Latch Vs Flip Flop

A latch and a flip-flop are two basic building blocks in digital electronics used to store binary data. The main difference between them is that a latch is level-sensitive and continuously updates its output based on the input, while a flip-flop is edge-sensitive and only updates its output on a clock edge.

The table below shows the principal comparisions to help understand better and then we diev into a few examples.

	Flip-Flop	Latch
What is the Basic Principle?	Edge triggered	Level triggered
Is it controlled by a Clock Signal?	Yes.	No.
What is it Designed Using?	Latches along with a clock.	Logic gates.
What is it Sensitive to?	Sensitive to the applied input and the clock signal.	Sensitive to the applied input signal- only when enabled.
Speed of operation	Slow.	Comparatively fast.
Classification	Can be classified into a synchronous or asynchronous flip-flop.	NA
Working	Works using the binary input and the clock signal.	Operates only using binary inputs.
Power Requirement	Requires more power.	Requires comparatively less power.
Analysis of Circuit	Easy to perform circuit analysis.	Analysing the circuit is quite complex.
Type of Operation Performed	Performs Synchronous operations.	Performs Asynchronous operations.
Robustness	Comparatively more robust.	Comparatively less robust.
Dependency of Operation	The operation relies on the present and past input bits along with the past output and clock pulses.	The operation depends on the present and past input along with the past output binary values.
Usage as a Register	Capable of working as a register as it contains clock signals in its input.	Cannot serve as a register as the register requires further advanced electronic circuits (EC). Time also plays an essential role here.
Types	J-K, S-R, D, and T Flip-flops.	J-K, S-R, D, and T Latches.
Area impact	Requires more area.	Requires less area.
Uses	Constitutes the building blocks of many sequential circuits such as counters.	Can be utilized for designing sequential circuits, but not generally preferred.
Input and Output	Checks the inputs and only changes the output at times defined by any control signal like the clock signal.	Responds to the changes in inputs continuously as soon as it checks the inputs.
Synchronicity	Synchronous, works based on the clock signal.	Asynchronous, does not work based on the time signal.
Is it fault  tolerant?	Protected against any fault.	Responsive to any occurring faults on the enable pin.

Here are some examples and usage of latches and flip-flops in real design, along with code snippets in SystemVerilog, VHDL, and Verilog.

Latch:

A latch is a level-sensitive device that maintains its output state as long as the input is held at a particular level. There are two types of latches: SR latch and D latch.

Example usage:

A latch can be used as a buffer in a digital design. In this example, a D-latch is used to buffer an input signal and output it to a display:

Verilog code:

module d_latch(input d, clk, output q);

    always @ (clk) begin

        if (clk)

            q <= d;

    end

endmodule

SystemVerilog code:

module d_latch(input logic d, clk, output logic q);

    always_latch begin

        if (clk)

            q <= d;

    end

endmodule

module display(input logic signal);

    // Display the signal

endmodule

module top(input logic input_signal, input logic clk);

    logic signal_buffer;

    d_latch latch(signal_buffer, clk, input_signal);

    display display(signal_buffer);

endmodule

VHDL code:

entity d_latch is

    port (

        d : in std_logic;

        clk : in std_logic;

        q : out std_logic

    );

end entity;

architecture behavior of d_latch is

begin

    process (clk)

    begin

        if rising_edge(clk) then

            q <= d;

        end if;

    end process;

end architecture;

entity display is

    port (

        signal : in std_logic

    );

end entity;

architecture behavior of display is

begin

    -- Display the signal

end architecture;

entity top is

    port (

        input_signal : in std_logic;

        clk : in std_logic

    );

end entity;

architecture behavior of top is

    signal signal_buffer : std_logic;

begin

    latch : entity work.d_latch

        port map (d => input_signal, clk => clk, q => signal_buffer);

    display : entity work.display

        port map (signal => signal_buffer);

end architecture;

Flip-flop:

A flip-flop is an edge-triggered device that stores a value on a clock edge. There are two types of flip-flops: D flip-flop and JK flip-flop.

