 = V i2 V o2 V i2 = V o1 Pro. Kaushik Roy

6 Bi-Stability and Meta-Stability V i2 =V o1 A V i2 =V o1 A C C B B d V i1 =V o2 d V i1 =V o2 Gain larger than 1 ampliies the deviation rom C Gain less than 1 reduces the deviation rom A Pro. Kaushik Roy

7 SR-Flip Flop NOR-based SR lip-lop, positive logic Schematic Logic Symbol Characteristic table NAND-based SR lip-lop, negative logic Forbidden state Schematic Logic Symbol Characteristic table Pro. Kaushik Roy

8 JK- Flip Flop Schematic Logic Symbol Characteristic table Clock input to synchronize changes in the output logic states o lip-lops Forbidden state is eliminated, But repeated toggling when J = K = 1, need to keep clock pulse small < propagation delay o FF Pro. Kaushik Roy

9 Other Flip-Flops Toggle or T lip-lop Delay or D lip-lop Pro. Kaushik Roy

10 Race Problem A lip-lop is a latch i the gate is transparent while the clock is high (low) Signal can raise around when is high Solutions: Reduce the pulse width o Master-slave and edge-triggered FFs Pro. Kaushik Roy

11 Master-Slave Flip-Flop Either master or slave FF is in the hold mode Pulse lengths o clock must be longer than propagation delay o latches Asynchronous or synchronous inputs to initialize the lip-lop states Pro. Kaushik Roy

12 One-Catching or Level-Sensitive Q M Q S Pro. Kaushik Roy

13 Propagation Delay Based Edge-Triggered Depend only on the value o In just beore the clock transition Pro. Kaushik Roy

14 Edge Triggered Flip-Flop Pro. Kaushik Roy

15 Flip-Flop: Timing Deinitions Pro. Kaushik Roy

16 Maximum Clock Frequency t pff t p, comb t setup T Pro. Kaushik Roy

17 CMOS Clocked SR Flip-Flop V DD S Q Q M2 M4 Q R Q S M6 M5 M1 M3 M8 M7 R Pro. Kaushik Roy

18 Transistor Sizing o SR Flip-Flop Assume transistors o inverters are sized so that V M is V DD /2, mobility ratio m n /m p = 3 (W/L) M1 = (W/L) M3 = 1.8/1.2 (W/L) M2 = (W/L) M4 = 5.4/1.2 To bring Q rom 1 to 0, need to properly ratio the sizes o pseudo-nmos inverter (M7-M8)-M4 V OL must be lower than V DD /2 k n, M 78 V V 2 DD DD DD DD V 2 8 DD Vtn k p, M 4 VDD Vtp 2 8 p ( W / L) M 78 ( W / L) M 4 ( W / L) M 3 n m m ( W / L) M 2( W / L) M 78 2( W / L) M 3 7 (3.6/1.2) V V 2 Pro. Kaushik Roy

19 Flip-Flop: Transistor Sizing Pro. Kaushik Roy

20 Propagation Delay Pseudo- NMOS inverter (M5-M6)-M2 Inverter M3-M4 Pro. Kaushik Roy

21 Complementary CMOS SR Flip-Flop V DD S M10 M12 R M9 M11 Q M2 M4 Q M6 M1 M3 M8 S M5 M7 R Eliminates pseudo-nmos inverters Faster switching and smaller transient current Pro. Kaushik Roy

22 6-Transistor SR Flip-Flop V DD R Q M2 M4 Q S M1 M3 Pro. Kaushik Roy

23 CMOS D Flip-Flop V DD D Q Q Q Q D D Pro. Kaushik Roy

24 Master-Slave D Flip-Flop V DD Q Q D D Pro. Kaushik Roy

25 CVSL-Style Master-Slave D-FF V DD V DD Q Q D Pro. Kaushik Roy

26 Charge-Based Storage D In D Pseudo-static Latch Pro. Kaushik Roy

27 Layout o a D Flip-Flop Q In Q In Q Q Pro. Kaushik Roy

28 Master-Slave Flip-Flop In A B D Overlapping clocks can cause race conditions undeined signals Pro. Kaushik Roy

29 2 Phase Non-Overlapping Clocks D In 1 A t p12 2 Pro. Kaushik Roy

30 Asynchronous Setting In 1 A 2 D 2 -Set 1 Pro. Kaushik Roy

31 2-phase Dynamic Flip-Flop In 1 A 2 D Input sampled Output Enabled 1 2 Pro. Kaushik Roy

