Keypad HDL encoder behavior: Does the “freeze bit” primarily detect when a key is released, or is it intended to latch the code upon a key press to stabilize the output?
-
AIncorrect — it latches on press, not on release
-
BCorrect — it detects release events
-
COnly valid for matrix keypads with diodes
-
DApplies solely to mechanical key bounce
Answer
Correct Answer: Incorrect — it latches on press, not on release
Explanation
Introduction / Context: In keypad encoders, a “freeze” or “latch” mechanism is frequently used to stabilize the reported key code and to avoid flicker during scanning and debounce intervals. This question distinguishes between release detection and press-latch behavior.
Given Data / Assumptions:
- Matrix keypads are scanned by activating rows/columns and reading the opposite lines.
- Mechanical switches bounce, creating brief, spurious transitions.
- A “freeze bit” conventionally captures the first valid key press and holds the code.
Concept / Approach: The freeze function is designed to latch the keypad output when a valid press is detected, often until the system acknowledges it or until no key is detected for a defined interval. It is not primarily a release detector; release is usually inferred when scanning no longer detects a closed contact and the freeze is cleared or a new event is permitted.
Step-by-Step Solution:
Scan rows/columns to detect a pressed key.On a valid press, set the freeze bit and latch the BCD/encoded value.Hold the value stable during bouncing or while the key remains pressed.Upon release or acknowledge, clear freeze to accept the next key press.Verification / Alternative check:
HDL implementations typically gate further updates while freeze=1, confirming press-latch semantics.Why Other Options Are Wrong:
Correct — it detects release events: Misstates the primary role; release is secondary.Only valid for matrix keypads with diodes: Freeze is logical; diodes prevent ghosting but are unrelated to latch semantics.Applies solely to mechanical key bounce: Freeze also aids in display stability and user interface logic.Common Pitfalls:
Treating freeze as a debounce-only flag instead of a code latch.Failing to clear freeze properly, which blocks subsequent key events.Final Answer:
Incorrect — it latches on press, not on release