Keypad encoder freeze detection: In a typical keypad encoder design with active-low column lines, the logic that activates the “freeze” bit only when exactly one column is low is implemented using ________.

Introduction / Context:
Matrix keypads are scanned by driving or reading rows and columns. In active-low schemes, a pressed key pulls one column line low while others remain high. To capture a valid keypress and prevent chatter or ghosting, many encoders assert a “freeze” (or latch) signal only when exactly one column indicates activation.

Given Data / Assumptions:

• Columns are active-low: pressed key yields a logic 0 on one column line.
• Only a single key should be recognized at a time (no rollover).
• Freeze asserts when the detection logic confirms one-hot behavior.

Concept / Approach:
NAND gates are convenient for detecting combinations and ensuring that only one low is present. For example, by appropriately combining signals (possibly after inversion), a NAND network can produce a HIGH (asserted) freeze only when one column is low and all others are high. Additional gating can reject two-or-more simultaneous lows.

Step-by-Step Solution:

Verification / Alternative check:
Truth-table simulation confirms freeze = 1 for one-low cases and 0 for none or multiple-low cases. Gate-level schematics from synthesis will show NAND-rich implementations for compactness.

Why Other Options Are Wrong:

Common Pitfalls:
Allowing ghost keys due to shared row/column paths; failing to debounce; not masking multiple simultaneous keypresses.

Final Answer:
NAND columns