TTL-to-CMOS interfacing — ensuring a valid HIGH at the CMOS input When interfacing a TTL output to a CMOS input (with CMOS powered at 5 V), what must be done to ensure the CMOS input sees a valid HIGH level?

Introduction / Context:
Standard TTL guarantees VOH(min) around ~2.4 V under load, while 5 V CMOS may require VIH(min) near ~3.5 V (≈ 0.7 * VCC). Without conditioning, a TTL HIGH may be insufficient for CMOS, leading to metastability or undefined inputs. A simple pull-up provides the additional sourcing to raise the node to a valid CMOS HIGH.

Given Data / Assumptions:

• TTL output to CMOS input, both on a 5 V rail.
• TTL HIGH sourcing is weak; CMOS requires a higher VIH(min).
• A resistor pull-up to 5 V can raise the node when TTL releases the line HIGH.

Concept / Approach:

The pull-up resistor provides current to charge the node toward VCC so that VOH at the interface exceeds CMOS VIH(min). The resistor also sets the sink current when TTL pulls LOW, so select RP to satisfy VOL(max) and power limits while ensuring acceptable rise time with the CMOS input capacitance.

Step-by-Step Solution:

Verification / Alternative check:

Alternatives include using HCT devices (TTL-compatible inputs) or level translators; both avoid the resistor’s RC rise-time penalty.

Why Other Options Are Wrong:

Using a dropping resistor on a supposed 12 V CMOS rail is irrelevant if CMOS is at 5 V; Zeners across TTL inputs are inappropriate; claiming direct interfacing “as long as VCC is 5 V” ignores the VIH(min) mismatch.

Common Pitfalls:

Choosing RP too small (excess current at LOW) or too large (slow rise times). Always check VOL(max), noise margin, and timing.

Final Answer:

Add a pull-up resistor from the TTL output/CMOS input node to VCC; choose RP based on how many CMOS inputs are on the node.