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| 1 | + |
| 2 | +yosys -- Yosys Open SYnthesis Suite |
| 3 | +=================================== |
| 4 | + |
| 5 | +This is a framework for RTL synthesis tools. It is highly |
| 6 | +experimental and under construction. The goal for now is |
| 7 | +to implement an extensible Verilog-2005 synthesis tool. |
| 8 | + |
| 9 | +The aim of this tool is to generate valid logic netlists |
| 10 | +from HDL designs in a manner that allows for easy addition |
| 11 | +of extra synthesis passes. This tool does not aim at generating |
| 12 | +efficient logic netlists. This can be done by passing the |
| 13 | +output of Yosys to a low-level synthesis tool such as ABC. |
| 14 | + |
| 15 | +Yosys is free software licensed under the ISC license (a GPL |
| 16 | +compatible licence that is similar in terms to the MIT license |
| 17 | +or the 2-clause BSD license). |
| 18 | + |
| 19 | + |
| 20 | +Unsupported Verilog-2005 Features |
| 21 | +================================= |
| 22 | + |
| 23 | +The following Verilog-2005 features are not supported by |
| 24 | +yosys and there are currently no plans to add support |
| 25 | +for them: |
| 26 | + |
| 27 | +- Non-sythesizable language features as defined in |
| 28 | + IEC 62142(E):2005 / IEEE Std. 1364.1(E):2002 |
| 29 | + |
| 30 | +- The "tri", "triand", "trior", "wand" and "wor" net types |
| 31 | + |
| 32 | +- The "library" and "configuration" source file formats |
| 33 | + |
| 34 | +- The "disable" and "primitive" statements |
| 35 | + |
| 36 | +- Latched logic (is synthesized as logic with feedback loops) |
| 37 | + |
| 38 | + |
| 39 | +Verilog Attributes and non-standard features |
| 40 | +============================================ |
| 41 | + |
| 42 | +- The 'full_case' attribute on case statements is supported |
| 43 | + (also the non-standard "// synopsys full_case" directive) |
| 44 | + |
| 45 | +- The "// synopsys translate_off" and "// synopsys translate_on" |
| 46 | + directives are also supported (but the use of `ifdef .. `endif |
| 47 | + is strongly recommended instead). |
| 48 | + |
| 49 | +- The "nomem2reg" attribute on modules or arrays prohibits the |
| 50 | + automatic early conversion of arrays to seperate registers. |
| 51 | + |
| 52 | +- The "nolatches" attribute on modules or always-blocks |
| 53 | + prohibits the generation of logic-loops for latches. Instead |
| 54 | + all not explicitly assigned values default to x-bits. |
| 55 | + |
| 56 | +- In addition to the (* ... *) attribute syntax, yosys supports |
| 57 | + the non-standard {* ... *} attribute syntax to set default attributes |
| 58 | + for everything that comes after the {* ... *} statement. (Reset |
| 59 | + by adding an empty {* *} statement.) The preprocessor define |
| 60 | + __YOSYS_ENABLE_DEFATTR__ must be set in order for this featre to be active. |
| 61 | + |
| 62 | + |
| 63 | +TODOs / Open Bugs |
| 64 | +================= |
| 65 | + |
| 66 | +- Write "design and implementation of.." document |
| 67 | + |
| 68 | +- Add brief sourcecode documentation to: |
| 69 | + |
| 70 | + - Most passes and kernel functionalities |
| 71 | + |
| 72 | +- Implement missing Verilog 2005 features: |
| 73 | + |
| 74 | + - Signed constants |
| 75 | + - ROM modelling using "initial" blocks |
| 76 | + - Builtin primitive gates (and, nand, cmos, nmos, pmos, etc..) |
| 77 | + - Ignore what needs to be ignored (e.g. drive and charge strenghts) |
| 78 | + - Check standard vs. implementation to identify missing features |
| 79 | + |
| 80 | +- Actually use range information on parameters |
| 81 | + |
| 82 | +- Implement mux-to-tribuf pass and rebalance mixed mux/tribuf trees |
| 83 | + |
| 84 | +- TCL and Python interfaces to frontends, passes, backends and RTLIL |
| 85 | + |
| 86 | +- Additional internal cell types: $bitcount, $pla, $lut and $pmux |
| 87 | + |
| 88 | +- Subsystem for selecting stuff (and limiting scope of passes) |
| 89 | + |
| 90 | +- Support for registering designs (as collection of modules) to CellTypes |
| 91 | + |
| 92 | +- Kernel support for collections of cells (from input/output cones, etc) |
| 93 | + |
| 94 | +- Smarter resource sharing pass (add MUXes and get rid of duplicated cells) |
| 95 | + |
| 96 | +- FSM state encoding and technology mapping |
| 97 | + |
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