RTL Design Practice Guide I - Sequential Logic - D flip-flop

 

2.2 Flip-Flops

2.2.1 D Flip-Flop

A D flip-flop, also known as data flip-flop or delay flip-flop, is a fundamental building block in digital logic design. It is a sequential logic device that stores and transfers a single data bit. The D flip-flop is widely used in various digital systems, including microprocessors, memories, and communication circuits.

The D flip-flop consists of a data input (D), a clock input (CLK), and an output (Q). The data input (D) represents the bit of information that we want to store or transfer. The clock input (CLK) serves as a control signal to synchronize the operation of the flip-flop. When a rising or falling edge of the clock signal occurs, the value present at the input is captured and stored.

The behaviour of a D flip-flop can be summarized as follows: When the clock signal transitions, the stored value at the data input (D) is transferred to the output (Q) of the flip-flop. This transfer happens instantly, making the output reflect the value of the data input at the clock edge. The output remains unchanged until the next clock transition occurs.

The D flip-flop is often used for holding or delaying data, as well as for implementing registers and data storage elements in digital circuits. It can be combined with other logic gates and flip-flops to create more complex sequential circuits, such as counters, shift registers, and state machines.

The D flip-flop is an essential component in digital logic design offering a simple and efficient means of storing and transferring data. Understanding its operation and characteristics is crucial for designing and analysing sequential logic circuits, ensuring the correct functionality and timing synchronization of digital systems.

2.2.2 Block Diagram

The block diagram of a D flip-flop with both synchronous and asynchronous reset can be represented as follows:

Figure 2.8 D Flip-Flop with a synchronous reset

Figure 2.9 D Flip-Flop with an asynchronous reset

 

2.2.3 RTL Schematic

In VLSI (Very Large-Scale Integration) digital design, an RTL (Register-Transfer Level) schematic refers to a high-level representation of a digital circuit. It and captures the behaviour and interconnections of the circuit components at a register-transfer level, focusing on the flow of data between registers and the operations performed on that data. The RTL schematic provides a functional view of the circuit without specifying the detailed implementation or gate-level connections.

Below are the representations of the synchronous and asynchronous reset RTL schematic of a D flip-flop.

 

Figure 2.10 D flip-flop with a synchronous reset

 

Figure 2.11 D flip-flop with an asynchronous reset

 

2.2.4 RTL Synthesis

RTL synthesis is a process in digital design where a high-level RTL (Register-Transfer Level) description of a digital circuit is transformed into a gate-level representation using a synthesis tool. The synthesis tool translates the RTL description, usually written in a hardware description language (HDL) like Verilog or VHDL, into a network of gates, flip-flops, and other components that implement the desired functionality.

During RTL synthesis, the synthesis tool analyses the RTL description, optimizes the logic, and maps it onto a library of standard cells or primitive elements. The goal is to generate a gate-level netlist that meets certain design constraints such as area, power and timing requirement.

Below are the representations of the synchronous and asynchronous reset synthesis schematic of a D flip-flop using FPGA.

Figure 2.12 D flip-flop with a synchronous reset

 

Figure 2.13 D flip-flop with an asynchronous reset

During the synthesis process, the synthesis tool maps the RTL description of the D flip-flop to the available LUTs and flip-flops within the FPGA fabric. It optimizes the logic placement and routing to achieve the desired functionality while meeting the timing requirements.

The exact configuration and resources used for implementing the D flip-flop in an FPGA may depend on the specific FPGA architecture, technology, and synthesis tool settings.

2.2.5 FPGA Implementation

FPGA (Field-Programmable Gate Array) implementation refers to the process of designing and configuring a digital circuit of system on an FPGA device. An FPGA is a programmable integrated circuit that contains an array of configurable logic blocks and programmable interconnects, allowing designers to implement custom digital logic.

The following is the D flip-flop implementation:

Reference Device: xcau15p-ffvb676-2-e

                                                LUT           -    1

                                                FF              -    1

                                                BRAM       -    0

                                                URAM       -    0

                                                DSP            -    0

                                                IO               -   4

                                                GT              -    0

                                                BUFG         -    1

                                                MMCM      -     0

                                                PLL            -     0

                                                PCIe           -     0

FPGA Implementation of a D flip-flop

 

2.2.6 Hardware Description Language

A hardware description language (HDL) is a specialized programming language used for designing and describing digital electronic systems at the register-transfer level (RTL) or behavioural level. HDLs provide a way to model and describe the behaviour, structure, and interconnections of digital circuits or systems.

