Design Logic Circuit

Vorgehensweise

Schritt 1: Angeben Circuit Function

Definieren the circuit's interface and behavior vollstaendig vor any synthesis:

1. Input signals: Auflisten all input signals with their names, bit widths, and valid ranges. For multi-bit inputs, specify bit ordering (MSB-first or LSB-first).
2. Output signals: Auflisten all output signals with their names and bit widths.
3. Truth table: Schreiben the complete truth table mapping every input combination to the corresponding outputs. For circuits with many inputs, express die Funktion algebraically or as a set of minterms/maxterms stattdessen.
4. Don't-care conditions: Identifizieren input combinations that cannot occur in der Praxis (e.g., BCD inputs 1010-1111) and mark them as don't-cares.
5. Timing requirements: Note any propagation delay constraints, though combinational circuits have no clock -- timing here refers to worst-case gate delay durch the critical path.

## Circuit Specification
- **Name**: [descriptive name]
- **Inputs**: [list with bit widths]
- **Outputs**: [list with bit widths]
- **Function**: [verbal description]
- **Truth table or minterm list**: [table or Sigma notation]
- **Don't-care set**: [d(...) or "none"]

Erwartet: A complete, unambiguous specification where every legal input combination maps to exactly one output value.

Bei Fehler: If the specification is ambiguous (e.g., missing cases, conflicting outputs for the same input), request clarification. Do not assume don't-care for unspecified inputs unless explicitly told to.

Schritt 2: Derive Minimal Boolean Expression

Obtain the simplest expression fuer jede output using the evaluate-boolean-expression skill:

1. Single-output functions: Fuer jede output bit, apply evaluate-boolean-expression to get the minimal SOP (or POS, abhaengig von which yields fewer gates).
2. Multi-output optimization: If multiple outputs share common sub-expressions, identify shared product terms that kann factored out. This reduces total gate count at the expense of slightly more complex routing.
3. XOR detection: Scannen for XOR/XNOR patterns in the truth table (checkerboard patterns in the K-map). XOR gates are expensive in NAND/NOR-only implementations but efficient in standard libraries.
4. Erfassen the expressions: Dokumentieren the minimal expression fuer jede output, noting the literal count and the number of product/sum terms.

## Minimal Expressions
| Output | Minimal SOP | Literals | Terms |
|--------|-------------|----------|-------|
| F1     | [expression] | [count] | [count] |
| F2     | [expression] | [count] | [count] |
- **Shared sub-expressions**: [list, if any]

Erwartet: A minimal Boolean expression fuer jede output, with shared sub-expressions identified for multi-output circuits.

Bei Fehler: If the expressions appear non-minimal (more literals than expected for die Funktion's complexity), re-run the K-map or Quine-McCluskey step from evaluate-boolean-expression. For functions with more than 6 variables, use Espresso or a similar heuristic minimizer.

Schritt 3: Abbilden to Gate-Level Schematic

Konvertieren the Boolean expressions into a network of logic gates:

1. Direct mapping (SOP): Each product term becomes a multi-input AND gate. The sum of products becomes an OR gate fed by the AND gates. Each complemented variable requires a NOT gate (or use NAND/NOR to absorb inversions).
2. Gate assignment: Fuer jede gate, record:
   • Gate type (AND, OR, NOT, XOR, NAND, NOR)
   • Input signals (by name or from die Ausgabe of another gate)
   • Output signal name
   • Fan-in (number of inputs)
3. Fan-in decomposition: If a gate exceeds the maximum fan-in constraint, decompose it into a tree of smaller gates. For example, a 4-input AND with a 2-input constraint becomes two 2-input ANDs feeding a third 2-input AND.
4. Schematic notation: Draw the circuit using text-based notation or describe the netlist in a structured format.

## Gate-Level Netlist
| Gate ID | Type | Inputs       | Output | Fan-in |
|---------|------|-------------|--------|--------|
| G1      | NOT  | A           | A'     | 1      |
| G2      | AND  | A', B       | w1     | 2      |
| G3      | AND  | A, C        | w2     | 2      |
| G4      | OR   | w1, w2      | F      | 2      |

- **Total gates**: [count]
- **Critical path depth**: [number of gate levels from input to output]

Erwartet: A complete gate-level netlist where every output kann traced back to primary inputs durch a chain of gates, with no floating (unconnected) inputs or outputs.

Bei Fehler: If the netlist has dangling wires or feedback loops (which are invalid in combinational circuits), recheck the mapping. Every signal must have exactly one driver and every gate input must connect to either a primary input or another gate's output.

