Kazi Fall 2006 EEGN 4941 EEGN-494 HDL Design Principles for VLSI/FPGAs Khurram Kazi

2Kazi Fall 2006 EEGN 494 Fundamental Steps to a Good design If you have a good start, the project will go smoothly Partitioning the Design is a good start Partition by: Functionality Don’t mix two different clock domains in a single block Don’t make the blocks too large Optimize for Synthesis

3Kazi Fall 2006 EEGN 494 Partitioning

4Kazi Fall 2006 EEGN 494 Recommended rules for Synthesis When implementing combinatorial paths do not have hierarchy Register all outputs Do not implement glue logic between block, partition them well Separate designs on functional boundary Keep block sizes to a reasonable size Separate core logic, pads, clock and JTAG

5Kazi Fall 2006 EEGN 494 Avoid hierarchical combinatorial blocks The path between reg1 and reg2 is divided between three different block Due to hierarchical boundaries, optimization of the combinatorial logic cannot be achieved Synthesis tools (Synopsys) maintain the integrity of the I/O ports, combinatorial optimization cannot be achieved between blocks (unless “grouping” is used).

6Kazi Fall 2006 EEGN 494 Recommend way to handle Combinatorial Paths All the combinatorial circuitry is grouped in the same block that has its output connected the destination flip flop It allows the optimal minimization of the combinatorial logic during synthesis Allows simplified description of the timing interface

7Kazi Fall 2006 EEGN 494 Register all outputs Simplifies the synthesis design environment: Inputs to the individual block arrive within the same relative delay (caused by wire delays) Don’t really need to specify output requirements since paths starts at flip flop outputs. Take care of fanouts, rule of thumb, keep the fanout to 16 (dependent on technology and components that are being driven by the output)

8Kazi Fall 2006 EEGN 494 NO GLUE LOGIC between blocks Due to time pressures, and a bug found that can be simply be fixed by adding some simple glue logic. RESIST THE TEMPTATION!!! At this level in the hierarchy, this implementation will not allow the glue logic to be absorbed within any lower level block.

9Kazi Fall 2006 EEGN 494 Separate design with different goals reg1 may be driven by time critical function, hence will have different optimization constraints reg3 may be driven by slow logic, hence no need to constrain it for speed

10Kazi Fall 2006 EEGN 494 Optimization based on design requirements Use different entities to partition design blocks Allows different constraints during synthesis to optimize for area or speed or both.

11Kazi Fall 2006 EEGN 494 Separate FSM with random logic Separation of the FSM and the random logic allows you to use FSM optimized synthesis

12Kazi Fall 2006 EEGN 494 Maintain a reasonable block size Partition your design such that each block is between gates (this is strictly tools and technology dependent) Larger the blocks, longer the run time -> quick iterations cannot be done.

13Kazi Fall 2006 EEGN 494 Partitioning of Full ASIC Top-level block includes I/O pads and the Mid block instantiation Mid includes Clock generator, JTAG, CORE logic CORE LOGIC includes all the functionality and internal scan circuitry

14Kazi Fall 2006 EEGN 494 Synthesis Constraints Specifying an Area goal Area constraints are vendor/library dependent (e.g. 2 input-nand gate, square mils, grid etc) Design compiler has the Max Area constraint as one of the constraint attributes.

15Kazi Fall 2006 EEGN 494 Timing constraints for synchronous designs Define timing paths within the design, i.e. paths leading into the design, internal paths and design leading out of the design Define the clock Define the I/O timing relative to the clock

16Kazi Fall 2006 EEGN 494 Define a clock for synthesis Clock source Period Duty cycle Defining the clock constraints the internal timing paths

17Kazi Fall 2006 EEGN 494 Timing goals for synchronous design Define timing constraints for all paths within a design Define the clocks Define the I/O timing relative to the clock

18Kazi Fall 2006 EEGN 494 Constraining input path Input delay is specified relative to the clock External logic uses some time within the clock period and i.e. TclkToQ(clock to Q delay) + Tw (net delay) ->{At input to B} Example command for this in synopsys design compiler: Dc_shell> set_input_delay –clock clk 5 (where 5 represents the input delay) (This command is Synopsys centric)

19Kazi Fall 2006 EEGN 494 Constraining output path Output delay is specified relative to the clock How much of the clock period does the external logic (shown by cloud b) use up? Tb + Tsetup; The amount to be specified as the output delay

20Kazi Fall 2006 EEGN 494 Timing paths

21Kazi Fall 2006 EEGN 494 Combinatorial logic may have multiple paths Static Timing Analysis uses the longest path to calculate a maximum delay or the shortest path to calculate a minimum delay.

22Kazi Fall 2006 EEGN 494 Schematic converted into a timing graph

23Kazi Fall 2006 EEGN 494 Calculating a path’s delay