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
Discussion & Comments