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Jk flipflop logicworks7/13/2023 ![]() ![]() Generally, synchronous counters count on the rising-edge which is the low to high transition of the clock signal and asynchronous ripple counters count on the falling-edge which is the high to low transition of the clock signal. Synchronous Counters use edge-triggered flip-flops that change states on either the “positive-edge” (rising edge) or the “negative-edge” (falling edge) of the clock pulse on the control input resulting in one single count when the clock input changes state. We could quite easily re-arrange the additional AND gates in the above counter circuit to produce other count numbers such as a Mod-12 counter which counts 12 states from”0000″ to “1011” (0 to 11) and then repeats making them suitable for clocks, etc. Thus, the count is reset and starts over again at “0000” producing a synchronous decade counter. Flip-flop FF0 toggles on every clock pulse. The additional AND gates detect when the counting sequence reaches “1001”, (Binary 10) and causes flip-flop FF3 to toggle on the next clock pulse. If we enable each JK flip-flop to toggle based on whether or not all preceding flip-flop outputs ( Q) are “HIGH” we can obtain the same counting sequence as with the asynchronous circuit but without the ripple effect, since each flip-flop in this circuit will be clocked at exactly the same time. These additional AND gates generate the required logic for the JK inputs of the next stage. The J and K inputs of flip-flop FFB are connected directly to the output Q A of flip-flop FFA, but the J and K inputs of flip-flops FFC and FFD are driven from separate AND gates which are also supplied with signals from the input and output of the previous stage. Then the synchronous counter follows a predetermined sequence of states in response to the common clock signal, advancing one state for each pulse. Therefore, a bit counter can also be called a mod 4 counter.It can be seen above, that the external clock pulses (pulses to be counted) are fed directly to each of the J-K flip-flops in the counter chain and that both the J and K inputs are all tied together in toggle mode, but only in the first flip-flop, flip-flop FFA (LSB) are they connected HIGH, logic “1” allowing the flip-flop to toggle on every clock pulse. ![]() Thus, for a 2-bit counter, there is a mod 4 counter by the mechanism of 2 to the power n. These numbers of states are asserted as mod numbers. ![]() Like the 2-bit counter, a counter having n number of flip-flops can have 2 to the power n states. There is the placement of each counter to correspond to the account value. The above two-bit ripple counter has four states. The clock pulse given into the first flip-flop is rippled through the other counters after the propagation delay. There is a small delay between the clock and the first and second transitions in the above counter.Īll the clear inputs are connected together so that a single pulse can clear all the flip-flops before the counting of bits. This phenomenon occurred here although it is an asynchronous counter. The transitions of Q 0, Q 1, and clock pulse in the figure of the timing diagram above are simultaneous. Therefore, the triggering of flip-flops cannot be simultaneous. Due to an essential propagation delay in the circuit through a flip-flop, the change in the input clock pulse and change of the Q output of the first flip-flop can never occur at the same time giving the exact result. But the second flip-flop changes only when it is triggered by the Q output of the first flip-flop. So, this is why the first flip-flop changes the state at the quick falling edge of the clock pulse. The external clock is connected to the clock input of the first flip-flop. Mechanism of working of 2-bit Ripple CounterĪ 2-bit ripple counter is shown in the above figure. We can use them as both Up and Down Counter. 2-Bit Asynchronous Counter is the type of ripple counter which has only 2 flip-flops in its design.
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