Chapter 2 - Sequential systems

What is a sequential logic system?

A system where outputs depend on input and the previous state (feedback)

What is a D-type flip-flop?

A bistable circuit with two stable states, used for data transfer, storage, and counting

What are the main uses of D-type flip-flops in electronics?

Data transfer, latches, counters

What does the 'D' stand for in D-type flip-flop?

Data (data input terminal)

What does the 'Q' output do in a D-type flip-flop?

Shows the data value stored (output)

What does the 'Q bar' output do?

Always the opposite of Q

What is the 'clock' input in a flip-flop circuit?

Controls when the flip-flop samples the D input for transfer to output

What is meant by 'rising-edge triggered'?

Data is stored at Q only when the clock signal goes from 0 to 1

When does the D-type flip-flop transfer the data input to output?

Only on the rising edge of the clock pulse

If D = 1 when the clock rises, what happens to Q?

Q becomes 1

How long does Q stay at that value?

Indefinitely, until reset or another clock pulse occurs

What are 'Set' (S) and 'Reset' (R) inputs for?

S sets Q to 1, R resets Q to 0, regardless of other inputs

In most GCSE circuits, which input is shown: Set or Reset?

Only Reset is usually included/shown

What does it mean to 'latch' a value using a D-type flip-flop?

The state (high or low) is stored and held even after inputs change, until reset

How is a D-type used as a memory latch?

D is held at 1; a clock pulse stores ‘1’ at Q; Q stays at 1 until reset

How do you reset a D-type latch in a circuit?

Activate (briefly connect) the reset input (logic 1 if active high)

What is a practical use for a D-type latch?

Alarm that remains on after triggered until reset (e.g., safe or burglar alarm)

What is a timing diagram?

Graph showing how logic levels at inputs and outputs change with time

How do you show D and clock pulses on a timing diagram?

Draw D and clock as square waveforms; mark outputs accordingly as signals change

In a D-type, what does the output Q do in response to a rising clock edge?

Takes the value of D at that moment

What is a binary counter?

Circuit that counts in binary, often using flip-flops

How do you build a 1-bit binary up-counter with D-type?

Feed Q bar to D; output toggles each clock pulse, dividing frequency by two

How do you build a 2-bit binary up-counter?

Connect two D-types: 1st gives LSB; its Q output clocks 2nd for MSB

What is frequency division in counters?

Output of counter changes at fractions (half, quarter…) of input clock frequency

What do 'LSB' and 'MSB' stand for?

Least Significant Bit (changes most often); Most Significant Bit (changes least)

What are typical outputs of a 2-bit binary counter (sequence)?

00, 01, 10, 11, then repeat

How do you reset a binary counter at a specific number?

Use combinational logic (AND/NAND gate) taken from counter outputs to trigger reset

What happens to a binary counter when reset is activated?

All outputs go to zero

What is the function of a counter integrated circuit (IC)?

Counts pulses and outputs binary or BCD sequence, often with built-in reset

What is a 4-bit counter’s maximum count?

15 (binary 1111)

How do you connect the reset in a 4017 decade counter?

Connect reset input to active logic (see datasheet); sequence ends when required output is reached

How does a 4017 decade counter work?

10 outputs; each output goes high sequentially with each clock pulse; after 10, resets to start again

What is a ‘decade counter’?

Counter that counts from 0 to 9, then resets

What is a practical use of a 4017 counter?

Sequencers like light chasers, dice, display circuits

How do you design a sequencer for a particular pattern length?

Connect required output (“end” step) to the reset input

What is a BCD counter?

Binary Coded Decimal; counts from 0–9 then resets (BCD code for decimal display)

What is a 7-segment display?

Display device using seven LEDs; can show digits 0–9 and some letters

How is a 7-segment display wired?

Each LED segment controlled separately; patterns form numbers/letters

What is the difference between common anode and common cathode 7-segment displays?

Common anode: all anodes to +V; common cathode: all cathodes to 0V

What does a decoder/driver IC do for a 7-segment display?

Converts BCD (or binary) from counter into seven segment signals; may boost current

Why are current-limiting resistors used with 7-segment displays?

Prevent LED burnout; set safe brightness

How do you use a BCD counter and 7-segment driver for a single-digit display?

BCD counter outputs to decoder/driver IC, which connects to display

How is a two-digit 7-segment display counter constructed?

Two BCD counters and two decoder/drivers used together; higher BCD counts tens, lower counts units

What’s needed in the logic to display special letters on 7-segment display?

Custom logic circuits; decoder/driver IC can usually only display 0–9

What is a state diagram?

Diagram showing how a logic system changes states (outputs) with each clock pulse

How can you show system behaviour using a truth table?

List all input combinations, draw corresponding system outputs

How can you automate a traffic light sequence using a counter?

Use a counter’s outputs as inputs to combinational logic that switches lights

How can you automate a ‘light chaser’ (LED sequence) system?

4017 counter; each output lights a different LED (or pattern) in sequence, reset after desired steps

What does EN (enable) do in 4017 counter?

When high, holds output state (“freezes” sequence)

What does R (reset) do in counters?

Instantly returns counter to start (all outputs zero or first state)

How do you change the speed of a sequencer?

Adjust RC values in clock pulse (astable) circuit, or use variable-frequency generator

What is BCD?

Binary Coded Decimal: 4-bit code for each decimal digit (0000 = 0, 1001 = 9)

How does a binary counter differ from a BCD counter?

Binary counts 0–15 (4-bit), BCD only 0–9 before reset/repeat

How would you count events with a display users can read?

Use BCD counter, decoder/driver IC, and 7-segment display

How do you display 3 on a 7-segment?

Illuminate segments a, b, c, d, g

How do you make a dice or heads/tails electronic game with a decade counter?

Use outputs to indicate numbers; reset gives required max number (6); ‘freeze’ EN to stop at random

What does “toggle” mean in flip-flops/counters?

Change state from 0 to 1 or 1 to 0 with each clock pulse

What’s a possible real application of a sequential logic system?

Car park entry/exit counter, traffic light, score board, timer

What are the advantages of digital counters over mechanical?

Faster, no wear, can directly control electronics/displays, easier logic operations