Interfacing CMOS to TTL: What condition enables direct connection without special buffers?

Difficulty: Easy

Correct Answer: As long as the CMOS supply voltage is 5 V, they can be interfaced; however, the fan-out of the CMOS is limited to two TTL gates.

Explanation:


Introduction / Context:
CMOS and TTL use different input thresholds and output drive characteristics. Designers frequently need to connect CMOS outputs to TTL inputs. Knowing when this is safe—without extra buffering—saves cost and board space.



Given Data / Assumptions:

  • Both families operate at a nominal 5 V supply when directly interfaced.
  • Classic CMOS outputs can reach rail-to-rail (near 5 V) HIGH, satisfying TTL VIH(min).
  • CMOS IOH (source current) is limited; driving many TTL inputs in parallel can overload the CMOS source.


Concept / Approach:
At 5 V, a CMOS HIGH level comfortably meets TTL HIGH threshold, so DC-level compatibility exists. Fan-out is constrained by the total current required by the TTL inputs; a typical rule of thumb is that a CMOS output should not drive more than about two standard TTL inputs without buffering.



Step-by-Step Solution:
Ensure both at 5 V → meets logic-level requirements.Evaluate drive → CMOS IOH must exceed the sum of TTL IIH across loads.Adopt conservative fan-out (≈2) → safe operation without special buffers.Therefore, select the statement describing direct 5 V interfacing with limited fan-out.



Verification / Alternative check:
Data sheets list IIH for TTL inputs. Summing IIH for two standard TTL loads typically remains within CMOS IOH capability; more loads may require a buffer or TTL-compatible CMOS families (HCT).


Why Other Options Are Wrong:

  • a: A “dropping resistor” from 12 V is unsafe and imprecise for supply regulation.
  • c: A Zener across TTL inputs is unnecessary and potentially harmful.
  • d: Special buffers are not mandatory when both sides are at 5 V and fan-out is modest.


Common Pitfalls:
Ignoring dynamic loading and wiring capacitance; even with correct DC levels, heavy capacitive loads slow edges and may violate timing.


Final Answer:
As long as the CMOS supply voltage is 5 V, they can be interfaced; however, the fan-out of the CMOS is limited to two TTL gates.

More Questions from Integrated-Circuit Logic Families

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion