In asynchronous counters we find the output of one stage connects directly to the clock input of the next stage.
This setup causes a problem called “Propagation Delay.” Here the timing signal is delayed slightly as it passes through each flip-flop.
In contrast synchronous counters have all their stages clocked together at the same time. We connect the external clock signal to the clock input of every flip-flop in the counter. This means all flip-flops are activated simultaneously creating a fixed time relationship.
With this synchronization all output bits change state at exactly the same moment in response to the clock signal. Theres no ripple effect and no propagation delay.
Understanding Binary 4-bit Synchronous Up Counter
We can see that the external clock pulses which we want to count, go directly to each J-K flip-flop in the counter chain. In this setup the J and K inputs are connected in toggle mode.
However only the first flip-flop called FFA (the least significant bit) has its J and K inputs set to HIGH, or logic “1.” This allows FFA to toggle with each incoming clock pulse. As a result the synchronous counter moves through a set sequence of states, advancing one state for every clock pulse it receives.
The J and K inputs of flip-flop FFB connect directly to the output QA of flip-flop FFA. For flip-flops FFC and FFD, their J and K inputs are connected to different AND gates.
These AND gates take signals from both the input and output of the previous stage. They create the necessary logic for the J-K inputs of the next stage.
By allowing each J-K flip-flop to toggle only when all preceding flip-flop outputs (Q) are HIGH we can achieve the same counting sequence as in an asynchronous circuit.
However this method removes the ripple effect since each flip-flop is clocked at the same time ensuring synchronized operation.
The maximum working frequency of this kind of frequency counter is substantially greater compared to the frequency of an equivalent asynchronous counter circuit as synchronous counters have no intrinsic propagation delay since each of the counter stages are activated simultaneously in parallel.
Timing Diagram for the 4-bit Synchronous Counter Waveform
Here we have a 4-bit synchronous counter that counts up. It increases its count with each clock pulse. The output goes from 0 (0000 in binary) to 15 (1111 in binary). And so we call this a 4-bit Synchronous Up Counter.
We can also make a 4-bit Synchronous Down Counter. To do this, we connect AND gates to the Q outputs of the flip-flops.
This setup creates a timing diagram that is the opposite of the up counter. The down counter starts with all outputs set to HIGH (1111 in binary) and counts down with each clock pulse until it reaches zero (0000). Then, it starts counting again.
Understand the Binary 4-bit Synchronous Down Counter
Now, we build synchronous counters by linking flip-flops together. We can connect any number of these flip-flops to create a “divide-by-n” binary counter. The concept of the modulo or “MOD” number applies here just like it does for asynchronous counters.
This means that we can design a Decade counter or a Binary-Coded Decimal (BCD) counter that counts from 0 to 2n-1 and also handle truncated sequences. To increase the MOD count of a synchronous counter we just add another flip-flop and an AND gate.
4-Bit Decade Synchronous Counter
We can create a 4-bit Decade Synchronous Counter using synchronous binary counters. This counter will count from 0 to 9.
By adding some extra logic we can change a standard binary counter into a decade counter which corresponds to the decimal value of 10.
When the counter reaches “1001” it resets and starts again at “0000” making it a decade or Modulo-10 counter.
Simplified Explanation of AND Gates and Synchronous Counters
We use additional AND gates to detect when the counting sequence reaches the binary value “1001” which is 9 in decimal.
When this count is reached then AND gates trigger flip-flop FF3 to change its state with the next clock pulse. Flip-flop FF0 on the other hand, toggles with every clock pulse it receives. This setup allows the count to reset and start again from “0000” making it a synchronous decade counter.
Modifying Counting Sequences
We can also rearrange the extra AND gates in this counter circuit to create different counting sequences like a Mod-12 counter.
This configuration lets the counter go through 12 states from “0000” to “1011” (representing decimal values 0 to 11) before starting over. This design is useful for applications like clocks.
Operation of Synchronous Counters
Synchronous counters use edge-triggered flip-flops that change states based on the clock pulse. They respond to either the “positive-edge” (low to high transition) or the “negative-edge” (high to low transition) of the clock signal.
This means that a count is registered every time theres a change in the clock input.
Typically the synchronous counters count on the rising edge of the clock signal, while asynchronous ripple counters count on the falling edge.
We might find it unusual that ripple counters use the falling edge of the clock cycle to change states.
However this design helps us connect multiple counters together. Specifically the most significant bit (MSB) of one counter can act as the clock input for the next counter.
This method works on the principle that the next bit needs to change its state when the previous bit goes from high to low.
This transition indicates when a carry should be passed to the next bit. In contrast synchronous counters have carry-out and carry-in pins. This feature allows us to link counters smoothly without any delays in propagation.
Conclusions
We can build synchronous counters using either Toggle or D-type flip-flops.
The design of synchronous counters is generally simpler than that of asynchronous counters.
The term “synchronous” comes from the fact that all flip-flops receive the same clock signal at the same time. This ensures that, all output states change at once.
Since the clock inputs are connected together, so there is no inherent propagation delay which is a significant advantage.
Synchronous counters are often called parallel counters because the clock signal is sent in parallel to all flip-flops.
They have a built-in memory circuit, that keeps track of the current state of the counter.
The counting sequence is controlled, using logic gates.
This design allows synchronous counters to operate potentially faster, than asynchronous counters.
References:
synchronous, presettable, 8-bit up/down counters
Maximum frequency of the synchronous counter
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