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Electronic clocks are analogous to mechanical clocks. The escapement is represented by the oscillator, analogous to the mechanical oscillators of a mechanical clock.
The chain of gears moving the indicating hands is represented by a counter, which counts the oscillations of 74hc7a4n oscillator. In this page, I discuss the various counter IC’s and how to use them.
Dual D Flip-Flop With Set And Reset
Then, the construction of a digital clock with a Nixie display is described. Since it is very helpful to know ratasheet they work when using them, the fundamentals of electronic counters will be presented here, and the reader is encouraged to perform the experiments, which are simple but instructive.
A counter is a sequential logic machinesince the outputs depend on past inputs, not simply on the current values of the inputs. The basic component of sequential logic is the flip-flop or bistable circuit that serves as memory, since it persists in the last state to which it was set. You’ll build and test ripple binary, synchronous binary, and Johnson ring counters. A useful and frequently-used flip-flop is the 74LS74A datasheett D flip-flop.
The CMOS equivalent is the One of its units is represented in the diagram at the right. D is the data input. Level-triggered devices are more reliable, but are hard for logic designers to understand and use properly, so edge-triggering now dominates. The chip designers have done an excellent job in making the 74A a reliable device, and it is easy to use.
Controls that operate this way are called asynchronous. If they wait for the active clock edge, they are called synchronous. These distinctions are often used with counters, and are quite important in making a design. For our experiments, just leaving them unconnected is equivalent to tying them high. Therefore, at an active clock edge, the flip-flop will change state, or “toggle. However, you must guarantee that the clock edge is sufficiently sharp, or there will be trouble. Bad clock edges are the main source of difficulty with counters.
In case of doubt, use a 74LS14 Schmitt-trigger inverter to clean up the clock signal. Datsaheet toggle flip-flop, by itself, is a 1-bit counter with the states 0, 1. It divides the input frequency by 2, which it is frequently called upon to do. To test a toggle flip-flop or other counter circuits, a datasyeet pushbutton is essential. The circuit is shown at the right. Pin numbers for the 74 and the 00 are shown in the figure below.
The SPDT pushbutton is the hardest thing 74hc74xn find. What you want is one that is normally closed one way, and springs back when pressed datashset released. One output gives a negative pulse usually used for counter clocks and the other gives a positive pulse.
If you use a simple logic switch with an 74hx74an switch and a resistor, or a grounded wire, you will find that the state of a toggle after triggering will be random enough to be a good game of chance!
74hc744an The debounced pushbutton is useful enough to make a permanent circuit darasheet a small box, perhaps together with an LED logic probe or two. Let the number of flip-flops in a counter be N. Each digit corresponds to the state of one of the flip-flops. If we interpret them as a binary number, the first digit is the most significant bit msband 74hf74an second one the least significant bit lsb. The assignment is arbitrary, but if we count in this order, the lsb varies more rapidly than the msb.
In data sheets, bit inputs are represented by letters A, B, C It is important to get the bits in the correct order datashet constructing a circuit. In general, for N flip-flops there are 2 N different states, each of which is present in the complete binary counting sequence beginning with all zeros, and up to all ones. Two toggle flip-flops can be connected so they will count this way, as shown at the left. LED’s are provided to show the states. Pulses to be counted are applied to the clock input of the first flip-flop, which toggles on every pulse.
Eatasheet can be continued for any number of flip-flops, each one of which will represent datawheet binary digit. If the delay dwtasheet the clock edge of a flip-flop to the change in the output is t dthen the Nth flip-flop changes a time N – 1 t d after the first.
The changes ripple down the line of flip-flops, and this type of counter is called a binary ripple counter. For the LS74, the delay time is somewhere around 25 ns, so a bit counter will require about ns to settle down. In some applications, this delay is of no consequence.
More troublesome is the occurrence of glitches with ripple counters. This refers to the intermediate states while the action is rippling down the line, which may trigger unwanted events.
