Sequential logic (A-level only) Flashcards – AQA A-level Physics (7408)

What is a binary counter?

A sequential circuit whose output bits represent the current binary count.

How does an active clock edge change the count in an up-counter versus a down-counter?

An up-counter increases the count by one; a down-counter decreases it by one.

What does an up/down control select in a binary counter?

The direction of counting.

What does the reset input do to a binary counter?

It sets the counter to its initial state, normally zero.

How many states are in the full binary sequence of a counter with $k$ output bits?

$2^k$ states.

How do up-counters and down-counters wrap at the ends of their ranges?

An up-counter wraps from its largest value to zero; a down-counter wraps from zero to its largest value.

What do the four output bits of a BCD counter represent?

The current decimal digit in binary-coded decimal.

Which four-bit patterns are used in a BCD counter's normal counting sequence?

The patterns for decimal 0 through 9; the other six four-bit patterns are not used in the normal sequence.

How does a BCD counter wrap around at the ends of its counting sequence?

An up-counter returns from 9 to 0; a down-counter returns from 0 to 9.

What triggers a BCD counter to change its count?

The clock.

What does reset normally do to a BCD counter's output?

Returns it to zero.

What does the up/down input select on a BCD counter that has one?

The counting direction.

How is a Johnson counter constructed?

A chain of storage stages feeds the inverted output of the last stage back to the first.

What happens at each active clock edge in a Johnson counter?

The stored pattern shifts along the chain, and the feedback value enters the first stage.

Starting from a suitable reset state, how many distinct states does a Johnson counter with $k$ stages cycle through before repeating?

$2k$ distinct states.

Do Johnson counter stage outputs generally represent the count in conventional binary?

No. They show the current pattern in the sequence.

What is the role of reset in a Johnson counter?

It establishes a starting pattern.

Is direction control inherent in every Johnson counter?

No. It may be provided in a reversible design.

What is a modulo-$n$ counter?

A counter that cycles through $n$ states before its sequence repeats.

How can logic gates make a basic counter operate modulo $n$?

They detect a selected output pattern and drive the reset input, returning the counter to its starting state at the required point.

For a modulo-$n$ up-counter counting from zero, what count can trigger an asynchronous reset?

The first unwanted count, $n$, is detected and immediately resets the counter.

When must a synchronous reset return a modulo-$n$ up-counter to zero?

On the next active clock edge after state $n-1$.

What capacity must a basic counter have to be used as a modulo-$n$ counter?

It must have enough states to represent the required $n$-state sequence.

What must the detected pattern and reset logic match in a modulo-$n$ counter circuit?

The circuit’s reset timing and signal polarity.