Dynamic Logic Families

Transcription

1 Dynamic Logic Families C.K. Ken Yang UCLA Courtesy of MAH,JR 1

2 Overview Reading Rabaey 6.3 (Dynamic), (NORA) Overview This set of notes cover in greater detail Dynamic Logic Families and in particular Domino Logic. There is an extensive discussion on the noise issues in dynamic circuits and how they are resolved. A few variants of domino logic are introduced. 2

3 Domino Logic Family Outline Dynamic/domino logic Domino logic Timing of domino logic Noise issues and keepers Dual-rail domino logic (Dynamic DCVS) and other domino styles 3

4 Review: Pre-charged Logic (1) We saw before that pseudo-nmos logic s main disadvantage was the static current that it consumes. One way to get rid of it is to build a dual pmos stack to cut this static current path (CMOS). Another approach to eliminate this static current is pre-charging. What do you mean by pre-charging? Before each evaluation phase, pre-charge the output high Execution of Boolean expression either discharges output or leaves it high A single low-to-high transition on the input allowed, but NOT a high-to-low transition during evaluation static current Dual pmos Network precharge A non-overlapping (good, but not always possible) B precharge precharge evaluate evaluate evaluate Psuedo-nMOS CMOS Pre-Charge 4

5 Review: Precharged Logic (2) Implement the logic function with nmos pull-down stack as in pseudo-nmos Can use a single clock signal = pre-charge = evaluate clk clk clk clk These gates cannot be cascaded, even if complementary clocks are used for alternating stages Constrained by low-to-high transition requirement at the input during evaluation Need to put an inverting stage between them Domino Logic 5

6 Review: Domino Logic clk X precharged node rises monotonically clk This can be any static CMOS gate (NAND, NOR, etc.) During pre-charge: Output of dynamic stage (X) pre-charged high when clk is low Domino gate output driving input of another always low during pre-charge During evaluate: X is conditionally discharged during evaluation Output of static buffer rises monotonically Inverting gate can be any inverting static CMOS gate It is impossible for buffer output to go from H-to-L during evaluation 6

7 Review: Domino Chains nmos nmos nmos Cascaded gates can be switched from PRECHARGE to EVAL on the same clock edge Logic decisions propagate through the cascade (or chain) like a row of falling dominos Length of domino chains is limited by EVAL time Logic must propagate to the output before falls Inputs to domino stage must be held stable during EVAL Domino gates are ratioless All domino gates are NONINVERTING (no XOR function) 7

8 Review: Delay in Domino Circuits clk Eliminating fat slow pmos transistors allows less input capacitance for same drive strength (lower logical effort) Less input capacitance for same drive strength Reduces diffusion capacitances Domino gate has lower switching threshold, so it starts switching sooner No contention between pull-up and pull-down 8

9 Review: Logical Effort of Dynamic Gates LE= 1 LE=2/3 What about the foot transistor? Does it need to be sized the same? NAND structure might not need a footing transistor. 9

10 Review: Precharged NAND Decoder Generally Built with NAND gates If you don t use clocked transistors Can get lower logical effort CLK W/2 4W If we used NAND gates with skewed inverters afterward 2W Assume inputs are pulses 2W W Average Logical Effort is Sqrt(2/3 * 5/6) =

11 Monotonic Edge Optimization Care most about evaluation speed, so skew static gate to favor input falling edge (output rising edge) Use high-skewed CMOS gates (pmos >> nmos) Caveats: degraded noise margins, slower pre-charge time Structuring logic into dynamic and static gates is an art form Static gate favors NAND (since series pmos slow) Dynamic stage allows more series devices clk dyanmic stage static stage 11

12 Clocked Evaluation Transistor The clocked evaluation transistor is not strictly necessary. Can remove if all the inputs are provably low during pre-charge Other domino gate outputs satisfy this condition Also okay if high inputs are in series with provably low input Delay pre-charge edge to reduce power burned at start of pre-charge clk clkd clkdd clk clkd clkdd L H L H clk clkd clkdd 12

