positive and negative edge triggered flip flop

Just before pulse c the D input goes low, so at the positive going edge of pulse c, Q goes low.īetween pulses c and d the asynchronous S input goes low and immediately sets Q high. 5.3.5.Īt the positive going edges of clock pulses a and b, the D input is high so Q is also high. A timing diagram illustrating the action of a positive edge triggered device is shown in Fig. The ‘Edge triggered D type flip-flop with asynchronous preset and clear capability’, although developed from the basic SR flip-flop becomes a very versatile flip-flop with many uses. 5.3.5 Typical Schematic Symbols for D Type Edge Triggered Flip-Flops The name Data Latch refers to a D Type flip-flop that is level triggered, as the data (1 or 0) appearing at D can be held or ‘latched’ at any time whilst the CK input is at a high level (logic 1).Īs can be seen from the timing diagram shown in Fig 5.3.2, if the data at D changes during this time, the Q output assumes the same logic level as the D.įig. If D = 0 then R must be 1 and S must be 0, causing Q to be reset to 0.

5.3.1, if D = 1, then S must be 1 and R must be 0, therefore Q is SET to 1. Provided that the CK input is high (at logic 1), then whichever logic state is at D will appear at output Q and (unlike the SR flip-flops) Q is always the inverse of Q). 5.3.1 is called a level triggered D Type flip-flop because whether the D input is active or not depends on the logic level of the clock input. 5.3.1 shows this as a ‘don’t care’ state (X). Operation.Īs long as the clock input is low, changes at the D input make no difference to the outputs. The S and R inputs are now replaced by a single D input, and all D type flip-flops have a clock input. This simple modification prevents both the indeterminate and non-allowed states of the SR flip-flop. The simplest form of D Type flip-flop is basically a high activated SR type with an additional inverter to ensure that the S and R inputs cannot both be high or both low at the same time. To avoid the ambiguity in the title therefore, it is usually known simply as the D Type. 5.3.1 together with its truth table and a typical schematic circuit symbol, may be called a Data flip-flop because of its ability to ‘latch’ and remember data, or a Delay flip-flop because latching and remembering data can be used to create a delay in the progress of that data through a circuit. its indeterminate output and non-allowed logic states) described in Digital Electronics Module 5.2 is overcome by the D type flip-flop. The major drawback of the SR flip-flop (i.e.

Positive and negative edge triggered flip flop