2.2.6.1 System Verilog: D Flip-Flop with Synchronous Reset

The following is the system Verilog implementation of a D flip-flop with a synchronous reset.

module dff_synchronous_sv (

  input logic clk,

  input logic din,

  input logic rstn,

  output logic qout

);

always_ff@(posedge clk) begin

  if (!rstn) begin

    qout <= 0;

  end else begin

    qout <= din;

  end

end

endmodule

 

In the code above, the ‘dff_synchronous_sv’ module represents a D flip-flop with synchronous reset. It has four inputs: ‘clk’ for the clock signal, ‘rstn’ for the synchronous reset signal, and ‘din’ for the input data. The output is ‘qout’, representing the output of the flip-flop.

Inside the ‘always’ block, the flip-flop is sensitive to the positive edge of the clock (‘posedge clk’) and the negative edge of the reset signal. When the reset signal is asserted (‘rst’ is low), the output ‘qout’ is set to logic 0 (‘1’b0’). Otherwise, when the reset is not asserted, the output ‘qout’ follows the input data ‘din’.

 

2.2.6.2 System Verilog: D Flip-Flop with Asynchronous Reset

The following is the system Verilog implementation of an asynchronous reset D flip-flop.

module dff_asynchronous_sv (

  input logic clk,

  input logic din,

  input logic rstn,

  output logic qout

);

  always_ff @(posedge clk or negedge rstn) begin

    if (!rstn) begin

      qout <= 0;

    end else begin

      qout <= din;

    end

  end

endmodule

 

In the code above, the ‘dff_asynchronous_sv’ module represents a D flip-flop with an asynchronous reset. It has four inputs: ‘clk’ for the clock signal, ‘rstn’ for the asynchronous reset signal, and ‘din’ for the input data. The output is ‘qout’ representing the output of the flip-flop.

Inside the ‘always’ block, the flip-flop is sensitive to the positive edge of the clock (‘posedge clk’) and the negative edge of the reset signal (‘negedge rstn’). When the reset signal is asserted (‘rstn’ is low), the output ‘qout’ is set to logic 0 (‘1’b0’). Otherwise, when the reset is not asserted, the output ‘qout’ follows the input data ‘din’.

2.2.7 Questions to ask design perspective

• Ø  How do you define the functionality and behaviour of a D flip-flop
• Ø  How do you describe the D flip-flop in a hardware description language (HDL) such as Verilog or VHDL?
• Ø  What are the timing requirements and constraints for the D flip-flop? How do you ensure proper setup and hold times?
• Ø  How do you handle clock signals in the D flip-flop design? How do you synchronize the clock to avoid metastability issues?
• Ø  What are the considerations for implementing a synchronous reset or asynchronous reset in a D flip-flop?
• Ø  How do you verify the correctness of the D flip-flop design? What kind of testbenches and simulation techniques can be used?
• Ø  How do you optimize the design for area, power, and timing? What design trade-offs can be made?
• Ø  How does the D flip-flop fit into a larger system design? How do you interface it with other components or modules?
• Ø  How do you ensure the reliability and robustness of the D flip-flop design? What are the possible sources of errors or hazards?

2.2.8 Problem statements for RTL design practice:

1. v  Design a D flip-flop with synchronous reset and verify its functionality using a testbench.
2. v  Implement a D flip-flop with an asynchronous reset and validate its behaviour through simulation.
3. v  Design a D flip-flop with an enable signal, allowing data to be stored only when the enable signal is asserted.
4. v  Implement a D flip-flop with an active-high and active-low output, providing both representations of the stored value.
5. v Create a D flip-flop with a data enable signal, allowing data to be stored only when data enable signal is asserted; otherwise, maintain the previous value.
6. v  Design a D flip-flop with a control input that switches between “hold” and “update” modes, holding the current value or storing the input data, respectively.
7. v  Implement a D flip-flop with a synchronous clear input and verify its functionality through simulation.
8. v  Create a D flip-flop with a synchronous pre-set input and validate its behaviour using test cases.
9. v  Design a D flip-flop with an additional clock enable input, allowing the clock signal to be gated and controlled externally.
10. v  Implement a D flip-flop with a transparent latch enable input, enabling the D input to directly propagate to the output without being clocked.