If the clock is High for a time interval less than the propagation delay of the flip flop then racing around condition can be eliminated. The propagation delay (delta t) should be made greater than the duration of the clock pulse (T).īut it is not a good solution as increasing the delay will decrease the speed of the system. There are three methods to eliminate race around condition as described below: Increasing the delay of flip-flop Methods to eliminate race around condition This condition also exists in T flip-flop since T flip-flop also has toggling options. This problem is called Race around the condition. To put it in words, “ For JK flip-flop if J, K and Clock are equal to 1 the state of flip-flop keeps on toggling which leads to uncertainty in determining the output of the flip-flop. This condition is called as Race around condition. Therefore, whenever Clock is equal to 1 there are consecutive toggling. Here, propagation delay has also been reduced, so the output will be given out at the instant input is given. Let us look at the timing diagram of JK flip-flop when the race around condition is considered.Īs you already know, when J, K and Clock are equal to 1, toggling takes place. This is where Race around condition comes into the play. This is what was expected, but the output may not be like this all the time. The delay between input and output is called a propagation delay. Here, T is the time period of the clock whereas delta t is the propagation delay. Now, let us look at the timing diagram of JK flip-flop.
The next state will be equal to the complement of the present state. Here, Q is the present state and Q’ is the next state.Īs you can see, when J, K and Clock are equal to 1, toggling takes place, i.e. When clock is high the output does not change, it remains in the previous state which was at the end of the negative clock pulse.This site uses Just the Docs, a documentation theme for Jekyll. Similarly, in negative triggering the clock samples the input line as the clock is negative and sets/resets the flip flop according to the state of the input lines. When clock is low the outputs does not change it remains in the previous state which was at the end of the positive clock pulse. In the positive triggering the clock samples the input line as the clock pulse is positive, and sets/resets the flip flop according to the state of the input lines. The level triggering may be of two types: Figure 3: Negative Edge Triggered Flip Flop A small circle is put before the arrow head to indicate negative edge triggering. A symbolic representation of negative edge triggering has been shown in Figure 3. The output of the flip flop is set or reset at the negative edge of the clock pulse. In negative edge triggered flip flops the clock samples the input lines at the negative edge (falling edge or trailing edge) of the clock pulse. Figure 2: Positive Edge Triggered JK Flip Flop Negative Edge Triggered Flip Flop The arrow head symbol is termed as dynamic signal indicator. The arrow head at clock terminal indicates positive edge triggering.
A symbolic representation for positive edge triggering has been shown in Figure 2.
This state of the output remains for one clock cycle and the clock again samples the input line on the next positive edge of the clock. The state of the output of the flip flop is set or reset depending upon the state of the input at positive edge of the clock. In positive edge triggered flip flops the clock samples the input line at the positive edge (rising edge or leading edge) of the clock pulse. A circuit clocked by the leading edge, as in Figure 1 (b) is referred to as being positive edge triggered while another circuit triggering on the trailing edge, as in Figure 1(c) is negative edge triggered. Some flip flop are other logic units are triggered when the clock reaches prescribed voltage levels or goes from one voltage level to another usually without regard to voltage rise or fall time. The particular flip flop specifications will provide this information as we shall see. Some flip flop circuits are triggered by the clock leading edge while other units are triggered on the clock trailing edge.
EDGE TRIGGERED FLIP FLOP TIMING DIAGRAM FULL
Figure 1: Clock Waveformįigure 1: Clock Waveform (a) Full Clock Pulse (b) Leading edge (c) Trailing edge For positive logic operation we define the low to high transition as the leading edge of the clock signal (Figure 1(b)) while the transition from high to low is called the clock trailing edge (Figure 1(c)). A clock signal as seen in Figure 1(a) has two transitions, one from low to high level the other from high to low level. The pulse goes from a low level 0 volt, the positive logical 0 condition, to a high level ( +5 volts, the positive logic logical 1 condition going between the two logic levels at a fixed frequency rate. A clock pulse used to operate a flip flop is illustrated in Figure 1(a).