Keypad scanner enable behavior In the keypad HDL encoder, if all column inputs remain HIGH (no key pressed), the ring counter remains enabled and continues scanning.

Introduction / Context:
Ring counters are often used to sequentially activate rows in a matrix keypad. With pull-ups on columns, a released keypad presents all HIGHs. The scanner should continue cycling through rows until a keypress pulls a column LOW, at which point detection logic can freeze the scan and latch the code.

Given Data / Assumptions:

• Columns idle HIGH; a pressed key causes an active LOW on the corresponding column during its row’s activation.
• The ring counter controls which row is driven at any moment.
• Enable for the ring counter is deasserted only when a keypress is detected.

Concept / Approach:
The enable logic is typically derived from the combined column status. When all columns are HIGH, the “no key” condition holds and the ring counter continues running. If any column goes LOW (key event), a freeze or hold condition disables the ring, preventing the scan from skipping over the pressed key while the system latches and debounces.

Step-by-Step Solution:

Verification / Alternative check:
Simulate a long period without keypresses and observe continuous row stepping. Introduce a keypress and confirm the ring halts and the keycode is stable until release.

Why Other Options Are Wrong:
“Incorrect” contradicts typical scanner design. “True only with pull-down resistors” reverses polarity; the principle still holds with consistent logic. “Only in asynchronous ring counters” is irrelevant; synchronous rings behave similarly.

Common Pitfalls:
Failing to debounce; forgetting to re-enable the ring after key release; sampling columns outside the valid window for the driven row.

Final Answer:
Correct