32 Flip-lop Insensitive to Clock Overlap V DD V DD M2 M6 In M4 X M8 D M3 C L1 M7 C L2 M1 M5 -section -section C 2 MOS Latch or Clocked CMOS Latch Pro. Kaushik Roy

33 C 2 MOS Latch Avoids Race Conditions V DD V DD M2 M6 In X D C L1 1 M3 1 M7 C L2 M1 M5 Cascaded inverters: needs one pull-up ollowed by one pulldown, or vice versa to propagate signal (1-1) overlap: Only the pull-down networks are active, input signal cannot propagate to the output (0-0) overlap: only the pull-up networks are active Pro. Kaushik Roy

34 Clocked CMOS Logic Replace the inverter in a C 2 MOS latch with a complementary CMOS logic V DD V DD In1-3 In1-3 PUN M4 M3 PDN C L1 X PUN M8 M7 PDN C L2 Divide the computation into stages: Pipelining Pro. Kaushik Roy

35 REG REG REG REG REG REG REG REG Pipelining a Non-pipelined a Pipelined log log b b T min t p, reg t p, logic tsetup, reg T min, pipe t p, reg max( t p, adder, t p, abs, t p,log) tsetup, reg Pro. Kaushik Roy

36 Pipelined Logic using C 2 MOS V DD V DD V DD In out F G C 1 C 2 C 3 NORA CMOS (NO-RAce logic) Race ree as long as all the logic unctions F and G between the latches are non-inverting Pro. Kaushik Roy

37 Example V DD V DD V DD In Number o static inversion should be even Pro. Kaushik Roy

38 NORA CMOS -module = 0 = 1 Logic Precharge Evaluate Latch Hold Evaluate Pro. Kaushik Roy

39 NORA CMOS -module = 0 = 1 Logic Evaluate Precharge Latch Evaluate Hold Pro. Kaushik Roy

40 NORA Logic NORA data path consists o a chain o alternating and modules Dynamic-logic rule: single 0 1 (1 0) transition or dynamic n-block (p-block) C 2 MOS rule: I dynamic blocks are present, even number o static inversions between a latch and a dynamic block Otherwise, even number o static inversions between latches Static logic may glitch, best to keep all o them ater dynamic blocks Pro. Kaushik Roy

41 Doubled C 2 MOS Latches Doubled n-c 2 MOS latch Doubled p-c 2 MOS latch Pro. Kaushik Roy

42 TSPC - True Single Phase Clock Logic Including logic into the latch Inserting logic between the latches Pro. Kaushik Roy

43 Simpliied TSPC Latches A and A do not have ull logic swing Pro. Kaushik Roy

44 Master-Slave Simpliied TSPC Flip-Flops Positive edge-triggered D lip-lops Reduces clock load Pro. Kaushik Roy

45 Further Simplication Pro. Kaushik Roy

46 Schmitt Trigger VTC with hysteresis Restores signal slopes Pro. Kaushik Roy

47 Noise Suppression using Schmitt Trigger Sharp low-to-high transition Pro. Kaushik Roy

48 CMOS Schmitt Trigger Moves switching threshold o irst inverter Pro. Kaushik Roy

49 Sizing o M3 and M4 V M+ = 3.5V k2 2 M1 and M2 are in saturation; M4 is in triode region k V V V k V V V V DD M tp V M DD V tn tp 2 DD M V DD V 2 M 2 V M- = 1.5V M1 and M2 are in saturation; M3 is in triode region Pro. Kaushik Roy

50 Schmitt Trigger: Simulated VTC Pro. Kaushik Roy

51 CMOS Schmitt Trigger In = 0, at steady state, V Out = V DD, V X = V DD - V tn In makes a 0 1 transition Saturated load inverter M1-M5 discharges X M2 inactive until V X = V in - V tn Use V in to approximate V M- M1-M5 in saturation k1 2 k V V V V 2 M tn DD M Pro. Kaushik Roy

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