CMOS–TTL interconnection limits: Because CMOS logic has extremely low static power consumption, can an arbitrary number of CMOS and TTL gates be interconnected without any fan-out limits or special interface considerations?

Introduction / Context:
Mixing and chaining logic gates is common in digital design. Complementary metal–oxide–semiconductor (CMOS) gates draw extremely low static power, which tempts beginners to assume there are no practical limits when interconnecting CMOS and transistor–transistor logic (TTL). This question probes the realistic constraints: fan-out, drive current, and logic-level compatibility.

Given Data / Assumptions:

• CMOS gates have very high input impedance and near-zero static input current.
• TTL outputs have asymmetric drive capability: relatively strong sink at LOW and relatively weak source at HIGH (especially in standard TTL).
• Interfacing may involve 5 V families such as LS, HC, HCT, etc.

Concept / Approach:
Even with low power, every output has finite drive current and specified fan-out. Fan-out is the maximum number of standard inputs a gate output can reliably drive while meeting VOL/VOH and noise-margin specs over temperature and supply tolerance. Cross-family connections must also satisfy VIH/VIL thresholds (for example, HC vs TTL-level HCT).

Step-by-Step Solution:

Verification / Alternative check:
Datasheets list fan-out numbers (e.g., LS-TTL fan-out of 10 into LS inputs). For CMOS, fan-out is often limited by capacitive loading and rise/fall times; excessive loads slow edges and can cause timing failures and increased dynamic power.

Why Other Options Are Wrong:

• “Any number can be tied together” ignores finite drive and logic thresholds.
• “Only if VCC > 10 V” is irrelevant; most logic families are 3.3–5 V.
• “Only to open-collector TTL” confuses wired-OR usage with general interconnection rules.

Common Pitfalls:
Assuming static power implies unlimited loading, forgetting level-compatibility between TTL and CMOS, and ignoring capacitive fan-out which impacts speed.

Final Answer:
Does not apply — fan-out and level-compatibility still impose limits.