Floor Planning and Power Planning Interview Questions


  1. What is Floor planning?
  2. How can you say a floor plan is good?
  3. Is timing information mandatory for floor planning?
  4. What are the inputs for floor planning stage?
  5. What are the outputs from floor planning?
  6. What are the floor planning control parameters?
  7. Differentiate Static and Dynamic power? What are the different techniques to save static and dynamic power?
  8. What do you mean by Aspect ratio?
  9. What is Sanity check?
  10. What is cell delay?
  11. What is net delay?
  12. What are the difference between standard cell and macro?
  13. Do static and dynamic power have any relation with power supply?
  14. What is core utilization?
  15. What is total chip utilization?
  16. What is Pre-routing? Why do you need to do it before placement?
  17. What is core limited and pad limited design?
  18. Can macro be placed between core and die boundary or in I/O pad?
  19. How is macro placement done in floor planning? What are the guidelines for macro placement?
  20. What is placement blockage? What are the different types of placement blockages?
  21. What is Halo? How it is useful?
  22. Can we rotate the macro in lower technologies?
  23. What do you mean by Flylines? How it is useful in floorplanning?
  24. What is core utilization percentage?
  25. What are the datas getting from synthesis team? How do you validate them?
  26. What is mean by wire bond and flip chip?
  27. What is the need for sanity checks?
  28. Can area recovery be done by downsizing the cells at path with positive slack?
  29. How to reduce congestion?
  30. What are the inputs requred for floor planning? Explain each?
  31. What is Power planning?
  32. Why do we use alternate routing approach HVH /VHV (Horizontal-Vertical-Horizontal)/(Vertical-Horizontal-Vertical)?
  33. What is the main reason to use flip chip over wire bond package?
  34. How to find out the minimum spacing between two macros?
  35. What are the steps to be taken care while doing floor planning?
  36. Why power stripes routed in the top metal layers?
  37. What would you do in order to not use certain cells from the library?

20 comments:

  1. Can you please post answers ..

    ReplyDelete
  2. Q37> Set don’t use attribute on those library cells.

    ReplyDelete
  3. Q36> Power routes generally conduct a lot of current. In order to reduce effect of IR drop, we need to make these routes less resistive. Top metal layers are thicker and offer lesser resistance. This helps to reduce IR drop.

    ReplyDelete
  4. Q35> Following are the steps to be taken care while doing floor planning.
    1. Die Size Estimation
    2. Pin/pad location
    3. Hard macro placement
    4. Placement and routing
    5. Location and area of the soft macros and its pin locations
    6. Number of power pads and its location

    ReplyDelete
  5. Q34>
    The distance between macro
    = (no. of pins of macros*pitch*2)/no. of available routing layers

    Metal pitch = Metal Width + Metal Space

    ReplyDelete
    Replies
    1. the pitch varies for different metal layers. which metal layer pitch we need to consider? for e.g: in layerstack with 8 metal layers with 5 1x, 2 2x and 1 ultra thick metal, width and space is different for each of these categories.

      Delete
    2. If macro pins include many metal, We should choose the pitch of TOP metal( For example, If macro have pin M4 and M6, We choose pitch of M6)

      Delete
  6. This comment has been removed by the author.

    ReplyDelete
  7. Q33> Flip-chip assembly and wire bonding are the principal methods for interconnecting ICs.
    Following are the Process Advantages of Flip-Chip Design
    1. Device Speed
    2. Power and Ground Distribution
    3. I/O Density with Area Array
    4. Package Size /Form Factor
    5. Low Stress over active Area
    6. Reliability

    ReplyDelete
  8. Q32>
    1. No. of routing resources is increases.
    2. To avoid crosstalk
    3. To reduce RC value

    ReplyDelete
  9. Q31> Power Planning is used to distribute power to each part of design equally.

    ReplyDelete
  10. Q30> Inputs are
    1. Synthesized Netlist
    2. SDC
    3. Floor control parameters
    4. Physical Partitioning information of design

    ReplyDelete
  11. Q29> Following are the congestion reduction techniques
    1. Placement Blockage
    2. Module Constraints( Fence, Region, Guide)
    3. Macro Padding ( Halos)
    4. Cell Padding ( help to remove pin density)
    5. Scan Chain Re-ordering

    ReplyDelete
  12. Q28>
    Yes, Area recovery can be done by downsizing cells at path with positive slack. Also deleting unwanted buffers will also help in area recovery

    Q27>
    The main reason for using Sanity check is used to checking/qualifying the design for further acceptance at each stages of the physical implementation.
    It qualifies the netlist in terms of timing, checks the issues related to library files, constraints files etc.
    Following are the sanity checks carried out in physical design flow:
    (a) check_library: It performs consistency checks between logical and physical libraries.
    (b) check_timing: This is to check any unconstrained paths present in the design. check whether the pins/ports has it's corresponding I/O delays and also checks for the clock definition exists for all flop pins.
    (c) check_design: This check is to report problems like undriven input ports, unloaded output ports, nets/ports with multiple drivers, unloaded nets, pins mismatch, cells or instances with out I/O pins/ports etc.

    ReplyDelete
  13. Q26>
    Flip-chip assembly and wire bonding are the principal methods for interconnecting ICs.

    Q24>
    Core utilization = (standard cell area+ macro cells area)/ total core area
    A core utilization of 0.8 means that 80% of the area is available for placement of cells, whereas 20% is left free for routing.

    Q23>
    Flylines means check net connections from macro to macro and macro to standard cells. If there is more connection from macro to macro, place those macros nearer to each other preferably nearer to core boundaries.