13 Pre-charge Properties Many domino gates can evaluate in one half-cycle, so it should be easy to pre-charge a single domino gate in the other half-cycle. But The domino gate must pre-charge enough to flip the high skew gate, then the high skew gate must fall below V t by sufficient noise margin before evaluation starts again To speed up domino evaluation, we want a small pre-charge transistor (small diffusion parasitic capacitances) Makes pre-charge slow High skew gate falls very slowly Delaying the clock to avoid pre-charge contention in un-clocked pull-down stacks reduces pre-charge time for clkdd domino gate Cycles are getting shorter Advanced domino methodologies are stretching the length of evaluation phase at the expense of pre-charge time Bottom line: pre-charge time is becoming an important issue. Size for roughly equal pre-charge and evaluate times 13

14 Domino Logic Family Outline Dynamic/domino logic Domino logic Timing of domino logic Noise issues and keepers Dual-rail domino logic (Dynamic DCVS) and other domino styles 14

15 Clocking for Domino Circuits (1) Make sure that the half-cycle during pre-charge is not wasted. Use clk for one domino chain, and clk_b for the 2 nd domino chain. Data transfers from one phase (chain) to the next. Need a latch between the phases since data is gone during precharge. If pre-charge comes early, we may lose the data. clk clk_b Clk_b Latch Clk_b domino Static Clk_b domino Static Clk_b domino Clk Latch Clk domino Static Clk domino Clk Static domino Legend: Domino: One inverting dynamic gate Static: One inverting static gate Latch: Inverting tristate latch Source: D. Harris 15

16 Clocking for Domino Circuits (2) Domino doesn t look so attractive in the context of a traditional pipeline. Pay clock skew twice in each cycle. Balancing short phases is difficult since there is no time borrowing. Latches become a significant fraction of the cycle time. clk clk_b Clk_b Latch Clk_b domino Static Clk_b domino Static Clk_b domino Clk Latch Clk domino Static Clk domino Static Clk domino Legend: Domino: One inverting dynamic gate Static: One inverting static gate Latch: Inverting tristate latch Source: D. Harris 16

17 Domino-clocking Evaluation Let T = cycle time = 16 FO4 delays; t skew = 2; t setup = 1 Difficult filling cycle exactly (no time borrowing) -> t imbalance = 1 T phase-logic = T/2 - t skew -t setup -t imbalance Baseline Design: T phase-logic = 50% of the phase is wasted in overhead! Slower than static! Optimized Design: Define clock domains and use t skew-local = 1 Work hard to balance logic between phases: t imbalance = 0 (optimistic) T phase-logic = Still, 25% of the phase is overhead! Source: D. Harris 17

18 Early Enhancements Good designers have recognized this problem for years. The largest problem is the hard edges set by the latches. A variety of latches soften this edge: from domino SR Latch Dual-Monotonic LatchTSPC Latch Source: D. Harris 18

19 Skew-tolerant Domino Clocking How much clock skew could we tolerate given N clock phases? Divide logic into N phases of T/N duration each. Overlapping clocks eliminates need for latches Extra overlap accommodates clock skew and time borrowing As with other domino techniques, budget skew on the transition from static to domino static domino static domino static domino static domino static domino static domino static domino static domino 19

20 Skew Tolerance T = t e + t p t p = t prech + t skew; t e = T/N + t skew + t hold Hence t skew-max = [T(N-1)/N - t prech -t hold ] / t p 1a 1b t e must overlap by t hold 2a Effective Precharge Window 1a 1b 2a static domino static domino static domino 20

21 Time Borrowing If we overlap the phases some more, we can provide a region where we can allow time-borrowing between the phases. Both phases are high for longer period of time. Helps with logic granularity. t borrow t overlap t hold t skew 21

22 Numerical Example Assume that T cycle =16 Let t prech = 4, long enough to: Precharge domino gate Make subsequent skewed static fall below V t t hold is slightly negative for reasonable cell libraries Next phase can evaluate before precharge ripples through static gate N t skew t p Conservatively bound t hold at 0 Sweet spots: N=2 (fewest clocks), N=4 (good tolerance, 50% duty cycle) 22

23 Aside: 4-Phase Skew-Tolerant Domino Don t need to worry about data flowing through from within 1 cycle. No min-delay constraint. Lots of overlap for skew tolerance and time borrowing. 23

24 Some Design Issues State is no longer stored in the latch at the end of a phase Instead, it is held by the first domino gate in the phase Use a full keeper to allow stop-clock operation from 1 block 2 weak All systems with overlapping clocks require min-delay checks Domino paths are presumably critical anyway, so few mindelay errors 4-phase has effectively no min-delay risk Overlap of all four phases is at most very small A minimum of 8 gates are in the cycle anyway 24

