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Verilog · Chapter 12.2.4 · Switch-Level Modeling

Resistive MOS Switches in Verilog — rnmos, rpmos, rcmos & rtran

The final switch-level primitive family is the resistive switches, including rnmos, rpmos, rcmos, and the bidirectional rtran, rtranif0, and rtranif1. Each behaves exactly like its non-resistive counterpart, with the same conduction conditions and the same direction, but with one difference. A resistive switch reduces the strength of the signal it passes, modeling a high-resistance transistor. A strong input becomes a pull, a pull becomes weak, and so on down the strength scale. This matters in drive-strength resolution, because a resistive switch's output is weaker than a non-resistive one, so where both drive a node the stronger one wins. Resistive switches model ratioed logic, charge-sharing structures, and circuits that deliberately weaken a signal. This page covers the variants and the strength reduction they apply.

Foundation11 min readVerilogResistive SwitchrnmosStrengthSwitch-Level

Chapter 12 · Section 12.2.4 · Switch-Level Modeling

1. The Engineering Problem

Some switch-level circuits need a device that passes a signal but weakens it — a high-resistance transistor whose output should lose to a stronger driver on a shared node. Verilog provides resistive switch primitives for exactly this:

The resistive switches (rnmos, rpmos, rcmos, rtran*) behave like their counterparts but reduce the strength of the passed signal by one level. Their output therefore loses, in resolution, to a non-resistive driver of the original strength.

This short page drills the resistive variants and the strength reduction they apply.

2. Mental Model — Same Switch, Lower Strength

3. The Resistive Variants

resistive.v
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Snippet
   rnmos (out, data, gate);    // like nmos, output strength reduced
   rpmos (out, data, gate);    // like pmos, output strength reduced
   rcmos (out, data, ng, pg);  // like cmos, output strength reduced
 
   rtran    (a, b);            // like tran, strength reduced
   rtranif1 (a, b, ctrl);      // like tranif1, strength reduced
   rtranif0 (a, b, ctrl);      // like tranif0, strength reduced

Same syntax, same conduction, same direction as the non-resistive forms — only the passed strength is lowered. A strong 1 through an nmos stays strong (subject to the NMOS weak-1 caveat); through an rnmos it is reduced to a pull (or lower) strength.

4. Strength Reduction in Resolution

The point of a resistive switch is that its output loses to a stronger driver. On a node driven by both a resistive and a non-resistive switch carrying different values, the non-resistive (stronger) one wins; carrying the same value, the result takes the stronger strength. This is how ratioed circuits work — a weak (resistive) pull-up or keeper holds a node, but a strong driver overrides it. The full resolution mechanics are 12.3; here the takeaway is that resistive = weaker = loses.

Visual A — resistive reduces strength

Resistive switch lowers the passed strength

data flow
Resistive switch lowers the passed strengthstrong inpute.g. strong 1rnmos / rpmos / …reduce one levelweaker outpute.g. pull 1loses to non-resistive driverloses tonon-resistive…in resolution (12.3)
A resistive switch passes the signal but drops its strength by one level, so its output is weaker than a non-resistive switch's. Where both drive a node, the non-resistive (stronger) driver wins. This models deliberately-weak devices in ratioed and charge-sharing circuits.

5. Common Mistakes

  1. Expecting a resistive switch to pass full strength — it reduces strength by one level (§2/§3).
  2. Using a resistive switch where a strong driver is needed — its output loses to non-resistive drivers (§4).
  3. Confusing resistive with non-resistive in resolution — the strength difference decides the node (§4, 12.3).

6. Interview Q&A

7. Exercises

Exercise 1 — Strength reduction

A strong 1 is passed through an rnmos. What strength does the output have (one level lower on the strength scale)?

Exercise 2 — Who wins?

A node is driven by an nmos passing a 0 and an rpmos passing a 1. Which value wins, and why?

Exercise 3 — Choose the primitive

You need a weak keeper that holds a node but is overridden by any strong driver. Which kind of switch (resistive or non-resistive) fits, and why?

8. Summary

The resistive switch primitives behave like their counterparts but reduce signal strength:

  • rnmos / rpmos / rcmos / rtran* — same conduction and direction as the non-resistive forms, but the passed signal is one strength level weaker.
  • In resolution, resistive loses — a resistive output is weaker than a non-resistive one, so the non-resistive driver wins; this models ratioed and charge-sharing circuits.

This completes the 12.2 switch primitives. The chapter now turns to the mechanism that decides node values: Chapter 12.3 Drive Strength and Resolution explains how the strengths of multiple drivers combine to determine the value on a switch-level node — the heart of switch-level simulation.