![d type positive edge triggered flip flop d type positive edge triggered flip flop](https://learnabout-electronics.org/Digital/images/D-Type-edge-trig.gif)
When these occur, a change of state has been initiated, but before it has been completed the flip-flop returns to its original state. It is also possible to have transient outputs, referred to as partial sets and resets. Additionally, if these two factors are not taken into consideration during the circuit design stage, data may be missed, and required transitions may not take place. The set-up and hold times represent important timing constraints which have some influence on the maximum clock frequency at which the device can be operated.
![d type positive edge triggered flip flop d type positive edge triggered flip flop](https://slideplayer.com/slide/2557193/9/images/3/Positive+Edge+Triggered.jpg)
Hence S ¯ and R ¯ remain unchanged, as do the outputs Q and Q ¯ of the flip-flop. The consequence of such a transition is that the output of g 4 makes a 0 → 1 transition which is transferred to one of the inputs of both g 1 and g 3 without affecting their outputs. If a change in D from 1 → 0 takes place after the hold time has elapsed and while the clock is still high, there will be no further change in the flip-flop output. The changes taking place in the circuit after the clock transition from 0 → 1 are recorded in Figure 6.21(c). It is essential that there should be no change in D during this period. The time taken for S ¯ to change 1 → 0 is the hold time and is equal to the gate delay of g 2. This is because the lower input of g 3 made a 1 → 0 transition during the set-up time. It should be noticed that there is no change in R ¯ as a consequence of the clock going high. The change in S ¯ initiates a change of state in the output latch and Q makes a 0 → 1 transition followed by a 1 → 0 transition in Q ¯. The inference is that there should be no change in Ck until after the elapse of the set-up time.Īfter the set-up time, the clock is allowed to go high, and as a consequence the output of g 2, S ¯, makes a 1 → 0 transition. The time delay before this change occurs is equal to the sum of the gate delays g 4 and g 1 and is the set-up time for the flip-flop. When the data D is changed from 0 → 1 during the asynchronous period then the output of g 4 changes from 1 → 0 which initiates a 0 → 1 transition at the output of g 1 and that change is transferred to the input of g 2 as shown in Figure 6.21(b). If additionally D = 0, then the remaining signals at different parts of the circuit can easily be determined, and they have been inserted in Figure 6.21(b). In order to maintain the output latch in a stable state, both S ¯ and R ¯ must be held at 1 and this is achieved when the clock Ck = 0 since the outputs of g 2 and g 3 are then 1.
![d type positive edge triggered flip flop d type positive edge triggered flip flop](https://i.stack.imgur.com/jUiPW.png)
The output latch is formed by gates g 5 and g 6. The three latches are interconnected as shown in Figure 6.21(b), with g 1 and g 2 comprising one latch while g 3 and g 4 comprise a second latch. The diagram shows the effect of a 0 → 1 transition on the D line (c) Effect of a 0 → 1 transition on the clock line (a) The basic S ¯ R ¯ flip-flop (b) Edge triggered flip-flop. A low logic level on the preset or clear inputs will set or reset the outputs regardless of the logic levels of the other inputs.Figure 6.21. The data on the D input may be changed while the clock is LOW or HIGH without affecting the outputs as long as the data setup and hold times are not violated. The triggering occurs at a voltage level and is not directly related to the transition time of the rising edge of the clock. The information on the D input is accepted by the flip-flops on the positive going edge of the clock pulse. The 74LS74 device contains two independent positive-edge-triggered D flip-flops with complementary outputs.