Digital engineering

D-Type Flip-Flop: From Basic Construction to Master-Slave Variations.

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The D-type flip-flop addresses a significant drawback of the original SR NAND gate bistable circuit. The problematic indeterminate state where both the SET and RESET inputs are at “0”. In this state, both outputs will be driven to logic “1”, overriding the feedback mechanism and leading to unpredictable behavior. The input that reaches logic “1” first will dominate, while the other input at logic “0” determines the final state of the latch.

d-type-flip-flop

 

To avoid this problem, an inverter can be introduced between the SET and RESET inputs. it creating a different type of flip-flop circuit known as a data latch. the delay flip-flop or D-type bistable, simply D flip-flop.

 

The D flip-flop is an important among all clocked flip-flops. By incorporating an inverter (NOT gate) between the SET and RESET inputs. The S and R inputs become complementary. To ensuring that they are never equal (both “0” or both “1”) simultaneously. This configuration allows the flip-flop to be controlled by a single D (data) input, simplifying its operation and improving its reliability.

In this setup, the data input labeled “D” replaces the “set” signal. While the inverter generates the complementary “reset” input. This modification turns a level-sensitive SR latch into a level-sensitive D-type flip-flop. Where now S = D and R = NOT D.

D-type Flip-Flop Circuit

A basic SR flip-flop requires two inputs. One to “set” the output and other is “reset”. By adding an inverter (NOT gate) to the SR flip-flop. we can manage both “set” and “reset” functions using only one input. since the two input signals become complementary to each other. This setup eliminates the ambiguity of the SR latch’s indeterminate state when both inputs are low.

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The single input is called the “data” input. When data input is high flip-flop are “set” and when it are low flip-flop is “reset”. However using only this data input would result in constant toggling with every change in the input.

To address this, an additional input known as the “clock” or “enable” input is introduced. This input isolates the data input from the flip-flop’s latching mechanism once the data is stored. As a result, the data input state is transferred to the output Q only when the clock input is HIGH. This setup forms the basis of the D flip-flop.

In a D flip-flop, the output reflects the logic level applied to the data terminal while the clock input is HIGH. When the clock input goes LOW both the “SET” and “RESET” inputs are held at logic HIGH preventing any change in state and allowing the flip-flop to retain the data that was present before the clock transition. Essentially, the output is “latched” at logic “0” or “1”.

Truth Table for the D-type Flip Flop

Clk D Q Q Description
↓ » 0 X Q Q Memory
no change
↑ » 1 0 0 1 Reset Q » 0
↑ » 1 1 1 0 Set Q » 1

Note that: ↓ and ↑ indicates direction of clock pulse as it is assumed D-type flip flops are edge triggered

Master-Slave D Flip-Flop

The basic D-type flip-flop can be extended by including a second SR flip-flop. Which is activated by the complementary clock signal. Resulting in a “master-slave D-type flip-flop.” In this configuration the first stage is known as the “master,” captures the input state on D at the leading edge of the clock signal (low-to-high). While the output stage remains inactive.

At the trailing edge of the clock signal (high-to-low), the second stage or “slave”. becomes active and latches to the output of the master stage. As a result, the output of master-slave D flip-flop appears to be triggered by negative edge of the clock pulse. This setup effectively creates one flip-flop by cascading two latches. that are controlled by opposite clock phases.

The Master-Slave D Flip Flop Circuit

As described above the leading edge of the clock pulse, the master flip-flop is activated, loading data from the D input. This means that the master is “on” during this phase. At the trailing edge of the clock pulse, the slave flip-flop is activated, or turned “on,” loading data from the master.

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Thus, at any given time either the master or slave flip-flop will be “on” but never both simultaneously. As a result, the output Q shows the value of D only after one full clock cycle (0-1-0) has been completed.

Many D flip-flop ICs are available in both TTL and CMOS packages. A common example is the 74LS74. a dual D flip-flop IC that contains two separate D-type bistables in the same chip. It allowing the creation of either single or master-slave toggle flip-flops.

74LS74 Dual D-type Flip Flop

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Other Popular D-type flip-flop ICs

Device Number Subfamily Device Description
74LS74 LS TTL Dual D-type Flip Flops with Preset and Clear
74LS175 LS TTL Quad D-type Flip Flops with Clear
74LS273 LS TTL Octal D-type Flip Flops with Clear
4013B Standard CMOS Dual type D Flip Flop
40174B Standard CMOS Hex D-type Flip Flop with Master Reset

D Flip-Flops as Data Latches

In addition to frequency division, D flip-flops are also valuable as data latches. A data latch acts as a device to hold or retain data present at its inputs functioning similar to a single-bit memory device. Integrated circuits such as the TTL 74LS74 or CMOS 4042 are available in quad format specifically for this application.

By connecting four 1-bit data latches to their clock inputs all of the latches can be “clocked” together. This setup creates a simple “4-bit” data latch, as shown below.

D-Type Flip-Flop Summary

D-type flip-flops can be built using two back-to-back SR latches, with an inverter (NOT gate) placed between the S and R inputs. So that a single D (data) input is enabled.

Further enhancement is achieved by adding a second SR flip-flop to the output. Which is activated by the complementary clock signal resulting in a “master-slave D flip-flop” configuration.

The main difference between D-type latch and D-type flip-flop is that the latch operates without a clock signal to change state. While a flip-flop requires a clock signal. The D flip-flop is edge-triggered. it means that it transfers data to the output queue on the rising or falling edge of the clock signal. In contrast, data latches. Such as data latches and transparent latches are level-sensitive devices.

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