Verification Presentation to SystemVerilog Basic Committee Peter Flake Nov 15, 2002

© 2002 Synopsys, Inc. (2) CONFIDENTIAL Agenda AM Interfaces Instantiation syntax Logic type Type use before definition

© 2002 Synopsys, Inc. (3) CONFIDENTIAL Interfaces Fundamental idea  Modules encapsulate behavior  Interfaces encapsulate communication Structural communication  Named bundles of wires Procedural communication  Prototype tasks and functions

© 2002 Synopsys, Inc. (4) CONFIDENTIAL Structural Communication Wires in interface default to inout Need to specify direction for documentation, linting, synthesis QOR So introduce modports  refer to names in enclosing interface

© 2002 Synopsys, Inc. (5) CONFIDENTIAL Procedural Communication Point to point messages  process in one module calls a task in another Needs hierarchical name - hard to maintain and re-use Interface prototype means name is local to module  import to calling module  export from called module

© 2002 Synopsys, Inc. (6) CONFIDENTIAL Broadcast Messages One module sends a message and does not know how many receive it forkjoin task Allows address to be coded in receiver module - correct encapsulation Allows registration of receivers e.g. count No slower than fork join and task call

© 2002 Synopsys, Inc. (7) CONFIDENTIAL Slippery Slope How about being able to translate task calls to values on wires? Put in tasks and continuous assignment  No new scheduling semantics How about being able to translate signals to task calls, and check bus activity? Put in always blocks

© 2002 Synopsys, Inc. (8) CONFIDENTIAL Conclusion Interfaces are not modules  The design intent is connectivity, checkers, testbench  They synthesize to P&R wires, not logic blocks Interfaces are not structures  Interfaces are static objects, never automatic  Interfaces can contain wires and modports  Interfaces can contain always blocks

© 2002 Synopsys, Inc. (9) CONFIDENTIAL Instantiation Syntax Parentheses required moduleType instance ( ); interfaceType instance ( );

© 2002 Synopsys, Inc. (10) CONFIDENTIAL Logic Type Two basic communication mechanisms  Resolved drivers - Verilog wire or VHDL signal  Shared variable - last write wins Verilog reg is a mixture (legacy)  Shared variable inside module  Gets converted to a wire through a port Reals and 2 state variables (e.g. bit) must be shared Useful to have a 4 valued data type that behaves like a shared variable through ports Must not be driven with strengths

© 2002 Synopsys, Inc. (11) CONFIDENTIAL Type Use Before Definition typedef foo; Minor convenience in SV 3.0 allows less attention to file order Type checking must be at elaboration time anyway  parameters, generate Essential with classes in Testbench donation

© 2002 Synopsys, Inc. (12) CONFIDENTIAL Agenda PM Time precision and time scale Time data type Typedef syntax Strings as array and structure literals Unions and 4 state members Masked and unmasked Short real to 32 bits Constants always_comb Increment and decrement

© 2002 Synopsys, Inc. (13) CONFIDENTIAL Time Precision and Time Scale For $root, use timeunit & timeprecision as first thing in source. `timescale does not affect $root timeunit & timeprecision override `timescale in their module or interface definition `timescale applies to any subsequent module or interface

© 2002 Synopsys, Inc. (14) CONFIDENTIAL Time Data Type Time variable is a logic [63:0] for backward compatibility Time literal is 3.4ns Time literals can be considered as floating point numbers scaled to the current time unit When passed as a parameter the floating point number may have to be re-scaled When used in an expression the number is rounded to the current time precision Only operators that are valid for reals can be used with time literals

© 2002 Synopsys, Inc. (15) CONFIDENTIAL Typedef Syntax ?

© 2002 Synopsys, Inc. (16) CONFIDENTIAL Strings as Array and Structure Literals A structure member or an array element should behave like a scalar of the same type A shortreal can be set to a real literal shortreal a = 4.5 An array can be set to a string

© 2002 Synopsys, Inc. (17) CONFIDENTIAL Unions and 4 State Members If a union contains a 2 state member and a 4 state member, the union is 4 state If the 2 state member is written and then the 4 state member is read, there is no Z or X Like assigment var4state = var2state

© 2002 Synopsys, Inc. (18) CONFIDENTIAL Masked and Unmasked Masked and unmasked are the terms in the Superlog manual and in the Co-Design donation They were changed to 4 state and 2 state by the Accellera committee Some changes were missed

© 2002 Synopsys, Inc. (19) CONFIDENTIAL Short Real to 32 Bits Real and short real should be cast to integer or 32 bits with rounding To get same bit pattern use union (like C)

© 2002 Synopsys, Inc. (20) CONFIDENTIAL Constants A parameter or localparam can only be set to an expression of literals, parameters or local parameters, or a constant function of these A const in a static scope can also be set to an expression containing a hierarchical name A const in an automatic scope can also be set to an expression containing arguments

© 2002 Synopsys, Inc. (21) CONFIDENTIAL always_comb Used to model combinational logic  indicates design intent  execution at time 0 is guaranteed Cannot drive from anywhere else - no resolution Functions can be used to partition the combinational logic & so add to sensitivity  useful with large case statement No timing or event controls should be allowed

© 2002 Synopsys, Inc. (22) CONFIDENTIAL always_comb continued Sensitivity list should be with functions inlined Static function locals should be treated like static variables in modules

© 2002 Synopsys, Inc. (23) CONFIDENTIAL Increment and Decrement Consider int i = 1; $display( "%d %d %d %d %d %d", i++, i++, ++i, --i, i--, i-- ); Current implementation gives Alternative is to limit to only one per variable per statement like C