Example usage:

A flip-flop can be used as a memory element in a digital design. In this example, a D flip-flop is used to store a counter value and output it to a display:

Verilog code:

module d_ff(input d, clk, reset, output reg q);

    always @ (posedge clk, negedge reset) begin

        if (!reset)

            q <= 0;

        else

            q <= d;

    end

endmodule

SystemVerilog code:

module d_ff(input logic d, clk, reset, output logic q);

    always_ff @(posedge clk, negedge reset) begin

        if (!reset)

            q <= 0;

        else

            q <= d;

    end

endmodule

module display(input logic signal);

    // Display the signal

endmodule

module top(input logic clk, reset);

    logic counter = 0;

    d_ff ff(counter, clk, reset, counter);

    display display(counter);

endmodule

VHDL code:

entity d_ff is

    port (

        d : in std_logic;

        clk : in std_logic;

        reset : in std_logic;

        q : out std_logic

    );

end entity;

architecture behavior of d_ff is

begin

    process (clk, reset)

    begin

        if reset = '0

            q <= '0';

        elsif rising_edge(clk) then

            q <= d;

        end if;

    end process;

end architecture;

entity display is

    port (

        signal : in std_logic

    );

end entity;

architecture behavior of display is

begin

    -- Display the signal

end architecture;

entity top is

    port (

        clk : in std_logic;

        reset : in std_logic

    );

end entity;

architecture behavior of top is

    signal counter : std_logic;

begin

    latch : entity work.d_ff

        port map (d => counter, clk => clk, reset => reset, q => counter);

    display : entity work.display

        port map (signal => counter);

end architecture;

Putting things differently, a latch is a circuit element that can hold a state (either 1 or 0) and remain in that state until a new input signal arrives. A flip-flop, on the other hand, is a type of latch that is designed to change state only when a specific set of conditions is met, such as a clock signal. In other words, a flip-flop is a synchronous device, meaning it changes state only at specific points in time, whereas a latch is an asynchronous device, meaning it can change state at any time. Additionally, flip-flops often have specific inputs for setting and resetting the state, whereas latches typically have a single input that determines the new state. 

Clocking:
Flip-flops are synchronous devices that change state only on the rising or falling edge of a clock signal, whereas latches are asynchronous and can change state at any time in response to the input signal.

Inputs:
Flip-flops typically have two inputs, a data input (D) and a clock input (CLK), whereas latches usually have a single input that determines the new state. Some flip-flops may also have a reset input (R) to initialize the circuit to a known state.

Output:
Flip-flops and latches both have two outputs, representing the two possible states (1 or 0). The output of a latch is a direct function of the input, whereas the output of a flip-flop depends on both the data input and the clock edge.

Timing:
Latches are considered to be faster than flip-flops because they don't wait for a clock signal to change state. However, the speed of a flip-flop is more predictable, since it changes state only on the rising or falling edge of a clock signal.

Usage:
Latches are used in applications where the timing of the input signal is not critical, such as in buffer circuits or level-sensitive data storage. Flip-flops are used in synchronous circuits where the timing of the data must be controlled, such as in digital clocks or digital counters.

Types:
There are several types of flip-flops, including SR, D, T, JK, and Master-Slave flip-flops, each with its own set of inputs and outputs, and specific uses in digital circuit design. Latches, on the other hand, come in two main types: SR and D latches.

A common implementation of a flip-flop is a pair of latches (Master/Slave flop).

Latches are sometimes called “transparent latches”, because they are transparent (input directly connected to output) when the clock is high.

The clock to a latch is primarily called the “enable”.

For more information have a look at the picture below.

Deprecated Hardware:

Latches:

1. Use flops, not latches
2.  Latch-based designs are susceptible to timing problems
3. The transparent phase of a latch can let a signal “leak” through a latch — causing the signal to affect the output one clock cycle too early
4. It’s possible for a latch-based circuit to simulate correctly, but not work in real hardware, because the timing delays on the real hardware don’t match those predicted in synthesis

Flip-flops:

1. Limit yourself to D-type flip-flops
2. Some FPGA and ASIC cell libraries include only D-type flip flops. Others, such as Altera’s APEX FPGAs, can be configured as D, T, JK, or SR flip-flops.

• For every signal in your design, know whether it should be a flip-flop or combinational. Examine the log file e.g. dc shell.log to see if the flip-flops in your circuit match your expectations, and to check that you don’t have any latches in your design.
• Do not assign a signal to itself (e.g. a <= a; is bad). If the signal is a flop, use an enable to cause the signal to hold its value. If the signal is combinational, then assigning a signal to itself will cause combinational loops, which are very bad.