Interfacing TTL outputs to CMOS inputs — solving VOH(min) mismatch If the VOH(min) of a TTL IC is too low to meet the VIH(min) requirement of a CMOS input, what is the usual simple fix?

Introduction / Context:
Interfacing TTL (5 V) outputs to CMOS inputs can fail when the TTL high-level output voltage (VOH) does not reach the CMOS input high threshold (VIH). A simple passive technique can often fix the DC level without active translators when speeds and loads allow.

Given Data / Assumptions:

• TTL VOH(min) may be around 2.4 V under load.
• CMOS VIH(min) (at 5 V CMOS) is typically ~0.7 * VCC ≈ 3.5 V.
• The interface is unidirectional and not extremely high-speed or high fan-out.

Concept / Approach:
Adding a pull-up resistor from the TTL output to VCC raises the output high level by providing additional current to the output node when the TTL transistor is off. This helps the node reach closer to VCC so the CMOS input sees a valid logic HIGH. Care must be taken regarding rise time and current draw.

Step-by-Step Solution:

Verification / Alternative check:
Some systems instead use buffers/level translators; for simple cases, the pull-up is sufficient and documented in interfacing app notes.

Why Other Options Are Wrong:

• A: A divider reduces voltage; it does not raise VOH.
• B: Not a solution—just avoidance.
• C: Pull-down makes the issue worse by lowering the high level.

Common Pitfalls:
Using too strong a pull-up causing excess current when low; using too weak a pull-up causing slow edges and timing failures.

Final Answer:
adding an external pull-up resistor to VCC