Schritt 4: Konvertieren to Universal Gate Basis (Optional)

Transformieren the circuit to use only NAND gates or only NOR gates:

1. NAND-only conversion:
   • Replace each AND gate with a NAND followed by a NOT (NAND with tied inputs).
   • Replace each OR gate using De Morgan: A + B = ((A')*(B'))' = NAND(A', B'), so use NOTs on inputs then NAND.
   • Replace each NOT gate with a NAND gate with both inputs tied together: A' = NAND(A, A).
   • Bubble pushing: Simplify by canceling adjacent inversions. Two NOTs in series cancel. A NAND feeding a NOT is equivalent to an AND.
2. NOR-only conversion:
   • Replace each OR gate with a NOR followed by a NOT.
   • Replace each AND gate using De Morgan: A * B = ((A')+(B'))' = NOR(A', B').
   • Replace each NOT gate with NOR(A, A).
   • Anwenden bubble pushing to cancel inversions.
3. Gate count comparison: Erfassen the gate count vor and nach conversion. NAND-only and NOR-only implementations typischerweise use more gates but simplify manufacturing (single gate type on a chip).

## Universal Gate Conversion
- **Target basis**: [NAND-only / NOR-only]
- **Gates before conversion**: [count]
- **Gates after conversion**: [count]
- **Gates after bubble-push optimization**: [count]
- **Conversion netlist**: [updated table]

Erwartet: A functionally equivalent circuit using only das Ziel gate type, with redundant inversions eliminated via bubble pushing.

Bei Fehler: If the converted circuit has more inversions than expected, re-examine the bubble-pushing step. A common mistake is forgetting that NAND and NOR are self-dual under complementation -- applying De Morgan consistently from outputs back to inputs avoids this.

Schritt 5: Verifizieren via Exhaustive Simulation

Bestaetigen the circuit produces correct outputs for every possible input:

1. Simulation approach: For circuits with up to 16 inputs (65,536 combinations), simulate every input exhaustively. For larger circuits, use targeted test vectors covering corner cases, boundary conditions, and random samples.
2. Propagate values: Fuer jede input combination, propagate signal values gate by gate from inputs to outputs, respecting the topological order (no gate is evaluated vor its inputs are ready).
3. Vergleichen gegen specification: Check each output gegen the truth table or expected function from Step 1. Don't-care outputs kann either 0 or 1.
4. Erfassen results: Log any mismatches with the failing input combination and the expected versus actual output.
5. Timing analysis (optional): Zaehlen the gate levels on the longest path from any input to any output. Multiply by the per-gate delay to estimate worst-case propagation delay.

## Simulation Results
- **Total test vectors**: [count]
- **Vectors passed**: [count]
- **Vectors failed**: [count, with details if any]
- **Critical path**: [gate sequence, e.g., G1 -> G3 -> G7 -> G9]
- **Critical path depth**: [N gate levels]
- **Estimated worst-case delay**: [N * gate_delay]

Erwartet: All test vectors pass. The circuit is functionally correct and the critical path depth is documented.

Bei Fehler: If any vector fails, trace the signal path for that input combination gate by gate to find the first gate producing an incorrect output. Common causes: a wire connected to the wrong gate input, a missing inversion, or an error in the NAND/NOR conversion.

Validierung

• All inputs and outputs are named and their bit widths are specified
• The truth table or minterm list covers all legal input combinations
• Boolean expressions are minimal (verified via K-map or Quine-McCluskey)
• Every gate in the netlist has all inputs connected and exactly one output
• No combinational feedback loops exist in the circuit
• Fan-in constraints are respected (all gates innerhalb the maximum fan-in)
• NAND/NOR conversion (if performed) preserves functional equivalence
• Bubble pushing wurde applied to eliminate redundant inversions
• Exhaustive simulation passes for all non-don't-care input combinations
• Critical path depth is documented

Haeufige Stolperfallen

• Combinational feedback loops: Accidentally connecting a gate's output back to its own input chain creates a sequential element (latch), not a combinational circuit. If state wird benoetigt, use the build-sequential-circuit skill stattdessen.
• Forgetting inversions in NAND/NOR conversion: The most common conversion error is dropping a NOT gate waehrend the De Morgan transformation. Always apply bubble pushing systematically from outputs to inputs, not ad hoc.
• Exceeding fan-in ohne decomposition: A 5-input AND gate ist nicht available in a 2-input library. Decompose into a balanced tree to minimize propagation delay, not a linear chain.
• Ignoring don't-cares: Failing to exploit don't-care conditions waehrend minimization leaves the circuit larger than necessary. Always include don't-cares when available.
• Confusing gate delay with wire delay: In introductory design, gate delay dominates. In real VLSI, wire delay (interconnect capacitance) can exceed gate delay. Note this limitation when estimating timing.
• Multi-output hazards: When multiple outputs share gates, changing one output's logic can inadvertently affect a shared sub-expression. Verifizieren all outputs nach any modification, not just the one being changed.

Verwandte Skills

• evaluate-boolean-expression -- derive the minimal Boolean expression used as input to this skill
• build-sequential-circuit -- add state elements (flip-flops) to create sequential circuits
• simulate-cpu-architecture -- use combinational blocks (ALU, mux, decoder) as datapath components