Synchronous “up/down” Counter

We can build a counter circuit with selectable between "up" and "down" count modes by having dual lines of AND gates detecting the appropriate bit conditions for an "up" and a "down" counting sequence, respectively, then use OR gates to combine the AND gate outputs to the J and K inputs of each succeeding flip-flop:

This circuit isn't as complex as it might first appear. The Up/Down control input line simply enables either the upper string or lower string of AND gates to pass the Q/Q' outputs to the succeeding stages of flip-flops. If the Up/Down control line is "high," the top AND gates become enabled, and the circuit functions exactly the same as the first ("up") synchronous counter circuit shown in this section. If the Up/Down control line is made "low," the bottom AND gates become enabled, and the circuit functions identically to the second ("down" counter) circuit shown in this section.

To illustrate, here is a diagram showing the circuit in the "up" counting mode (all disabled circuitry shown in grey rather than black):

 

Here, shown in the "down" counting mode, with the same grey coloring representing disabled circuitry:

Links:

• Synchronous Counters
• Different type of counters
• Synchronous “up” counter
• Synchronous “down” counter
• Application of “up/down” counter

Different type of synchronous counters

A synchronous counter, in contrast to an asynchronous counter, is one whose output bits change state simultaneously, with no ripple. The only way we can build such a counter circuit from J-K flip-flops is to connect all the clock inputs together, so that each and every flip-flop receives the exact same clock pulse at the exact same time.

In electronics, synchronous counters can be implemented quite easily using register-type circuits such as the flip-flop, and a wide variety of classifications exist:

• Binary counters
• Up/down counters
• Loadable counters
• BCD counters
• Ring counters
• Johnson counters

Binary counters

Binary counters are the simplest form of counters. An N-bit binary counter counts from 0 to (2^N - 1) and back to 0 again.

 

Links:

Different types of counters

Up/down counters

Loadable counters

BCD counters

Ring counters

Johnson counters

Why “synchronous”?

• The difference between asynchronous and synchronous counters.

In an asynchronous counter, an external event is used to directly SET or CLEAR a flip-flop when it occurs. In a synchronous counter however, the external event is used to produce a pulse that is synchronised with the internal clock. An example of an asynchronous counter is a ripple counter. Each flip-flop in the ripple counter is clocked by the output from the previous flip-flop. Only the first flip-flop is clocked by an external clock. Below is an example of a 4-bit ripple counter.

• Dangers of asynchronous counters.

Although the asynchronous counter is easier to implement, it is more "dangerous" than the synchronous counter. In a complex system, there are many state changes on each clock edge, and some IC's (integrated circuits) respond faster than others. If an external event is allowed to affect a system whenever it occurs, a small percentage of the time it will occur near a clock transition, after some IC's have responded, but before others have. This intermingling of transitions often causes erroneous operations. What is worse, these problems are difficult to test for and difficult to foresee because of the random time difference between the events.

 

Links:

• Why do we need counters?
• How are counters made?
• Synchronous Counter.
• Different type of counters

Why do we need counters ?

In a digital circuit, counters are used to do 3 main functions: timing, sequencing and counting.

A timing problem might require that a high-frequency pulse train, such as the output of a 10-MHz crystal oscillator, be divided to produce a pulse train of a much lower frequency, say 1 Hz. This application is required in a precision digital clock, where it is not possible to build a crystal oscillator whose natural frequency is 1 Hz.

A sequencing problem would arise if, for instance, it became necessary to apply power to various components of a large machine in a specific order. The starting of a rocket motor is an example where the energizing of fuel pumps, ignition, and possibly explosive bolts for staging must follow a critical order.

Measuring the flow of auto traffic on roadway is an application in which an event (the passage of a vehicle) must increment a tally. This can be done automatically with an electronic counter triggered by a photocell or road sensor. In this way, the total number of vehicles passing a certain point can be counted.

Links:

• How are counters made?
• Why “Synchronous”?
• Synchronous Counter
• Different type of counters

Synchronous Counter

The purpose of writing this is to collate information on Digital Synchronous Counters. Particular emphasis was placed on the following areas :

1. Types of Synchronous Counters and How they work

2. Fast Counter Techniques

3. Implementation of Counters :

4. Dedicated Hardware and Alternative Devices

According to the Oxford Encyclopædic Dictionary:

synchronous adj. existing or occurring at the same time.

counter n. an apparatus used for counting. || a person or thing that counts.

So a "synchronous counter" should mean "a person, thing or apparatus that counts at the same time" ?!?! Hmmm...

Let us take a look at the definition given by the IBM Dictionary of Computing instead.

synchronous (1) Pertaining to two or more processes that depend upon the occurrence of specific events such as common timing signals. (2) Occurring with a regular or predictable time relationship.

counter (1) A functional unit with a finite number of states each of which represents a number that can be, upon receipt of an appropriate signal, increased by unity or by a given constant. This device is usually capable of bringing the represented number to a specified value; for example zero.

So a "synchronous counter" is actually a functional unit with a certain number of states, each representing a number which can be increaced or decreased upon receiving an appropriate signal (e.g. a rising edge pulse), and is usually used to count to, or count down to zero from, a specified number N.

... and what it "really" means. OK! Enough of dictionary definitions.

Basically, any sequential circuit that goes through a prescribed sequence of states upon the application of input pulses is called a counter. The input pulses, called count pulses, may be clock pulses or they may originate from an external source and may occur at prescribed intervals of time or at random. The sequence of states in a counter may follow a binary count or any other sequence.

 

Read More

• Why do we need counters?
• How are counters made?
• Why “Synchronous”?
• Different type of counters

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