For example, a binary mod 8 counter has 8 countable states. They are from to So the mod 8 counter counts from 0 to 7. A binary mod 4 counter has 4 count states, from to So the mod 4 counter counts from 0 to 4. Counting means incrementing or decrementing the values of an operator, with respect to its previous state value. So to perform the mathematical operation we use no devices other than counters.
We cannot perform this action counting with any other logic devices rather than counters. Back to top. Stops count without resetting when at logic 1. TC can be used to detect the end of an up or down count, and as well as being available as an output, TC is used internally to generate the Ripple Carry output. Connecting Synchronous counters in cascade, to obtain greater count ranges, is made simple in ICs such as the 74HC by using the ripple carry RC output of the IC counting the least significant 4 bits, to drive the clock input of the next most significant IC, as show in red in Fig.
Although it may appear that either the TC or the RC outputs could drive the next clock input, the TC output is not intended for this purpose, as timing issues can occur. Although synchronous counters have a great advantage over asynchronous or ripple counters in regard to reducing timing problems, there are situations where ripple counters have an advantage over synchronous counters. When used at high speeds, only the first flip-flop in the ripple counter chain runs at the clock frequency.
Each subsequent flip-flop runs at half the frequency of the previous one. In synchronous counters, with every stage operating at very high clock frequencies, stray capacitive coupling between the counter and other components and within the counter itself is more likely occur, so that in synchronous counters interference can be transferred between different stages of the counter, upsetting the count if adequate decoupling is not provided.
This problem is reduced in ripple counters due to the lower frequencies in most of the stages. Also, because the clock pulses applied to synchronous counters must charge, and discharge the input capacitance of every flip-flop simultaneously; synchronous counters having many flip-flops will cause large pulses of charge and discharge current in the clock driver circuits every time the clock changes logic state.
This can also cause unwelcome spikes on the supply lines that could cause problems elsewhere in the digital circuitry. This is less of a problem with asynchronous counters, as the clock is only driving the first flip-flop in the counter chain.
Asynchronous counters are mostly used for frequency division applications and for generating time delays. In either of these applications the timing of individual outputs is not likely to cause a problem to external circuitry, and the fact that most of the stages in the counter run at much lower frequencies than the input clock, greatly reduces any problem of high frequency noise interference to surrounding components. Hons All rights reserved.
Revision Learn about electronics Digital Electronics. Number Systems 2. Digital Logic 3. Logic Families 4. Combinational Logic 5. Sequential Logic. Module 5. After studying this section, you should be able to: Understand the operation of digital counter circuits and can: Describe the action of asynchronous ripple counters using D Type flip flops. Understand the operation of synchronous counters. These are made of flip-fops as basic elements. There are two states included in the flip-flops.
The high state is represented by 1 and the low state by 0. The operation of the counter can be made possible by high state-driven flip-flops. Counters consist of modes that are represented by the number of countable states, for example a mod-8 counter. In this, the number of states countable is from to that is 0 to 7. Hence eight countable states name the counter as a mod-8 counter. Definition: The circuit is designed with digital logic to obtain information about the number of events that occurred.
This type of digital logic device can be defined as a Counter. The design of counters can be achieved by following various steps. The above steps are especially followed fro the design of the type of counter known as Synchronous Counters. The interconnection of the flip-flops results in the classification of the counters. Although the single clock signal applied to the counters. There is a difference among the operation based on a single cock applied to the flip-flops in the circuit or the signal applied to the main flip-flop.
The asynchronous counters are also referred to as Ripple counters. The simplest in design among the other counters is the ripple counter.
Flip flops, which are electronic circuits used to store binary data, in synchronous counters are triggered by the same clock signal. Synchronous counters are very simple in design. All the flip flops are interconnected and driven by the same clock signal. The state output of the preceding flip flop determines the state change of the succeeding flip flop.
Since the flip flops work synchronously, the synchronous counters do not require settling. There is a required number of logic gates to implement the synchronous counters and the operation is fast. Unlike the asynchronous counter, the synchronous counter has one global clock which drives each flip flop so output changes in parallel.
An advantage of synchronous counters over asynchronous counters is that it can operate on a higher frequency as it does not have cumulative delay from the clock. Asynchronous counters transition without having to depend on the clock signal input.
For this reason, asynchronous counters are also called ripple counters. It uses a smaller number of logic gates and its operation is very slow compared to synchronous counters. This counter will increment once every clock cycle and takes two clock cycles to overflow. Hence, it will alternate between a transition from 0 to 1 and a transition from 1 to 0 for every cycle. You can continue to add additional flip flops, always inverting the output to its own input, and using the output from the previous flip flop as the clock signal.
The result is called a ripple counter, which can count to 2 n -1, where n is the number of bits flip-flop stages in the counter. But they are useful in applications such as dividers for clock signals, where the instantaneous count is unimportant, but the division ratio overall is. The use of flip flop outputs as clocks lead to timing skew between the count data bits, making this ripple technique incompatible with normal synchronous circuit design styles.
0コメント