Keypad scanning with a ring counter — what does the preset state define? In a scanned-matrix keypad application that uses a ring counter to step through columns, what does the preset state of the ring counter determine at power-up?

Introduction / Context:
Matrix keypads are scanned by driving one axis (rows or columns) and reading the other. A ring counter conveniently cycles one active line at a time across the driven axis. Correct initialization ensures the scan begins on a known line so the encoder/decoder that interprets key positions operates predictably from the first cycle.

Given Data / Assumptions:

• Ring counter outputs correspond to individual column lines.
• One output is asserted (or de-asserted) at a time in sequence.
• An encoder maps the active column/row to a binary code.

Concept / Approach:
Presetting the ring counter forces a specific bit to be active on startup, aligning the scanning sequence with the column encoder’s expectations. If the first active line were ambiguous, early keypresses could be misinterpreted because the encoder would not know which column index corresponds to the instantaneous ring state.

Step-by-Step Solution:

Verification / Alternative check:
Practical firmware/HDL often synchronizes a debouncer and a timer with the ring counter so that the encoder index equals the known column position at each sample time—reliant on a deterministic preset state.

Why Other Options Are Wrong:

• NANDing of rows/columns: gate choice is implementation detail, not what the preset defines.
• Row encoder output: the preset controls the driven axis (columns in this setup), not the sensed axis.
• Pull-up strength: unrelated to ring preset.

Common Pitfalls:

• Forgetting to preset can yield a rotated scan pattern, breaking key mapping tables.

Final Answer:
The proper output of the column encoder