HDL keypad encoder behavior: In a keypad encoder design, which element must latch and hold its present state until the pressed key is released to ensure clean detection and prevent spurious transitions?

Introduction / Context:
Matrix keypads are common in digital projects. Rows and columns are scanned to detect which key is pressed. Because key contacts bounce and because scanning cycles can cross multiple states during a press/release, reliable designs include logic that “freezes” the detection result long enough for the system to register a single, clean key event. Understanding which HDL component provides this hold behavior is crucial for robust input handling.

Given Data / Assumptions:

• The keypad is scanned by a small FSM or counter-driven sequencer.
• Key bounce and scan timing can cause transient states.
• The design includes a mechanism to hold the recognized key until release.

Concept / Approach:
During a press, the system should detect a stable row/column combination and then inhibit further updates so that additional bounces or rescans do not generate multiple key codes. A control latch—often described as a “freeze bit”—captures the current code and prevents updates until a release is detected. This matches standard debouncing and single-event generation techniques in HDL designs.

Step-by-Step Solution:

Verification / Alternative check:
Waveform simulation shows that, with the freeze bit set, subsequent bounces do not alter the output code. Only after release does the circuit accept new input, ensuring one code per keypress.

Why Other Options Are Wrong:

• Ring/MOD-6/BCD counters: These are scanning or counting mechanisms, not responsible for the hold-until-release behavior.
• Edge detector: Helps find transitions, but does not enforce a persistent hold across the press interval.

Common Pitfalls:
Relying only on counters without a hold latch; ignoring bounce; allowing new scans to overwrite the currently reported key before it is processed.

Final Answer:
Freeze bit