Major building blocks of an HDL keypad emulator When emulating a matrix keypad in HDL, which major block is essential for generating the one-hot scanning pattern that steps through columns?
-
AA sequencer
-
BA clock
-
CA multiplexer
-
DA ring counter
Answer
Correct Answer: A ring counter
Explanation
Introduction / Context:Keypad emulation or scanning requires driving one column at a time while monitoring rows for closures. HDL designs accomplish this with a compact pattern generator to step through columns in a loop.
Given Data / Assumptions:
- Matrix keypad with multiple rows and columns.
- Goal is to minimize simultaneous column drive to avoid ghosting.
- Need a deterministic, cyclic one-hot pattern.
Concept / Approach:A ring counter is perfect for producing a circulating one-hot vector (for example, 0001 → 0010 → 0100 → 1000 → 0001 ...). This directly drives columns and simplifies the logic required to identify the active column during a key press.
Step-by-Step Solution:
Instantiate a ring counter of width equal to the number of columns.Seed it with a 1 at reset and rotate left/right each tick.Use the counter outputs to drive column lines; read rows synchronously.On detection, freeze the ring counter while encoding and debouncing.Verification / Alternative check:Simulate the ring counter sequence and confirm only one column is asserted at a time. In hardware, LEDs on columns will “walk” visibly.
Why Other Options Are Wrong:
- Sequencer: Sometimes used generically, but the specific, efficient block is a ring counter.
- Clock: Necessary, but not the scanning logic itself.
- Multiplexer: Useful for routing signals, not for generating the one-hot pattern.
Common Pitfalls:Forgetting to handle rollover or to synchronize row sampling to avoid metastability; not freezing the scan during a press.
Final Answer:A ring counter