Using Electric 9-9: Silicon Compiler

Chapter 9: TOOLS

9-9: Silicon Compiler

Electric has a silicon compiler called QUISC (the Queen's University Interactive Silicon Compiler). It is a powerful tool that can do placement and routing of standard cells from a structural VHDL description. The VHDL is compiled into a netlist which is then used to drive placement and routing. Also, because Electric is able to generate VHDL from a schematic, the silicon compiler can hide the VHDL and produce layout directly from schematics.

Be warned that the silicon compiler is rather old, and so it produces layout that alternates standard cell rows and routing rows. Modern silicon compilers use 3 and 4 metal processes to route over the standard cells, but this system does not.

The VHDL description is normally placed in the "vhdl" view of a cell. It can be created and edited entirely inside of Electric, or it can be read from disk by using the Read Text Cell... command of the Cells menu. See Section 4-10 for more on text editing. Automatic generation of VHDL from layout is done with the Make VHDL View command of the View menu.

Once the VHDL is created, it is compiled into a netlist and read into the silicon compiler. The netlist is normally placed in the "netlist-quisc-format" view of a cell. If the netlist is too cumbersome to retain in memory, it may be kept on disk by using the VHDL Options... subcommand of the VHDL Compiler command of the Tools menu and unchecking the "Netlist stored in cell" item. The "VHDL stored in cell" check controls the placement of VHDL text in a similar way.

When generating a schematic or VHDL description, it is important to know what primitives are available in the standard cell library. Electric comes with a CMOS cell library in the MOSIS CMOS ("mocmos") technology. This library is not correct, and exists only to illustrate the Silicon Compiler. These component declarations are available:

component and2 port(a1, a2 : in bit; y : out bit); end component;
component and3 port(a1, a2, a3 : in bit; y : out bit); end component;
component and4 port(a1, a2, a3, a4 : in bit; y : out bit); end component;
component inverter port(a : in bit; y : out bit); end component;
component nand2 port(a1, a2 : in bit; y : out bit); end component;
component nand3 port(a1, a2, a3 : in bit; y : out bit); end component;
component nand4 port(a1, a2, a3, a4 : in bit; y : out bit); end component;
component nor2 port(a1, a2 : in bit; y : out bit); end component;
component nor3 port(a1, a2, a3 : in bit; y : out bit); end component;
component nor4 port(a1, a2, a3, a4 : in bit; y : out bit); end component;
component or2 port(a1, a2 : in bit; y : out bit); end component;
component or3 port(a1, a2, a3 : in bit; y : out bit); end component;
component or4 port(a1, a2, a3, a4 : in bit; y : out bit); end component;
component rdff port(d, ck, cb, reset : in bit; q, qb : out bit); end component;
component xor2 port(a1, a2 : in bit; y : out bit); end component;

To use the silicon compiler, simply run the subcommands in the Silicon Compiler command of the Tools menu. The commands are organized in the menu so that, when run sequentially downward, they perform the compilation process. The steps are as follows:

Create a circuit to be compiled (there is a VHDL example in cell "tool-SiliconCompiler" of the "samples.txt" library, and you can read the library with the Readable Dump subcommand of the Import command of the File menu).
Use the Read MOSIS CMOS Library subcommand to read the cell library.

Use the Silicon Compiler Options... subcommand to set parameters.

Because cells are laid out in rows that run horizontally, the Vertical routing arc runs in and out of cells, while the Horizontal routing arc runs between the cells in the routing channel. The Power arcs run horizontally between the cells to connect both power and ground. The Main Power arcs run vertically along the sides of the circuit to connect the horizontal power and ground rails (you can choose which layer to use for the main power arcs). A block of P-well will be placed along the bottom of each cell and extend up to the P-Well height (if nonzero). A block of N-well will be placed along the top of each cell and extend down to the N-Well height (if nonzero). The offset of these blocks from the bottom or top can also be given. The Via size, Minimum metal spacing, Routing feed-through size, Minimum port distance, and Minimum active distance are rules that are used to place wires in the routing channel.

Finally, the Number of rows of cells specifies how many rows to use when creating layout. A one-row circuit may be exceedingly wide and short, so you may wish to experiment with this value. For a square circuit, the number of rows should be the square root of the number of instances in the circuit (the number of instances appears as the sum of the unresolved references, listed by the VHDL Compiler).

Use the Get Network for Current Cell subcommand to get the netlist associated with the current cell. This command will compile VHDL if the netlist is unavailable or out of date (note, however, that if the current cell is a netlist, it will be used without consideration of more recent VHDL or layout cells). This command will also generate VHDL from a layout if the VHDL is missing or out of date (note also that if the current cell is VHDL, it will be used without consideration of more recent layout cells).
Use the Do Placement subcommand to place the library cells.
Use the Do Routing subcommand to wire them together.
Use the Make Electric Layout subcommand to actually create the placed-and-routed circuit in the current library. The incremental design-rule checker is turned off at this point because the new cell will be vast and may be troublesome. The new cell is given the "layout" view.

The last subcommand in the Silicon Compiler command is Issue Special Instructions..., which does not normally need to be used. However, the silicon compiler system is so extensive that advanced users may wish to use it. After issuing this command, any sequence of silicon compiler instructions may be typed. Use the "Cancel" button to return to Electric.

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