IEEE SOCC 2011 paper

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65nm and beyond CMOS designs are commonly
implemented with “tapless” library cells which do not
provide built-in n-well or substrate taps, improving cell
density. This cell efficiency results in additional layout
complexity for power-gating designs. Three well
tapping methods are described for production powergating
designs considering design schedule, leakage
power, chip area and complexity.

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IEEE SOCC 2011 paper

  1. 1. WELL TAPPING METHODOLOGIES IN POWER-GATING DESIGN Kaijian Shi1 and David Tester2 1 2 Cadence Design Systems, Dallas, USA and Structured Custom, Cambridge, UK kaijians@cadence.com and david.tester@structured-custom.comABSTRACT The first method implements always-on tap cells to 65nm and beyond CMOS designs are commonly keep n-well biased at VDD when the design operatesimplemented with “tapless” library cells which do not in the shutdown mode, i.e. power gated.provide built-in n-well or substrate taps, improving cell The second method requires built-in taps in the PMdensity. This cell efficiency results in additional layout cells to maintain well biasing of the PM cells when thecomplexity for power-gating designs. Three well design is in the shutdown mode.tapping methods are described for production power- The third method partitions the design into always-gating designs considering design schedule, leakage on and shut-down regions. The PM cells are placedpower, chip area and complexity. exclusively in the always-on region to sustain required n-well biasing.I. INTRODUCTION Each method has advantages and shortcomings. Tapless designs have been popular in 65nm and The choice of method depends on the considerationsbeyond CMOS designs to increase cell density and and priorities on leakage power, silicon utilizationsilicon area efficiency. In tapless designs, the logic is efficiency and implementation complexity.implemented using cells which do not have built-in tapcontacts connecting n-well and p-substrate to power In the following part of the paper, the methods willand ground rails in the cells. To prevent latch-up and be described in detail. Considerations for reliablemaintain proper transistor back biasing, tap cells production power-gating designs will be discussed.which have built-in contacts to n-well and p-substrate Overheads and tradeoffs will be explained. Next, theare inserted in the layout at required intervals to three methods will be compared in terms of impact onconnect n-wells to VDD and p-substrate to VSS, leakage power, silicon utilization efficiency andbased on design rules defined in the technology DRC implementation complexity. Finally, recommendationsfile. The n-wells of tapless logic cells extend out of the will be provided for selection of a method based oncell boundaries to ensure n-well connections when design goals and priorities.the cells are placed next to each other. Consequently, II. ALWAYS-ON TAP CELL BASED METHODn-well and p-substrate regions of the logic cells areproperly biased by power and ground supplies. The first method provides dedicated power supplythrough the tap cells insertedin the design.. to n-wells using the tap cells to keep n-wells of the design biased to VDD in shutdown mode. This The tap insertion becomes complicated in power- requires an always-on tap cell, since the normal tapgating designs [1-6] where logic cells can be cells are not powered in the shutdown mode.powered-off while power management (PM) cells,such as power switch cells, isolation cells, retention A. Always-on tap cell vs. normal tap cellregisters and always-on logic cells, must remain The normal tap cell is a simple design (Fig. 1a)powered to maintain controllability and state retention which has two metal contacts; one connects n-well towithin the design. For power-gating designs that VDD rail and the other connects p-substrate to VSS.implement header switches to shut off power supply, When the tap cells are inserted into a design, theirthe main challenge is to maintain proper n-well bias in VDD and VSS rails are connected, by abutment, tothose logic cells that are powered-off and the PM cells the power and ground network of the design tothat remain alive. provide n-well bias. In the power-gating design, the Three well tapping methods are described in this power supply to VDD rails is shut-off in the shutdownpaper, addressing challenges in power-gating designs mode. To maintain the power supply to the n-well, theusing tapless standard cells. The methods have been n-well contact in the tap cell must be separated fromapplied successfully to production designs meeting the VDD rail and directly connected to the powerdifferent design goals and priorities. supply. This design change is depicted in Fig. 1b resulting in the always-on tap cell where the n-well
