Subject: Re: Question on ROM // Your opinion on reply
From: Shalom Bresticker (Shalom.Bresticker@motorola.com)
Date: Mon Jul 15 2002 - 07:59:49 PDT
In a message dated 7/13/02 2:44:55 PM Eastern Daylight Time,
VhdlCohen@aol.com writes:
> Subj:Question on ROM // Your opinion on reply
> Date:7/13/02 2:44:55 PM Eastern Daylight Time
> From:VhdlCohen@aol.com
> To:vlog-synth@eda.org
> Sent from the Internet
>
>
>
> Question
> 5.6.2, pg 13
> Can any variables be used in calculating memory values for a ROM? Can
> those variables be used in other synthesizable constructs after that?
> Is there any assumption that they obtain an initial value because of
> the calculation?
>
> My answer:
> Add the following to 5.6.2
> Local variables can be used for the calculation of memory values.
> Variables outside the scope of the initial statement shall not be
> allowed.
> Example 25: // <-- that would mess up the numbering of the other
> examples.
> module rom_2dimarray_initial (
> output wire [3:0] z,
> input wire [2:0] a); // address- 8 deep memory
> // Declare a memory rom of 8 4-bit registers. The indices are 0 to 7:
> (* synthesis, rom_block = "ROM_CELL XYZ01" *) reg [3:0] rom[0:7];
> initial
> begin : ROM_DEF
> integer i;
> for (i=0; i <= 255; i=i+1)
> if (i<100) rom[i] = (i + 1) % 16;
> else rom[i] = (i + 2) % 16;
> end
>
> assign z = rom[a];
> endmodule
>
> Rationale: Order of execution of initial statements is
> non-deterministic.
> Allowing non-local variables would allow one ROM (e.g., ROM1 defined
> with initial) to read content of another ROM or another
> non-initialized variable.
> Like:
> initial
> begin : ROM_DEF
> integer i;
> for (i=0; i <= 255; i=i+1)
> if (i<100) rom[i] = i + 1;
> else rom[i] = rom2[i]* 2; // rom2 is in another initial statement
> // THAT SHOULD NOT BE ALLOWED
> end
>
> Opinions?
>
> Below is same code with earlier vlog style for compilation checks.
> FYI
> ---
> // style (for code debug only. Compiles OK
> module rom(z, a);
> output [3:0] z;
> input [4:0] a;
>
> reg [3:0] rom[0:7];
> // integer i;
> initial
> begin : ROM_DEF
> integer i;
> for (i=0; i <= 255; i=i+1)
> if (i<100) rom[i] = (i + 1) % 16;
> else rom[i] = (i + 2) % 16;
> end
>
> assign z = rom[a];
> endmodule
>
>
> ---------------------------------------------------------------------------
>
> Ben Cohen Publisher, Trainer, Consultant (310) 721-4830
> http://www.vhdlcohen.com/ vhdlcohen@aol.com
> Author of following textbooks:
> * Real Chip Design and Verification Using Verilog and VHDL, 2002 isbn
> 0-9705394-2-8
> * Component Design by Example ", 2001 isbn 0-9705394-0-1
> * VHDL Coding Styles and Methodologies, 2nd Edition, 1999 isbn
> 0-7923-8474-1
> * VHDL Answers to Frequently Asked Questions, 2nd Edition, isbn
> 0-7923-8115
>
> -----------------------------------------------------------------------------
Ben:
My response would be not to allow variables (local), but to allow only
statically
computed constants - this is covered for your example. I believe the
question
was, can you have rom[i] = foo; where foo is a variable. We should
restrict foo
to be a statically computed constant.
- bhasker
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