Key release behavior in keypad simulation: In a keypad encoder testbench, just after the 4 ms mark the test releases the key by setting the 4-bit column value to ________, which represents no column pulled low and places the data output in Hi-Z.

Introduction / Context:
Matrix keypad interfaces typically use active-low columns. When a key is pressed, one column is driven or read as LOW; when no key is pressed, all columns read HIGH. In HDL simulations, we model this behavior with a 4-bit bus for columns, using hexadecimal shorthand. Releasing a key restores the bus to the “no key” condition: all ones.

Given Data / Assumptions:

• Columns are 4 bits, active-low.
• Pressed key forces one bit to 0; others remain 1 (one-hot low).
• Released state is all bits = 1 (1111b).

Concept / Approach:
If columns are represented as a 4-bit vector, no key pressed corresponds to 1111 in binary, which is F in hexadecimal. Many encoders use this state to tri-state (Hi-Z) or idle the data outputs until a valid keypress is detected, often with a freeze or debounce mechanism.

Step-by-Step Solution:

Verification / Alternative check:
Waveform inspection: After changing columns to F, the encoder should remove the valid code and drive its output bus to Hi-Z or idle, consistent with the design specification.

Why Other Options Are Wrong:

Common Pitfalls:
Forgetting active-low convention; mixing up row/column polarity; not modeling pull-ups in the testbench, which can leave buses at undefined levels.

Final Answer:
F hex