  2. 2. tap becomes a pin that can be connected to the chip logic never reaches close to ground because thepower supply. The p-substrate is still connected to switch cells are far from ideal and considerably leaky aVSS rail to maintain well bias as the VS rail remains SS in 40nm node and beyond. Also, the size ratio is Aconnected in the shutdown mode. determined by IR-drop constraints and power density of the design in normal oper ration mode. Table 1 shows the leakage penalty of always-on n-well biasing from SPICE simulation of a small test case in different technology nodes and Vth’s. Size ratio of the switch to logic cells is 0.096. Table I. Ioff in gated nwell vs always-on nwell s. Ioff_well_off Ioff f_well_on Node/Vth Ioff_ratio Figure 1 a) normal tap cell, b) always-o tap cell on (nA) (nA) 28HP/LVt 16.2 77.4 4.78B. Method description 28HP/SVt 15.78 153.4 9.72 For a single domain power-gating design, the g 28HP/HVt 12.9 57.65 4.47always-on tap cells are inserted at the intervalsdefined by the technology tapping rules. Then, the 40LP/LVt 0.169 1.06 6.27n-well pins of the tap cells are routed to connect the 40LP/SVt 0.166 1.43 8.61always-on VDD supply network in the design to get 40LP/HVt 0.143 1.05 7.33constant power supply. Since n-wells of the taplesslogic cells are overlapped with adjacen cells in each nt 65LP/LVt 0.082 0.443 5.40row in the layout, forming continuing n-w wells in the cell 65LP/SVt 0.081 0.439 5.42rows, the n-wells of the logic cells are biased by the 65LP/HVt 0.070 0.467 6.67always-on VDD through the tap cells’ n- -wells. The main advantage of the metho is that it is odsimple to implement, leveraging the e existing normal In power-gating designs with both always-on andtap insertion flow. It also results in highest silicon power gated power domains, th always-on tap cells heutilization efficiency compared with o other methods, are inserted in the power gated domains. For always- ddue to no additional well spacing requ uirement in cell on domains, normal tap cells are use to avoid VDD aplacement. Moreover, it does not impo constraints ose power routing needed for always-on taps. a Anon PM cell placement which helps phys sical synthesis. example of the method implem mented in a two powerHowever, the method incurs a leakage power penalty domain design is shown in Fi 2. The bottom left ig.in the shutdown mode because n-w wells of pMOS domain is always-on with norm tap cells inserted maltransistors are biased at VDD while the power supply e aligned to the rails. Always-on taps were implementedto the transistors is shutoff. This create a significant es in the rest of the design with tap n-well pins wbias from n-well to drain and gate of p pMOS resulting connected to the always-on VDD straps next to them. Din higher junction and gate leakag in pMOS.geConsequently, the shutdown mode leakage of a III. TAP PM CELL BASED MET THODdesign can increase up to 10 times depending on For leakage critical designs, the leakage penalty intechnology nodes, Vth cell types and s size ratio of the the always-on tap cell based method might not meet mswitch and logic cells. At smaller tech hnology nodes the leakage target. In that case, it is necessary to shut ,this causes a higher leakage penalt due to the ty off the power supply to the n-w taps. However, PM wellthinner tox and hence larger well lea akage. On the cells are active in shutdown mode so n-wells in the mother hand, lower Vth logic cells ha ave a smaller PM cells must remain being biased at VDD to beleakage penalty because the reductio of the sub- on functional. To address this issue, the tap PM cellthreshold leakage from the reversed back biasing based method has been developed.becomes more effective and the re elatively large The method implements sp pecially designed PMleakage makes the n-well leakage co ontributing part cells containing built-in n-well taps that connect torelatively less effective. As we look to th dependency he their internal always-on power straps (Fig. 3) keeping son the transistor size ratio of the sw witch and logic n-well biased in shutdown mode, This results in acells, a smaller ratio results in larger le eakage penalty mixed tap and tapless design where PM cells are tap wdue to lower shutdown voltage on the logic cells and cells and rest of the logic cells are tapless cells. In the ahence lower cell leakage and higher n-w leakage. It well shutdown mode, the power su upply to the tap cellsis worth mentioning that the shutdown voltage on the inserted in the tapless design re egions is shut off which