25 Pulse Stretching and Shrinking Stretch pulses by 2 inverter delays using an even number of inverters. Input transitions HIGH Output stays HIGH (inverted) after the 2 inverter delay. Create a pulse with only 3 inverter delay pulse-width. Input transitions HIGH Both inputs are HIGH (output LOW) for 3 inverter delays Each tick= t inv 25

26 Multiphase Clock Generation Generating precisely shaped clocks is not easy. Fortunately, it doesn t need to be terribly precise. 2-phase clocking 1 and 2 are nonoverlapping. In this design, length of ck non-overlap does not scale with frequency. Use pulse stretchers to guarantee overlap. Control overlap with inverters. 4-phase clocking often need well-controlled delay lines. ck in Clock complement Pulse widen 3 ¼ t per ¼ t per 1 2 Pulse widen Pulse widen

27 Example: 2 -Phase Time Borrowing Time borrowing in the Itanium (Rusu00) Use 4 clock phases Clkd overlaps with both clkb and clk to allow borrowing between Phase 1 and Phase 2. Instead of requiring exactly 180 o overlapping clocks 27

28 N-phase Skew-Tolerant Domino The idea is to delay the clock along with the data flow. Can t delay by too much (>T cycle /2 in case (a) >T cycle in case (b)) would cause improper timing. Last phase ( 6 ) needs to arrive before the next 1 arrives. Phases are not necessarily uniform. 28

29 Interfacing with Static Logic (1) When domino output is driven to a static logic. Pre-charge phase must be eliminated. Follow the pre-charge gate with the latch (Itanium 2) Evaluates low when clock transitions HIGH. When pre-charge data (X) evaluates, output transitions HIGH (or stays LOW). Stays stable during pre-charge because latch is non-transparent when clock is LOW. 29

30 Interfacing with Static Logic (2) When a static logic outputs are driven to the first domino stage. Capture the data with a F/F or latch so that the data do not transition during Evaluate. Or in some way so that only rising edges are allowed. Ultrasparc/Itanium 2 both use a latch that only allows the output to transition from L-H. The latch is pulsed. Only conducting LOW for 3 inverter delay time. A -input arrives before the rising edge is latched. Rising edge A -input that arrives during the pulse is also latched. This essentially gives a small degree of time borrow. 30

31 Domino Logic Family Outline Dynamic/domino logic Domino logic Timing of domino logic Noise issues and keepers Dual-rail domino logic (Dynamic DCVS) and other domino styles 31

32 Noise in Domino Design #1: Charge Leakage Out CLK A Subthreshold leakage Junction leakage V Out Precharge Evaluate Minimum clock rate on the order of khz 32

33 Noise in Domino Design #2: Coupling and Gnd Bounce Coupling Ground Bounce high skew gate V t The output of a dynamic gate is a floating node Coupling on the dynamic node can cause the static gate to glitch Input glitches can discharge dynamic node Portion of glitch >V t is important Ground bounce can cause a glitch or turn on the nmos pull down 33

34 Noise in Domino Design #3: Backgate Coupling A B out1 in out2 3 Dynamic NAND Static NAND 2 1 out1 0-1 in out2 Time, ns

35 Domino Noise Margin: Keepers weak minimum long Keeper for tiny domino gates Dynamic output may be corrupted by subthreshold leakage, -particles Use a weak keeper to make the dynamic node static Keeper doesn t help much with charge sharing and output coupling b/c it is so small Also degrades evaluation speed Prefer separate inverter for keeper Allows complex static gates, minimizes noise coupled onto keeper Dual-gate keeper minimizes load on tiny gates 35

36 Delayed Keepers Weakened keepers are not as effective at restoring the degraded voltage. To avoid fighting, we can turn on a stronger keeper after a small delay. (Alvandpour02), (Allam01), (Jung01) In (b), x floats momentarily. Key is to not delay by too much. Restore before too much charge is gone. But not start the keeper before all the inputs have arrived. Works best with the static logic interface (when all inputs are stable). 36

