![]() The inputs the NAND gate are from the Q output from FF1 and FF3 ( from FF3FF2FF1FF0)Ĩ An asynchronously clocked decade counter with asynchronous recycling.Īsynchronous Decade Counter An asynchronously clocked decade counter with asynchronous recycling. One way to make the counter recycle after the count of nine (1001) is to decode count ten (1010) with a NAND gate and connect the output of the NAND gate to the clear (CLR) inputs of the flip-flops. ![]() To obtain a truncated sequence, it is necessary to force the counter to recycle before going through all of its possible states. Counters with the states in their sequence are called decade counters. Counter can also be designed to have a number of states in their sequence that is less than the maximum of 2n. The Modulus of a counter is the number of unique states that the counter will sequence through. CLK PLUSE Q3 Q2 Q1 Q0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 REPEAT RIPPLE COUNTER UP – NGT AND ALL NON FIRST CLK RECEIVE CLK PLUSE FROM Q Clk pulse Q2 Q1 Q0 1 2 3 4 5 6 7 8 (REPEAT) RIPPLE COUNTER UP – PGT AND ALL NON FIRST CLK RECEIVE CLK PLUSE FROM Q’Ħ Four-bit asynchronous binary counter and its timing diagram.Īsynchronous/Ripple Counter Four-bit asynchronous binary counter and its timing diagram. Three-bit asynchronous binary counter and its timing diagram for one cycle. Propagation delays in a 3-bit asynchronous (ripple-clocked) binary counter. Only the first FF receive clock pulse from the source ( clock genarator), others FFs receive clock pulse from either Q or Q’ of prior FF So the effect of an input clock pulse “ripples” through the counter, taking some time, due to propagation delays, to reach the last flip-flop. Cannot get to the second flip-flop (FF1) immediately because of the propagation delay through FF0. * Digital counters have the following important characteristics, Maximum number of count Up-Down Count Asynchronous or Synchronous Operation Free-Running or Self-StoppingĪsynchronous counter are commonly referred to as ripple counter because the effect of the input clock pulse is first “felt” by first flip-flop (FF0). * They are Sequential logic circuits because timing is obviously important and they need a memory characteristic. It works exactly the same way as a two-bit asynchronous binary counter mentioned above, except it has eight states due to the third flip-flop.2 Counters * Counters are important digital electronic circuits. The term recycles it refers to the transition of the counter from its final state back to its original state. The fourth pulse it recycles to its original state (Q0=0, Q1=0). The 2-bit counter exhibits four different states, as you would expect with two flip-flops (22= 4). After the leading edge of CLK4, Q0=0 & Q1=0. The positive-going edge of CLK4 causes Q0 to go LOW, while ̅Q0 goes HIGH and triggers FF1, causing Q 1 to go LOW. Thus, after the leading edge of CLK3, ̅Q0=1 & Q1=1. Output ̅Q0goes LOW and has no effect on FF1. The positive-going edge of CLK3 causes Q0 to go HIGH again. ![]() After the leading edge of CLK2, Q0=0 & Q1=1. ̅Q0 goes HIGH and triggers FF1, causing Q1 to go HIGH. ![]() The positive-going edge of CLK2 causes Q0 to go LOW. After the leading edge of CLK1, Q0=1 & Q1=0. At the same time the ̅Q0 output goes LOW, but it has no effect on FF1 because a positive-going transition must occur to trigger the flip-flop. The positive-going edge of CLK1 (clock pulse1) causes the QĠ output of FF0 to go HIGH. Therefore, the two flip-flops are never simultaneously triggered, so the counter operation is asynchronous.Īpplying 4clock pulses to FF0, Both flip-flops are connected for toggle operation (J=1, K=1) and initially RESET (Q LOW). Because of the inherent propagation delay tie through a flip-flop, a transition of the input clock pulse (CLK) and a transition of the ̅Q0 output of FF0 can never occur at exactly the same time. But FF1 changes only when triggered by a positive-going transition of the ̅Q0 output of FF0. The second flip-flop, FF1, is triggered by the ̅Q0 out-put of FF0.FF0 changes state at the positive-going edge of each clock pulse. Notice that the clock (CLK) is applied to the clock input (C) of only the first flop-flop, FF0, which is always the least significant bit (LSB). An asynchronous counter is one in which the flip-flops within the counter do not change states at exactly the same time because they do not have a common clock pulse.Ī 2-Bit Asynchronous Binary Counter Fig1-1 shows a 2-bit counter connected for asynchronous operation. The term asynchronous refers to events that do not have a fixed time relationship with each other. Asynchronous counters called ripple counters, the first flip-flop is clocked by the external clock pulse and then each successive flip-flop is clocked by the output of the receding flip-flop. ![]()
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