  3. 3. Figure 3. Tap PM cell (double row) Figure 2. Domain-based tap insertion e example region based method describe in the next section. ed PM cells can be freely placed in optimal positions as iin turn shuts off power to the n-well of l logic cells. The long as their always-on VDD pin can be routed to the nsn-well of the PM cells is biased through internal taps always-on power straps. Ho owever, the methodto the always-on VDD, maintaining norm operation. mal requires custom PM cells and area wasted at left and a Since n-well of the PM cells is biased while right boundaries of the PM cell is significant loweringsurrounding n-well in the tapless lo ogic cells are silicon utilization efficiency cons siderably since power-shutoff, the n-well of PM cells can no longer overlap gating designs often impleme tens of thousands entn-well in surrounding tapless cells. Well spacing switches, always-on buffers and isolation cells. dbetween PM cells and tapless cells m must satisfy thehot-well spacing rule defined in the te echnology. It is IV. ALWAYS-ON REGION BAS SED METHODworth noting that the n-well of a tapless cell is This method has been developed to avoid theextended beyond the cell boundary so n-wells of the leakage penalty of the first met thod and the design oftapless cells are overlapped forming a continuous n- custom tap PM cells in the se econd method. In thiswell. This tapless cell n-well extension w intrude into will case, PM cells are all tapless cells requiring tap cellthe PM cell placed next to the tapless cell, and insertion to maintain n-well bias To address the need s.therefore must be considered in the ho ot-well spacing of separating n-wells of the PM cells from the n-wells Mcheck. Consequently, PM cells need considerable d of the logic cells, placement reg gions, called always-onspace at cell boundaries, consuming s silicon area. In regions, are created exclusively for PM cells. Eachproduction designs, cells are common mirrored on nly always-on region has its own dedicated always-onadjacent rows and n-wells of cells in the mirrored rows e VDD rails separated from rails outside the region. The ooverlap. This is leveraged in designin the tap PM ng n-well of the cells in the regio is connected to the oncells to occupy both mirrored rows to hide the n-wells always-on VDD through tap cells inserted in theof the PM cell from the top and bott tom of its cell region. These always-on region are placed cross the nsboundaries and hence eliminate needs of the hot-well chip based on the prediction of the needs and nspacing at top and bottom to improve a area efficiency. positions of the PM cell ls in the physicalFig. 3 shows an example PM cell. implementation. In the physic synthesis, the PM cal Only those PM cells that require pM MOS transistors cells are only allowed to be pla aced in the always-onto be active in shutdown mode need bu uilt-in well taps. regions. An illustration example is shown in Fig. 4.For those isolation-low cells which imp plement a pull- The always-on regions are shown in red. The top sdown nMOS at the output, there is no need for the tap and bottom regions are for switc cells. The region on chversion cells, because pMOS transistors of the cell do the right is where the output iso olation cells are placed.not contribute to the isolation in the shhutdown mode The four regions in the middle of the block are created oand so isolation-low cells can be taplesss. to place always-on repeaters.The main advantage of the method is low leakage Advantages of the method are low leakage power apower in shutdown mode, since n-we of the logic ell in shutdown mode and no need to create the custom dcells are not biased. Impact on the physical tap PM cells. However, the method introduces eimplementation is much smaller than the always-on e considerable physical implem mentation complexity.
