TTL-to-CMOS interfacing — reason for a pull-up resistor When connecting a TTL output to a CMOS input (same nominal VCC), why is a pull-up resistor commonly added on the TTL output line?

Introduction / Context:
TTL and CMOS families use different guaranteed logic-level thresholds. Standard TTL guarantees VOH(min) around 2.4 V at load, whereas many CMOS families (at 5 V) require VIH(min) near 0.7 * VCC ≈ 3.5 V. Without proper level conditioning, a TTL HIGH might not be recognized as a valid HIGH by CMOS inputs, risking indeterminate logic states.

Given Data / Assumptions:

• TTL VOH(min) is lower than typical CMOS VIH(min).
• Systems share the same 5 V supply in this scenario.
• Pull-up resistor connects the TTL output to VCC, assisting the HIGH level.

Concept / Approach:

A pull-up resistor sources additional current so the line rises closer to VCC when the TTL output goes HIGH (especially if the TTL device is an open-collector or has limited sourcing). This ensures the node exceeds the CMOS VIH(min) level reliably. When the TTL output is LOW, it sinks current through the pull-up to maintain VOL constraints, which the TTL output can handle by design if the resistor value is chosen appropriately.

Step-by-Step Solution:

Verification / Alternative check:

Use a level-shifter or TTL-compatible CMOS (HCT) as an alternative; both aim to ensure VIH thresholds are met without relying solely on a resistor.

Why Other Options Are Wrong:

Changing LOW level is not the goal; the problem is ensuring a sufficiently high HIGH level.

Reducing CMOS input capacitance is unrelated to a pull-up resistor.

Common Pitfalls:

Using too strong a pull-up (excess current at LOW) or too weak (slow edges); forgetting that HCT inputs accept TTL levels without pull-ups.

Final Answer:

to increase the output HIGH voltage