    Q22>
    In advanced Technology Nodes Macro Orientation is fixed since the Poly Orientation can’t vary, so there will be restrictions in Macro Orientation.

    Q21>
    Halo (Keep-out Region) is a region around boundary of fixed macro in desig in which no other macro or std cells can be placed. Placement of cells out of such region avoids congestion.

    ReplyDelete
  14. Q20>
    In order to avoid placement in some areas, they are used.
    There are 3 types of Placement blockages
    (a) Soft Blockage: allows only buffer & inverters to optimizes/ meet the timing. Used for Timing purpose only. Advantages is that Area is less, Timing is ease & less transition time.
    (b) Hard Blockage: Won't allow any cell. Used for Routing purpose only. Advantages is used to avoid congestion. Disadvatages is Area is wastage.
    (c) Partial Blockage: Some % of all celss can be used. No of available tracks can be increased uisng this. Advantages is to meet timing, routing easy & less IR drop.

    Q19> Macro placement can be done using Macro guidelines.
    1. Based on Flylines analysis (connections) between macros.
    2. Macro should be near to core boundaries.
    3. Communicating macros should be placed nearer.
    4. Macro cell pin direction should be towards std cell direction.
    5. Two Macros in between, we can placed/add soft bloackages
    6. MAcros alignment should be follow
    7. Criss Cross connection is not allowed.
    8. Notches( edges) is avoided,
    9. Orientation is not allowed.

    ReplyDelete
  15. Q18>
    Yes. Core is defined as the inner block, which contains the standard cells and macros. There is another outer block which covers the inner block. The I/O pins are placed on the outer block.

    Q17>
    1. Pad Limited Design :
    A design is called Pad limited design when there are large number of pad cells determining the die size
    2. Core Limited Design :
    A design is called Core limited design when the large number of standard cells and macros determine the die size.

    Q16>
    Prerouting means routing of PG nets. Power Planning also called Pre-routing as the Power Network Synthesis (PNS) is done before actual signal (net) routing and clock routing.
    For Area optimization, pre-routing is need to do it before placement(Flip Chip Concept)

    Q15>
    Total utilization T(F) of floorplan F is derived using the following equation
    T(F) = (A(m) + A(p) + A(s) ) / A
    where A(m) = Area occupied by macros,
    A(p) = Area occupied by Pads/ Pad fillers
    A(s) = Area occupied by Standard Cells

    ReplyDelete
  16. Q14>
    Core Utilization = (Stdcell area + Macro Cell Area)/ ( Total Core Area)

    Q13>
    Yes.
    Static Power: The charge power consumed when gate is in ideal/fixed state (0 or 1). (Leakage Current)
    Dynamic Power: The charge power consumed when gate is in switching state. (Short Circuit Current)

    Q12>
    Macros are intellectual properties that you can use in your design. You do not need to design it. For example, memories, processor core, serdes, PLL etc. A macro can be hard or soft macro.
    A standard cell is a group of transistor and interconnect structures that provides a boolean logic function (e.g., AND,OR, inverters) or a storage function (flipflop or latch).

    Q11>
    Net delay is the amount of delay from the output of a cell to the input of the next cell in a timing path. This delay is caused by the parasitic capacitance of the interconnection between the two cells, combined with net resistance and the limited drive strength of the cell driving the net.

    Q10>
    Cell Delay is the amount of delay from input to output of a logic gate in a path. PT calculates the cell delay from delay tables provided in the technology library for the cell.

    Q9>
    Sanity Checks mainly checks the quality of netlist in terms of timing. It also consists of checking the issues related to Library files, Timing constraints, IOs and Optimization Directives.

    Q8>
    Aspect Ratio gives/ decide the shape of block.
    Aspect Ratio = Vertical Resources/ Horizontal Resources

    Q7>
    Static Power: The charge power consumed when gate is in ideal/fixed state (0 or 1). (Leakage Current)
    Dynamic Power: The charge power consumed when gate is in switching state. (Short Circuit Current)
    Static power(SP) = Leakage current * supply voltage
    Leakage current = is (e ^qv/KT - 1)
    where is= reverse saturation current
    V = Diode voltage
    K = Boltzman constant
    T= Temperature
    Dynamic power consumption of CMOS circuitry is given by the formula:
    DP = C * V2 * f
    where P is the power, C is the effective switch capacitance, V is the supply voltage, and f is the frequency of operation.
    How to reduce dynamic power?
    1) reduce power supply voltage Vdd
    2) reduce voltage swing in all nodes
    3) reduce the switching probabilty (transition factor)
    4) reduce load capacitance

    Q6>
    Floor planning control parameters are:
    Aspect Ratio & Core Utilization

    Q5>
    Outputs of Floor Planning are:
    1. Die/ Block Area
    2. I/O pad placed
    3. Macro placed


    Q4>
    Inputs for floor planning stage are
    1. Synthezed Netlist
    2. SDC
    3. Floorplanning control parametes
    4. Physical partioningv information of design

    Q3>
    Yes. For Sanity check purpose we need timing information.

    Q2>
    1. Minimize chip area
    2. Making routing easy
    3. Reduce IR drop across design

    Q1>
    Floorplanning is process placing macros in chip/ core area.

    ReplyDelete
  17. Does answer to ques 18 makes sense ? How macro can be placed between core and die area boundary ? Can you please elaborate ?

    ReplyDelete
  18. macros can not be placed between core and die area. But some IPs are exception which can be placed. Generally IPs which owns BUMPs are allowed to intersect core to die area

    ReplyDelete