Chapter 9: Tools
9-7: Network Consistency Checking (NCC)
9-7-3: Preferences

NCC options are available in the NCC Preferences (in menu File / Preferences... , "Tools" section, "NCC" tab).

Figure 9.22

Operation Section

This section allows you to select what kind of NCC operation to perform. You can either compare hierarchically, compare flat, or list all the NCC annotations in the design.

It is recommended that you use hierarchical comparison because it is faster and the mismatch diagnostics are much more precise and intelligible. However, transistor size checking limits what NCC can compare hierarchically because the size of a schematic transistor may depend upon the instance path.

The best way to use NCC is to initially perform all comparisons hierarchically. This will typically require many iterations. Once the circuit has passed hierarchical comparison, turn on size checking. This will report transistor size mismatches.

Size Checking Section

The "Size Checking" section controls how NCC compares transistor widths and lengths. This section affects two distinct NCC phases: netlist comparison and series / parallel combination.

Netlist comparison

After each topological comparison, NCC can optionally perform size checking. If NCC finds no topological mismatches, and if "Check transistor sizes" is checked, then NCC checks, for each pair of matching transistors, that the widths and lengths are approximately equal.

The two tolerance values allow the user to specify how much more the larger of the two matched transistors may be than the smaller before NCC reports a size mismatch. The "Relative size tolerance" is the difference in percentage. The "Absolute size tolerance" is the difference in units. NCC reports a size mismatch when both tolerances are exceeded.

If you choose "Check transistor sizes" and "Hierarchical Comparison" simultaneously then NCC restricts which cells it treats hierarchically to ensure a correct answer in the presence of automatically sized transistors. For this case it compares a pair of cells hierarchically if and only if each cell is instantiated exactly once.

Series / Parallel Combination

When NCC builds the netlist, it performs series / parallel combination. When NCC finds a number of transistors with the same channel length wired in parallel, NCC substitutes a single transistor whose width is the sum of the widths of those transistors. When NCC finds a number of transistors with the same channel width and channel length wired in series, NCC substitutes a single multi-gate transistor that represents all the series transistors.

NCC uses the "Relative size tolerance" and the "Absolute size tolerance" fields to determine how close transistor widths and lengths have to be before it will combine them in series or in parallel.

Body Checking Section

The check box "Check transistor body connections" allows the user to select whether NCC checks connections to the body port of transistors. By default, body checking is disabled and NCC ignores connections to transistor body ports.

If the user wishes to check body connections, then she must check this box. Then, the NCC will make sure that the schematics and layout have matching connections to all transistor body ports.

Note that only certain versions of schematic transistors have body ports. The designer must use those schematic transistors. In addition, in this version of Electric, layout transistors also have body ports. The designer must specify the connectivity of the body port of layout transistors using well arcs.

Note that the body port of the layout transistors are in the very center of the transistor and are "hard to select". If you wish to connect to the body port of a layout transistor you may need to push the "Toggle Special Select" button in the Electric tool bar (see the Section 2-1-5 for more).

At the moment, only the MoCMOS layout technology has been augmented to allow body connections. This is because this implementation of body checking is experimental. We'd like to get some feedback from users before we go to the effort of generalizing all other technologies.

Checking All Cells Section

In hierarchical mode, NCC attempts to compare all cells in the design starting with those at the leaves and working it's way toward the root. For that mode it is often best if NCC stops as soon as it finds an export or topology mismatch. To get this behavior the user should check "Halt after finding the first mismatched cell". Note that size mismatches never cause NCC to stop.

It is occasionally useful to continue checking even after mismatches have been detected. For example, the designer might find that although a cell mismatches, it cannot be fixed because someone else designed it. When asked to continue, NCC will do the following when comparing cells that use the mismatched one:

If the check box "Don't recheck cells that have passed in this Electric run" is checked, then NCC skips a cell if that cell passed NCC in a previous run and the designer hasn't since changed the cell.

Note that NCC only remembers when cells were last checked during a single run of Electric. If you run NCC, quit Electric, restart Electric, and rerun NCC, all cells will be checked.

Reporting Progress Section

This panel controls how verbose NCC is in reporting its progress. Most users should leave this at 0.

Error Reporting Section

The error reporting section controls how many error messages are printed when the Local Partitioning algorithm has failed to find a mismatch but the Gemini algorithm has. Most users will want to leave these at the default setting of 10.

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