Keypad encoder signal detection in HDL projects: In the keypad encoder design, which element is responsible for detecting that a key press has occurred and for latching the code for stable output?
Digital Electronics
Digital System Projects Using HDL
Difficulty: Easy
Choose an option
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Afreeze bit
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Bring counter
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CBCD counter
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DMOD-6 counter
Answer
Correct Answer: freeze bit
Explanation
Introduction / Context: A keypad encoder scans rows and columns to identify which key is pressed. To stabilize the output despite bounce and scanning transitions, encoders commonly use a latching mechanism often referred to as a “freeze bit.”
Given Data / Assumptions:
- Matrix keypad scanning cycles through lines with a ring or Johnson counter.
- Mechanical switches bounce briefly upon actuation.
- A freeze bit latches the detected key code until the system is ready for the next event.
Concept / Approach: The freeze bit does not generate the scan but acts on the result, asserting when a valid key press is detected and holding the encoded output steady. The scanning mechanism (e.g., ring counter) and numeric counters assist the process but do not perform the latching/press acknowledgment role themselves.
Step-by-Step Solution:
Use a ring counter to drive rows/columns during scanning.Sense active intersections to detect a pressed key.When a valid press is found, set the freeze bit and latch the key’s encoded value.Maintain the code until release/acknowledge clears the freeze.Verification / Alternative check:
Simulation waveforms show stable outputs while freeze=1 despite ongoing scan activity.Why Other Options Are Wrong:
ring counter: Generates scan timing but does not latch press events.BCD counter / MOD-6 counter: Count mechanisms are general-purpose and not specific to press detection and latching.Common Pitfalls:
Clearing the freeze prematurely, leading to flickering codes.Failing to handle simultaneous or ghost key conditions if diodes are not used.Final Answer:
freeze bit