Keypad HDL encoder freeze logic In a scanned keypad HDL encoder, NAND-combining the column sense lines can be used to generate a “freeze” signal that latches a detected keypress.

Introduction / Context: Matrix keypads are commonly scanned by driving rows and reading columns (or vice versa). When no keys are pressed, pull-ups typically keep all column inputs HIGH. Pressing a key pulls the corresponding column LOW during the active row drive. Designers often need a simple combinational indicator that “some key is active now” to latch (freeze) the code and stop further scanning glitches.

Given Data / Assumptions:

• Columns idle HIGH via pull-ups; a pressed key forces the active column LOW.
• Freeze is asserted HIGH when any column detects a LOW (key event).
• We consider NAND of all column signals as a candidate for the freeze indicator.

Concept / Approach: For active-low key detection, the AND of all columns is 1 only when every column is HIGH (no keys). If any column goes LOW, the AND drops to 0. Negating this with a NAND produces 1 whenever any LOW appears—exactly the desired “key present” signal to freeze the encoder or halt the ring counter.

Step-by-Step Solution:

Verification / Alternative check: A wired-AND (via pull-ups) with a single inverter yields the same logic; in HDL, a reduction-AND followed by NOT is equivalent to a multi-input NAND.

Why Other Options Are Wrong: “Incorrect” ignores the standard active-low detection technique. “True only with OR gates” is logically wrong for active-low sense lines. “Valid only if columns are active HIGH” reverses the assumed polarity and would require different logic.

Common Pitfalls: Forgetting to debounce; not disabling scanning during freeze, causing ghosting; mismatched polarity between hardware and HDL.

Final Answer: Correct