Since we are using the D flip-flop, the next step is to draw the truth table for the counter. Counter is a sequential circuit. Also, For the truncated sequence count, when it is not equal to , extra feedback logic is needed. It is a group of flip-flops with a clock signal applied. It contains 3 flip-flops, Q0, Q1, Q2 are the outputs of the flip-flops. Asynchronus does not mean that the circuit does not have clock . Timing Diagram of Asynchronous Decade Counter and its Truth Table In the above image, a basic Asynchronous counter used as decade counter configuration using 4 JK Flip-Flops and one NAND gate 74LS10D. The J B and K B inputs are connected to Q A. The clock inputs of the remaining flip-flops have the outputs of their preceding flip-flops as inputs. The J A and K A inputs of FF-A are tied to logic 1. A 4-bit down counter is a digital counter circuit, which provides a binary countdown from binary 1111 to 0000. A digital circuit which is used for a counting pulses is known counter. We will be using the D flip-flop to design this counter. This is quite less compared to the asynchronous counters. We will be using the D flip-flop to design this counter. Every counter has a limit with regards to the number they can count up or down to. The way to achieve the ability to count in both the directions is by combining the designs for the up and the down counters and using a switch to alternate between them. Asynchronous Up-Down Counters Figure 2.5 : Asynchronous Up-Down Counter In certain applications a counter must be able to count both up and down. MOD-4 Counter State Diagram We can see from the truth table of the counter, and by reading the values of QA and QB, when QA = 0 and QB = 0, the count is 00. All J and K inputs are connected to Logic 1. Let’s construct the truth table for the 4-bit up counter using D-FF The below image is showing the timing diagram and the 4 outputs status on the clock signal. In fact, in an asynchronous counter, only the first flip-flop is given a clock (CLK) input. The only difference is that for the up counter the output is taken at the non-inverting output ports of the flip-flops. We will use Kmaps to find the logic equations for the remaining flip-flops. So, in this case, we will calculate the equation for only Qn1 to be fed back to Q1. These flip-flops will have the same RST signal and the same CLK signal. He is currently pursuing a PG-Diploma from the Centre for Development of Advanced Computing, India. The only difference in the construction will be that in the 2-bit synchronous down counter, the output will be taken from the inverted outputs of the flip-flop. Up-down counters can count both upwards as well as downwards. Depending on the type of clock inputs, counters are of two types: asynchronous counters and synchronous counters. Above table is created as per follow : When Q 4 =0 which is present state and Q 4 ‘=0 which is next state then T 4 become 0 [As per excitation table, have a look ] Similarly, if Q 4 is 0 and Q 4 ‘ is 1 then T 3 become 1. Since its a Parallel In Serial Shift counter, we first need to initiate it by giving it an input. Since we are using the D flip-flop, the next step is to draw the truth table for the counter. Qn1 is high when Q1 is low AND Q0 is high, OR Q1 is high AND Q0 is low. What are up counters, down counters and up-down counters? This is our complete and definitive guide to digital counters and all their types. If we take the outputs from the MSB and LSB flip-flops and connect them to an AND gate, we can get a logic 1 at the count of 9. A counter is made by cascading a series of flip-flops. About the authorUmair HussainiUmair has a Bachelor’s Degree in Electronics and Telecommunication Engineering. For up-counters, the non-inverted output, Q, is connected to the display. How to design a 4-bit synchronous down counter and 4-bit synchronous up-down counter? These flip-flops will have the same RST signal and the same CLK signal. Hence the input to the fourth flip-flop will have the following logic expression, Therefore from the Kmap, the input equation for the third flip-flop is, And the equation for the for the second flip-flop is. Truth Table – The 3-bit ripple counter used in the circuit above has eight different states, each one of which represents a count value. Since this is a 2-bit synchronous counter, we have two flip-flops. The settling time or the time taken for all the flip-flops to get activated is equal to the sum of all the times needed to activate the last flip-flop. As there is a maximum output number for Asynchronous counters like MOD-16 with a resolution of 4-bit, there are also possibilities to use a basic Asynchronous counter in a configuration that the counting state will be less than their maximum output number. The only difference between an up-counter and a down counter stems from the ports that are connected to the display. On the other hand, 74LS390 is another flexible choice which can be used for large divide by a number from 2 to 50,100 and other combinations as well. For example, a 4-bit synchronous up-counter had 16 states. More precise crystal oscillators can produce precise high frequency other than the signal generators. 3) Different combination of LEDs lit up for different combination of inputs. Those Flip-flops are serially connected together, and the clock pulse ripples through the counter. This means that for every clock pulse, all the flip-flops will generate an output. 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