| Add Proposal | Add Analysis | Edit Class, Environment, or Release |
| Number | 593
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| Category | enhancement
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| Synopsis | Continuous assignment case expressions
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| State | open
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| Class | enhancement
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| Arrival-Date | Jul 19 2004
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| Originator | Shalom Bresticker <Shalom.Bresticker@freescale.com>
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| Release | 2001b
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| Description |
This comes from B12 from the old 1364-2001 enhancement requests, by Adam Krolnik. This wanted the ability to write something like assign data_out = case(mux_select) 2'h0: mux_1; 2'h1: mux_2; 2'h2: mux_3; 2'h3: mux_4; endcase It is nice and probably not hard to implement, but does not add new functionality, so we gave it priority MEDIUM To quote from the old database: If one wants to write a case expression, they must use an always block. This is different from the style where assign statements are used to implement combinational logic. One may decide to use the ternary operator to express a pseudo-case structure, but it would seem that this doesn't directly fit the mold of synthesizable mux structures. assign data_out = (mux_select == 0) ? mux1 : (mux_select == 1) ? mux2 : (mux_select == 2) ? mux3 : mux4; This doesn't really look like a mux - more like a priority select block. One has to write this to have something look like a mux. always @(mux_select or mux1 or mux2 or mux3 or mux4) begin case (mux_select) 2'h0 : data_out = mux1; 2'h1 : data_out = mux2; 2'h2 : data_out = mux3; 2'h3 : data_out = mux4; endcase end I would like to propose that a case structure be available to an assign statement. The above logic could then be written as: assign data_out = case(mux_select) 2'h0: mux_1; 2'h1: mux_2; 2'h2: mux_3; 2'h3: mux_4; endcase This case statement returns a value equal to the size of the expression selected. Motivations: 1. Looks more like a mux structure (1 selector) than equivalent ternary operator (possibly multiple selectors) structure. 2. Simpler than creating an always block with a mux inside. 3. Simpler mux structure; only 1 lhs per structure rather than possible multiple. [NOTE: the case_statement has three forms 'case', 'casex' and 'casez' all which are applicable for the continuous version.] |
| Fix |
Unknown |
| Unformatted |
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