Which one of the following denotes a sequential electronic circuit that is used to store 1-bit of information ?

Correct — C, (c) Flip-flop. A flip-flop is a bistable circuit: built from logic gates with the output fed back to the input, it has two stable states and will sit in whichever one it was last put into, which is precisely what storing a single binary digit means. It holds a 0 or a 1 indefinitely as long as power is supplied, and changes only when its inputs tell it to, so its present output depends not just on its present inputs but on what happened to it earlier. That dependence on history is the definition of a sequential circuit, as against a combinational circuit such as an adder, a multiplexer or a decoder, whose output is fixed entirely by the inputs present at that instant. The flip-flop is therefore the elementary memory element of digital electronics and the smallest sequential circuit there is. Four types are standard. The SR flip-flop has set and reset inputs and one forbidden input combination. The D or data flip-flop stores whatever is on its single data input at the clock edge, and is the type registers are made of. The JK flip-flop removes the forbidden combination by making it toggle the stored bit. The T or toggle flip-flop changes state on every clock pulse and is what counters are built from. A flip-flop is distinguished from a latch by being edge-triggered — it acts at the instant the clock changes — where a latch is level-sensitive and follows its input for as long as the enabling level is held. Everything larger is an assembly of these: an n-bit register is n flip-flops clocked together, a shift register passes a bit along a chain of them, and a counter is a chain wired to advance a binary number.

Why the others are wrong

• (a)Register — A register is sequential, and it does store information, so the elimination has to be made on the number of bits. A register holds a group of bits — a byte, a word, an address — and is physically a row of flip-flops sharing a clock, one flip-flop per bit. The question asks for the circuit that stores one bit, which is the component a register is assembled from rather than the assembly itself. The relationship is worth remembering in both directions: to know what a 32-bit register is, count 32 flip-flops.
• (b)Transistor — A transistor is a single electronic component, not a circuit, and on its own it stores nothing — it acts as a switch or an amplifier and follows its input from moment to moment. Transistors are the material from which logic gates are made, and gates are what a flip-flop is made of, so a transistor sits two levels below the answer. A static memory cell that holds one bit typically takes about six transistors precisely because storing a bit needs a bistable arrangement rather than a single device.
• (d)Capacitor — A capacitor is a passive component that stores electrical charge, and the association with memory is not accidental: a dynamic random-access memory cell stores one bit as charge on a tiny capacitor controlled by a single transistor. But a capacitor is a component rather than a sequential circuit, it has no clock and no logic, and the charge leaks away, which is why DRAM has to be refreshed thousands of times a second while a flip-flop holds its bit for as long as the power is on. That contrast — capacitor-based DRAM needing refresh against flip-flop-based SRAM which does not — is itself a favourite examination point.

Digital circuits divide into two families. A combinational circuit produces an output that is a function only of its present inputs: half and full adders, encoders and decoders, multiplexers and demultiplexers, comparators and code converters all belong here, and each can be written as a truth table. A sequential circuit adds feedback and a memory of its previous state, so its output depends on the present inputs and on the sequence of inputs that came before; almost all sequential circuits in a computer are synchronous, meaning that state changes only at the edges of a clock signal. The elementary memory element of a sequential circuit is the flip-flop, which stores exactly one bit in a bistable arrangement of gates. From flip-flops the rest of the machine's storage and control is built: registers hold words, shift registers move bits along, counters count clock pulses, and finite state machines sequence the steps of an instruction. The same idea reappears at the level of main memory in two forms. Static RAM holds each bit in a flip-flop-like cell of about six transistors and needs no refreshing, which makes it fast and expensive and therefore the material of cache. Dynamic RAM holds each bit as charge on a capacitor with one transistor, which makes it dense and cheap but requires periodic refreshing, and it is the material of main memory. Recognising which of a set of names is a component, which is a circuit and which is an assembly of circuits answers most questions in this area.

Key facts

• A flip-flop is a bistable sequential circuit that stores one bit of information and holds it until its inputs change it.
• A combinational circuit's output depends only on its present inputs; a sequential circuit's output also depends on its previous state.
• The standard flip-flops are SR, D, JK and T; the JK removes the SR's forbidden input combination by toggling instead, and the T toggles on every clock pulse.
• A flip-flop is edge-triggered and a latch is level-sensitive; both store a bit, but the latch follows its input while the enable level is held.
• An n-bit register is n flip-flops sharing a clock; shift registers and counters are also assemblies of flip-flops.
• Static RAM stores each bit in a flip-flop-like cell of about six transistors and needs no refresh, which is why it is used for cache.
• Dynamic RAM stores each bit as charge on a capacitor with one access transistor and must be refreshed periodically because the charge leaks.
• A transistor is a component, not a circuit; gates are made of transistors and flip-flops are made of gates.

Common traps

• Choosing 'register' because registers obviously store data. A register stores a word; the one-bit element inside it is the flip-flop.
• Treating a transistor or a capacitor as a circuit. Both are components, and the question asks for a circuit.
• Forgetting that a capacitor really does store a bit in DRAM — the option is chosen to reward that half-memory and then punish it, because DRAM needs constant refreshing and a capacitor alone is no circuit.
• Confusing a latch with a flip-flop when a question turns on clocking.
• Assuming all memory is sequential logic. ROM, for instance, is read as a combinational lookup.

• practice — not a real PYQ

  Which one of the following flip-flops toggles its stored bit when both of its inputs are held high ?

  • (a)SR flip-flop
  • (b)D flip-flop
  • (c)JK flip-flop
  • (d)Master-slave latch

  Answer(c) JK flip-flop

• practice — not a real PYQ

  Which one of the following is a combinational circuit ?

  • (a)Shift register
  • (b)Multiplexer
  • (c)Ripple counter
  • (d)D flip-flop

  Answer(b) Multiplexer