37 Issue in Domino Design #5: Charge Sharing Domino designs often fail due to charge sharing if internal nodes are not considered Occurs when internal node was low; capacitance divider with output formed Reduce charge sharing by reducing capacitance of internal nodes relative to capacitance of load High fanout gates suffer least from charge sharing Pre-charge internal nodes where necessary with secondary pre-charge devices (generally, every other node suffices) clk out clk in x C out in out goes to high skew gate 0 C x x let C x = C out 37

38 Pre-charging Internal Nodes Normally, internal nodes are pre-charged with small pmos devices Not crucial to get node to 100% of Vdd, just reduce noise Gates actually run faster when some charge sharing occurs Less capacitance needs to be pulled all the way down Sometimes pre-charge an internal node to Vdd-V t with an nmos device Maybe even pre-discharge an internal node to speed it up Worst case for speed is with node high, worst case for noise is with node low If we can tolerate the noise with node low, we might improve the speed by guaranteeing the node is low Use small nmos device (make sure it is off during evaluation) Only can pre-discharge a node if no path to Vdd possibly exists Must be sure that noise is tolerable for all cases when doing this! A B 2 O 38

39 Domino Pitfalls Review There are lots of ways that domino circuits can fail: Charge sharing and leakage Noise coupling onto the output (crosstalk). An -particle hit, sub-threshold leakage, or substrate charge injection on the dynamic node. Power supply noise (especially ground bounce). Fortunately, these are all relatively easy to check with ERC (Electrical Rule Check) and DRC (Design Rule Check) tools. Microprocessor companies routinely build reliable domino datapaths these days. 39

40 Domino Logic Family Outline Dynamic/domino logic Domino logic Timing of domino logic Noise issues and keepers Dual-rail domino logic (Dynamic CVSL) and other domino styles 40

41 Non-monotonic Logic Domino gate + high skew gate pair can only implement non-inverting ( monotonic ) functions. Many important functions are non-monotonic, such as XOR clk a b_b a_b b One solution: push non-monotonic function to end of logic cone Build first part of cone in domino gates Switch to static of transmission gate logic for non-monotonic part Example: carry select adder often uses static mux 41

42 Dual-Rail Domino clk clk F F out_h a a a out_l b b merge into a single pulldown network We can overcome this problem by computing both true and complementary outputs with dual rail domino. Also known as Differential Cascode Voltage Switch (DCVS) Compute out_h and out_l; may be able to share transistors out_h is asserted when the output is evaluated to be high out_l is asserted when the output is evaluated to be low Asserting both out_h and out_l is illegal Both out_h and out_l are unasserted during pre-charge 42

43 Keepers for DCVS F m1 Pull-down Tree m2 F F m1 Pull-down Tree m2 F Keepers are the same idea. Since we have differential, keepers can be cross coupled. 43

44 Multiple-Output Domino MODL (Hwang89) Opportunistic reuse of logic Particularly true of pre-charged carry-propagate chain Can be thought of as one big gate. 44

45 Diode-Footed Domino VDD CLK Out C L Diode-Foot Current Mirror CLK_b The stacking reduces leakage Current mirror and feedback increase the speed 45

46 Operation: Pre-Charge Phase VDD CLK = 0 0 -> VDD VDD -> 0 C L VDD -> 0 CLK_b= VDD 46

47 Operation: Evaluate Phase VDD VDD CLK = VDD VDD -> VDD C L 0 -> 0 CLK = VDD VDD -> C L 0 -> VDD Vx 0V V x CLK_b = 0 CLK_b = 0 V x has finite voltage due to leakage current. Stack of 2 reduce leakage. Initial discharge due to charge sharing Current mirror provide a faster discharge path. Feedback provide remaining discharge 47

48 Simulations Noise immunity test: Apply input noise pulse until noise is unity gain. Normal Operation 48

49 Noise Immunity of DFD 49

50 Summary Dynamic logic is based on optimizing for one edge of evaluation. To eliminate the other edge, a pre-charge phase is introduced. Timing is a critical element of the design Because one of the nodes is dynamic, noise is another critical design constraint. Large internal capacitance can lead to a bad delayrobustness tradeoff. Large fanin can be challenging (especially ANDs). Monotonicity forces us to build dual rail making ANDs unavoidable. Diode-footed is one attempt at pushing the tradeoff to a different point. (We ll see many more). 50