Gate Level Simulation - Various Issues

The typical RTL-to-gate-level-netlist flow is

GLS verification:-

• Verifying netlist which is in the form of gates
• TB setup of RTL will be modified upon
• done on
  • DC - Synthesized netlist Design Compiler
  • PD - Place and Route netlist
    • design close to a representation of the actual geometries we manufacture

Why do GLS?

verification	GLS
RTL	clock and reset circuits bring-up PLL clash with digital in conjunction to timing and delays Realistic power analysis with delays CDC and Synchronization checks
LEC	Done to check RTL simulation vs Synthesis equivalence DFT verification, since scan-chains are inserted after RTL synthesis Clock-tree synthesis For switching factor to estimate power Analyzing X state pessimism or an optimistic view, in RTL or GLS
STA	Dynamic Timing behaviour of chip logic The inability of STA to identify asynchronous interfaces Static timing constraint requirements, like false and multi-cycle paths Verifying system initialization and that the reset sequence is correct

GLS challenges:-

• Long compilation time with increasing netlist and SDF Annotations
• Shrinking Technology nodes:
  • 90nm -> 65nm -> 45nm
  • Different nodes often imply different circuit generations and architectures
  • Generally, the smaller the technology node means the smaller the feature size, producing smaller transistors which are both faster and more power-efficient
• Complexity of design reflected in SDF timing annotations
• Large Compile DB sizes which translate to
  • large Machine Memory requirements
  • frequent simulation hangs
• Longer simulation runtimes extending upto 3-4 days or even a week
• GLS Debug is cumbersum and time consuming
  • false failures due to x-propogation

GLS setup requrements:-

• Netlist design
• Technology library models gates, flops, HardMacro(IC in block form)
• SDF file for timing annotations
• Non-resettable flops list from STA engineer
• Special models for Analog/Mixed Signals like PLL Phase-locked loop

GLS steps:-

• 'GATESIM switch to instantiate Netlist DUT
• SV environment for GLS
  • adding 'GATESIM wherever we refer RTL hierarchy
    • optimized by the sysnthesis tool
  • Testcases, asserstions and memory paths differ with RTL
• Changing paths for preloading memories (RAM)
  • Compiled C code, .hex form, should be loaded into from where the processor starts fetching data
    • On-chip RAM, DDR RAM, Code-RAM, Data-RAM
    • hierarchy of these memories may change during sysnthesis
• SDF Annotations
• adding Force file
  • .do files prepared by GLS engineer from a list of non-resettable flops STA engineers
  • contains syntax to initialize non-resettable flops and memory outputs
  • force -feeze and -deposit
    • freeze will fix the value of a register throughout the simulation
    • deposit will just intializing allowing it to be driven later by the design
  • can be used to address simulation hangs by forcing some flop values
    • x propogation from memory models can be identified by forcing NO_CORRUPT bit on memory
    • clock enables can also be forced

GLS Testcase Selection:-

• we don't run entire RTL regression, because
  • GLS is time consuming
• must involve boot up or initialization
• atleast one testcase for each block in the design
• clock checking and source switching
• clock frequency scaling
• asynchronous paths in design
• to check entry/exit from different modes of the design
• Dedicated tests for timing exceptions in the STA
• Patterns covering multi clock domain paths in the design

For example

SoC Verification	GLS Verification
CPU's Power Manager Peripherals Buses Multi-media SubSystem etc..	Booting Cache access with MMU enabled Interrupt handling (Generic Interrupt controller) Power collapse entry and exit
CPU's total testcases might comprise of 200-300 testcases	number of tests shrink to 20-30
testcases will target different features of the CPU's	Peripherals may have one testcase each likeUART, I2C, SPI

GLS types:-

• Zero / Unit delay simulation no Annotations
  • early clean-up of netlist
  • can be done in the early design cycle
  • before SDF is ready or annotations added
  • checks reset sequence and system boot
  • checks if there are issues due to scan insertion
  • can identify zero delay loops and thus race conditions into the design
  • faster than SDF sim
• Timing simulation with SDF Annotations behaviour simulating real Silicon(Si)
  • SDF provides timing delay values for all cells and interconnects
    • min: best delay for ideal Process Voltage Temperature PVT conditions
      • for hold timing checks
    • max: worst case timing delay
      • for setup timing checks
    • typical: intermediate of min & max

GLS Issues

TB compile time Setup issues

- due to design optimization by sysnthesis tool
- nomenclature of hierarchies and signals may change
  - GenVar
  - Bus signals getting bit blasted
  - change/removal of signals due to optimization

X-propogation

• X optimism refers to how simulations may incorrectly exhibit determinate behaviour even when inputs to logic are X and have indeterminate value.
  • It can be dangerous and can mask real RTL bugs
  • posedge and negedge from x and z
• x pessimism: drives 'z or 'x even in determinate situations where having x is not ambiguous
  • mux example with sel as x
• Causes:-
  • Library models:-
    • Each gate has a corresponding library model containing its functionality
    • models might have specify blocks which might add delays or use #delay
    • they prevent x to updated or initialized in a given timing event
  • Design
    • non-resettable flops can't break from x-propogation
    • clock divider example where w/o reset can't break x and with reset can't generate the clock
  • Synthesis
    • resets might be boggled by logic
    • Incomplete Logic Optimization Interferes With Reset
  • SDF

Zero delay Simulation issues- Zero delay timing

Zero delay loops

• Combinational gate loops formed in netlist
  • Adding small delay will break these loops
  • tool dependent command-line options to generate logs
    • +vcs+loopreport=100000
    • +autofindloop in questa
• Scheduling semantics
  • circular logic with continuous assignments, either with forever or always_* blocks or assign statements
  • multi-bit circular logic can be evaded during sysnthesis
  • zero delay simulations do not have zero delay loops because there a netlist of gates is formed where timing of each gate is specified need to check
    • Ex:- circular logic with forever loop
    • Ex:- circular logic with always_comb
    • Ex:- circular logic in netlist gates
• Fixing requires adding delays in gate paths
  • can be done using SDF annotations
  • .tfile will add delays without changing design code

Functional bugs with Netlist

Multi driven signals

• FV(Formal Verification) will catch it

Glitches in simulation

• generally in clock

Unconnected ports

• 'hz (default value) causing x-propogation
• caught in FV

Simulation hang issues

Additional functional issues, found during debug

Simulation hang due to TB/model issues

• Memory data getting corrupted - initialization
• Memory data out signal X
• x-propogation from TB
• Non-resettable flops causing x-propogation

SDF simulation issues

discrepencies b/w SDF file and design chages should trigger SDF file regeneration netlist and SDF cells and nets names must match

Wishbone debugging:-

• wb_ack_o is used to validate the transaction.
• udp_dff was the cause of the problem. udp is User Defined Primitive