A flip-flop is a circuit used in digital electronics to store one bit of data information. It can be either 0 or 1. Unlike Multiplexers or decoders, it is a sequential circuit, which means the output depends not only on the current input but also on the past inputs as it has a memory to store them. Flip-flops are generally used in digital systems such as registers, memory device counters, etc.
Flip-flop can be understood as the most basic foundation of memory in digital electronics. Each flip-flop circuit can store a single binary digit (bit). To create a storage register, multiple flip-flops are grouped and are further organized to create RAM, cache, and other memory units of the computer. Let’s discuss the basics of how flip-flops work and where they are used in computer storage.
For example:
A register is made from 8 flip-flops and can store 1 byte of data (8 bits).
Flip-flops are used in the counter to count clock pulses in binary.
Clock and Edge Triggering
A clock is the alternating sequence of 0’s and 1’s. We can also call it a periodic digital signal. We call it periodic because a fixed pattern is repeated after a certain time (time period). The number of times that pattern is repeated in 1 second is called its frequency. Refer to the illustration below to get a good grasp.
Edge Triggering
In digital systems, edge triggering is a term used for the process where a flip-flop changes its output at the moment of clock signal transition.
There are two types of edges.
- Positive Edge Triggering. (Rising Edge)
This trigger occurs when the clock signal makes a transition from LOW(0) to HIGH(1). It is indicated by a small triangle on the clock in the diagram.
- Negative Edge Triggering (Falling Edge)
This trigger occurs when the clock makes a transition from HIGH (1) to LOW (0). It is often indicated by a circle on the clock diagram.
Why is Edge Triggering Important?
- Edge triggering makes sure that the flip-flop only responds to the input at some precise moment, avoiding unnecessary changes or fluctuation in the signal.
- In complex circuits, a flip-flop must take care that it is updated in sync with others. A shared clock ensures uniform timing.
- Without edge triggering, noisy signals can interrupt and make the flip-flop respond, causing false results.
- Edge triggering is also important for shift registers, counters, and other memory operations allowing safe data sampling.
Note: Flip-flop can also be called a clocked Latch. It is entirely similar to the latch, except the clock is replaced by an enable pin in the latch. For more information about latches, read Understanding Latches.
Working of a Flip-Flop
For the illustration of working, we have chosen an SR flip-flop, which is the most basic type of flip-flop.
- When a flip-flop updates, its state is controlled by the clock. When the clock is 1, then only the output changes according to the input. On receiving the clock as 1, the flip-flop processes the input. It changes or holds the output Q based on the logic/truth table.
- When the clock turns 0, the output becomes stable and doesn’t change with any input change.
- Once a value, either 0 or 1, is stored, it is retained until the next time the clock triggers 1. This cool bit retention idea makes the flip-flop the building block of the memory system.
- SR flip-flop is the most basic type of flip-flop, where there are two inputs and a clock. Here, S (Set) sets the output to 1, and R (Reset) resets the output back to 0.
Implementation
The simplest SR flip-flop can be implemented using two cross-coupled NOR gates. The output of NOR1 goes into the input of NOR2 and vice versa. This feedback system traps the data in a loop state.
At the rising edge of the clock, Q changes based on the inputs S, and R.
These changes can be shown in the table below.
| Clock Pulse | S | R | Q(next) | Description |
| 1 | 0 | 0 | No change | Memory State |
| 1 | 0 | 1 | 0 | Reset |
| 1 | 1 | 0 | 1 | Set |
| 1 | 1 | 1 | Invalid | Not Allowed |
| 0 | 0 | 0 | X (clock falling) | |
| 0 | 0 | 1 | X (clock falling) | |
| 0 | 1 | 0 | X (clock falling) | |
| 0 | 1 | 1 | X (clock falling) |
So if we remove the clock falling cases, in an asynchronous Version, when,
Set (S=1, R=0): Output Q becomes 1
Reset (S=0, R=1): Output Q becomes 0
Memory (S=0, R=0): Retains the previous state
Problem: Invalid (S=1, R=1): Both outputs become 0, hence, it breaks the complement rule
Applications: Where are Flip-Flops used in computer storage?
Since flip-flops are the basic building block of a sequential circuit, their ability to store 1-bit data is a crucial measure. This makes them essential for building complex systems. Here are the use cases of a flip-flop.
Memory Units
A single flip-flop stores 1 bit. Grouping multiple flip-flops together can store, 00,01, 10, 10010, and many more. As we know that the entire data in the universe is the combination of these 0’s and 1’s, we can create any bit memory unit by combining them. In RAMs, as well, flip-flops are utilized for temporary storage of memory addresses being read/written.
CPU Registers
Registers are the CPU’s small memory locations, which are nearest to the processing unit and come after the cache. Each bit of data inside a register utilizes one flip-flop (often D-Type). Registers use one flip-flop to store one bit of data
For example, a 64-bit CPU will use 64 flip-flops for storing one bit of data.
CPU cache
One key application of flip-flops is in the CPU cache in the form of Static RAM. Because S-RAM is so fast, it is installed in very small volumes inside the CPU chips. Again, for each memory bit, one flip-flop is utilized. Each memory cell in a CPU cache uses 6 transistors, which makes a flip-flop-like structure and stores one bit of data.
Counters
Flip-flops arranged in a particular sequential manner also count pulses. Each flip-flop toggles based on the state of others. Two types of counters are the Asynchronous Counter and the Synchronous Counter. It is used in digital clocks, frequency dividers, and in pedometers for event counting.
Compared to DRAM, SRAM is far faster but expensive and hard to scale. Now, DRAM is cheaper than SRAM and is popular in the primary computer memory because of its great performance-to-price ratio. S-RAM is expensive because there is no capacitor-based storage there, and only the transistors are utilized. We can say that Flip-Flops are making it expensive, but they are making them faster and stable as well.
Conclusion
A flip-flop may be as small as just storing a single bit of data, but they are the foundation of memory elements. A flip-flop has wide uses, from simple data storage to complex timing, counting, and shift operations in computer systems. Flip-flops can be considered the backbone of sequential logic.
Understanding the core concept of how they work not only tightens your grasp on digital electronics but also opens the door to designing powerful and unique circuits. In short, flip-flops are tiny in function but mighty.