For example, if an 8-bit ripple counter is atthe next clock produces, and so on before finally reachingthe next static state.
For this reason, it 74gc74an not usual to look at the intermediate bits of a ripple counter, and they are not satisfactory for clocks. To eliminate glitches as far as possible, all the flip-flops should change state at the same time. This is done by connecting all the clock inputs to one clock signal. Counters that work this way are called synchronous74nc74an everything happens at the effective clock edge.
In this case, logic must be used that produces the proper next state for each flip-flop, depending on the current state. This is a standard exercise in digital design, to texts of which the reader is referred for general methods.
Check this state by state to verify that it is correct. Synchronous binary counters require a lot of logic, and they are not made from discrete components, except as a school exercise. However, this one is easy to breadboard and test, and brings out the essentials. It is arranged so that the next clock when at 9 brings 74yc74an counter back to There is nothing strange at all in this. Any synchronous counter can be made to count in any sequence whatsoever through any of the possible states.
A decimal counter is simply one example. Indeed, many counters can be made to reset themselves when reaching a certain count, or even to load a 74c74an starting number at that point. Synchronous counters can even be made to count backwards datassheet forwards as desired.
They are very flexible sequence engines indeed. Another kind of synchronous counter eliminates glitches entirely.
74HC74A Datasheet, PDF – Qdatasheet
This is the Johnson counter that is based on a shift register of N stages. A shift register is a series of flip-flops with each output Q connected to the next input D, and the clock inputs connected together.
Any data presented to the first D input is shifted down the line of flip-flops one stage for each clock, and eventually appears at the output of the final flip-flop. In a Johnson counter, the final output is complemented inverted and applied to the first D input, making a closed ring.
There are 10 states, so this datssheet a decimal counter by nature, and the states can be assigned to the integers as shown.
Note that this is not the binary sequence! Should any of these appear by chance, then the counting sequence is destroyed, and the counter collapses. Therefore, 74hc74wn is necessary that a Johnson counter have a self-correction circuit that removes any erroneous states. The logic shown at the right, when used to feed the third flip-flop in a Johnson counter, eliminates bad states in a few clocks. For example, is eliminated at the next clock, requires two clocks, while needs four.
I do not know the source of this circuit, used in the series logic, but it is very ingenious. Textbooks seem to give useless methods of self-correction. The circuit is shown at the left. Note that Q is connected directly to D, so it is a shift register, and the feedback is correct. Here, the digit on the right corresponds to the flip-flop on the left in the diagram.
The states merely have to occur in order, so there is no significance in starting with We could have started with any other state, and proceeded in order. Note that the decoding is accomplished by detecting the “edge” between 1 and 0 or 0 and 1 with an AND gate, or with a NAND if an active low output is required.
Johnson counters are very satisfactory, but are given little mention in texts, and appeared only in the series logic family as the decimal and the octaland were ignored in TTL. They have been picked up in HC logic, and a variant HC has even been added. They are ideal for clocks, since the states are completely decoded and can be easily displayed. The usual way to implement decoded outputs is to use binary counters, and then decoder IC’s.
The LS or HC is a 3-to-8 decoder very useful in computers, the is a dual 2-to-4 decoder that could be used with the binary counters abovethe 41 a BCD-to decoder that gives high outputs for the unused codes A-F, and the a hex-to decoder in a pin DIP. All of these have active-low outputs, which is standard. A glance at the logic diagrams for these chips show that the decoding is not as simple as for a Johnson counter.
Incidentally, decoders are static logic, not sequential machines, and can be implemented with NAND gates and inverters. Again, this is a standard problem in digital design, but it is seldom worthwhile to make one’s own.
The three principal types of counters have been studied above, so datasheer you know their fundamentals, their advantages and their drawbacks. We can extend the study to the available MSI counter chips, and give some examples of their applications in practice, and of some of their peculiarities.
These experiments will follow here in due course. Although there daatsheet to be many counters available, there are really only a few principal types, and if these are understood, any counter is easy to use.