A Sequential Circuit is a type of digital circuit where the output depends on both the current inputs and the past history of inputs. Unlike combinational circuits, sequential circuits have memory elements to store previous states.
They are widely used in memory devices, counters, registers, and control systems.
Characteristics of Sequential Circuits
- Output depends on present input and previous state.
- Contains memory elements like flip-flops to store information.
- Uses feedback to maintain state.
- Can be synchronous (clocked) or asynchronous (unclocked).
Classification
- Synchronous Sequential Circuits
- Changes state only at the clock edge.
- Examples: Counters, Shift Registers
- Asynchronous Sequential Circuits
- Changes state immediately when inputs change, without clock.
- Examples: Simple latch circuits
Basic Memory Elements: Flip-Flops
Flip-flops are the fundamental storage elements in sequential circuits.
| Type | Inputs | Function | Description |
|---|---|---|---|
| SR Flip-Flop | S, R | Set-Reset | Stores 1 bit; S=1 → Q=1, R=1 → Q=0 |
| D Flip-Flop | D | Data/Delay | Stores input D at clock edge; Q=D |
| JK Flip-Flop | J, K | Toggle/Set-Reset | J=K=1 → toggles Q |
| T Flip-Flop | T | Toggle | T=1 → Q toggles, T=0 → Q unchanged |
Example: D Flip-Flop Truth Table
| D | CLK | Q(next) |
|---|---|---|
| 0 | ↑ | 0 |
| 1 | ↑ | 1 |
Registers
- Registers are groups of flip-flops used to store multiple bits of data.
- Example: 8-bit register stores one byte.
- Types: Shift Registers (left, right, circular)
Counters
- Counters are sequential circuits used to count pulses.
- Types:
- Asynchronous (Ripple) Counter – flip-flops triggered one after another
- Synchronous Counter – all flip-flops triggered simultaneously by the same clock
- Example: 3-bit up counter counts from 0 to 7 in binary.
Applications of Sequential Circuits
- Memory Devices: RAM, ROM, Registers
- Digital Watches / Timers: Counting seconds, minutes
- Control Systems: Traffic light controllers, elevators
- Data Storage & Transfer: Shift registers in communication systems
- State Machines: Finite state machines in computers and robots
Example Problem
Design a 2-bit Synchronous Up Counter:
- Use 2 T Flip-Flops (T1, T2)
- T1 = 1 (toggles every clock pulse)
- T2 = Q1 (toggles when Q1=1)
- Output sequence: 00 → 01 → 10 → 11 → 00 …
Key Notes
- Sequential circuits are memory-dependent, unlike combinational circuits.
- Synchronous sequential circuits are preferred in modern systems for stability and predictability.
- Mastering flip-flops and counters is essential for digital system design.
✅ Summary of Digital Electronics Topics Covered
- Number Systems: Binary, Decimal, Octal, Hex, conversions, applications
- Logic Gates: AND, OR, NOT, NAND, NOR, XOR, XNOR, symbols, truth tables, applications
- Boolean Algebra: Laws, De Morgan’s Theorems, simplification, circuit optimization
- Combinational Circuits: Adders, MUX/DEMUX, Encoders/Decoders, design steps
- Sequential Circuits: Flip-flops, registers, counters, synchronous/asynchronous circuits