  4. 4. Moreover, the region creation and p placement are timing models of the tapless PM cells, as modification Mhighly design dependent and difficult to predict. It t has minimal impact on fun nctional layout. Areacould often result in lower silicon utiliza ation efficiency overhead of the method is usua less than 5%. allyand negative impact on design timing an routability. nd The always-on region based method is much more complicated to implement and less predictable in its effect on the design. Moreove it often impacts the er, design timing and routability. The area overhead varies with the quality of the always-on region t planning and could be significant. However, the method does not introduce a leakage penalty, nor requires development of custom PM cells. m The choice of the method depends on the design d goals and priority in terms of design performance, f leakage power, schedule, and silicon area. If a development schedule and pe erformance are higher priorities than leakage power in shutdown mode, the n always-on tap based method is the choice. On the other hand, for battery operated designs where shutdown mode leakage is critic and area efficiency cal is less important, the tap PM cell based method is a c Figure 4. Always-on region based m method good choice. In the case where design resources are e not available to create the tap PM cells, the always-on P An always-on region can be created by either an region based method is an alter rnative method.exclusive region or a custom placeme site. In the entformer case, a special filler cell not containing n-well VI. SUMMARYis needed at region boundaries to se eparate the n- Three well tapping methods have been developedwells from outside. VDD rails in th region are he to address the challenges in the tapless power-gating eassigned to the always-on VDD and s separated from designs. The methods are described with arails outside of the region. implementation details. Each method has advantages m In practice, always-on region plannin is done after ng and shortcomings which are discussed andinitial physical implementation to scope how many PM compared. The proper choice of the method depends ocells are needed and where they should be placed. on design goals and priorities in terms of designAssuming PM cells can be clustered into regions, the performance, leakage power, development schedule, dalways-on regions are created and pla aced based on and silicon area. The method have been applied dssize and position requirements. To ensu the regions ure successfully to production pow wer-gating designs tocan hold the required PM cells, always- regions are -on meet different design goals and priorities.often created larger than actually needed. Thisreduces silicon utilization efficiency. REFERENCES [1] Kaushik Roy, Saibal Mukhopadhy yay, and Hamid Mahmoodi- Always-on regions also add placeme constraints ent meimand, “Leakage current mecha anism and leakage reductionand obstructions, impacting placemen and routing. nt techniques in deep-submicrometer CMOS circuits”, Proc.This could result in sub-optimal physical synthesis. IEEE Vol. 91, no. 2, Feb. 2003 [2] M. Anis, S. Areibi and M. Elmasry, “Design and optimization of ,V. COMPARISON AND RECOMMEND DATION multi-threshold CMOS (MTCMOS) circuits”, IEEE Trans. CAD- ) ICS, 2003 Each method is appropriate for volu ume production [3] Benton H Calhoun, Frank A Honore and Anantha Pdesigns and has advantages and shor rtcomings. The Chandrakasan, “A leakage re eduction methodology foralways-on tap cell based method is simple to distributed MTCMOS”, IEEE J. Solid-State Circuits, vol. 39, Simplement with little impact on timing, routability and no. 5, May, 2004, pp. 818-826 [4] Kaijian Shi and David Howard, “S Sleep Transistor Design andsilicon utilization efficiency at the expen of leakage nse Implementation – Simple Concepts Yet Challenges To Bepower. Optimum”, Proc.. IEEE VLSI-DAT, April, 2006 The tap PM cell method is easy to implement with [5] David Flynn, Michael Keating, Robert Aitken, Alan Gibbonslittle impact on timing and routability. I does require It and Kaijian Shi , “Low Power Methhodology Manual for System- on-Chip Design”, Springer, 2007custom design of tap PM cells, though they can be [6] Kaijian Shi, Zhian Lin, Yi-min Jian Lin Yuan “Simultaneous ng,relatively easy to create by adding well taps and Sleep Transistor Insertion and Po ower Network Synthesis forextending cell boundaries of the taples version PM ss Industrial Power Gating Design ns”, Journal of Computer,cells that are already available. It is fea asible to reuse Academy Publisher Vol. 3 No.3 March, 2008 M

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