Asynchronous Counter

Transcription

1 Asynchronous Counter Contents: Asynchronous/Ripple Counter Propagation Delay in Ripple Counter MOD Number Synchronous/Parallel Counter Counters < 2^N MOD Number Presettable Counters Decade Counter and BCD Counter MOD 60 counter Asynchronous Down Counter Prepared By Mohammed Abdul kader Assistant Professor, EEE, IIUC

2 Asynchronous Counter/Ripple Counter In asynchronous counter each FF output drives the clock input of next FF. The types of arrangement is called an asynchronous counter because the FFs don t change state in exact synchronism with the applied clock pulses. Only 1 st FF responds to the input clock pulses, other FF gets clock from the output of previous FF. This type of counter is also often referred to as a ripple counter due to the way the FFs respond one after another in a kind of rippling effect. The term asynchronous counter and ripple counter used interchangeably. 2

3 4-bit asynchronous counter

4 MOD Number MOD number of a counter as defined as the number of states that the counter goes through in each complete cycle before it recycles back to its starting state. The MOD number can be increased simply by adding more FFs to the counter. Where N is the number of flip-flop. Frequency Division In the basic counter each FF provides an output waveform that is exactly half the frequency of the waveform at its CLK input. In any counter, the signal at the output of the last FF (i.e. the MSB) will have a frequency equal to the input clock frequency divided by MOD number of the counter. 4

5 Counters with MOD Number<2 N

6 Decade Counters and BCD Counters The MOD-10 counter is also referred to as a decade counter. In fact, a decade counter is any that has 10 distinct states, no matter what the sequence. A decade counter as shown in fig, which counts in sequence from 0000 (decimal 0) through 1001 (decimal 9), is also commonly called a BCD counter, because it uses only the 10 BCD code groups 0000, 0001,, 1000 and To reiterate, any MOD-10 counter is a decade counter and any decade counter that counts in binary from 0000 to 1001 is a BCD counter. 6

7 MOD-60 Counter Example: A MOD-60 counter was needed to divide the 60 Hz line frequency down to 1 Hz. Construct an appropriate MOD-60 counter. Solution: 2 6 = 64, So we need six FFs to design a MOD-60 counter. The counter is to be cleared when it reaches the count of 60 (111100). Thus, the output of FFs Q5, Q4, Q3 and Q2 must be connected to the NAND fate. The output of flip-flop Q5 will have a frequency of 1Hz. 7

8 All of the counters we have looked at thus far have counted upward from zero, that is, they were up counters. Asynchronous Down Counter It is relatively simple matter to construct asynchronous (ripple) down counters, which will count downward from a maximum count to zero. The sequence of a 3-bit down counter is given below. 8

9 Asynchronous Down Counter

10 Propagation Delay in ripple counter In a ripple counter, because of the inherent propagation delay (t pd ) of each FF, the second FF will not respond until a time t pd after the first FF receives an active clock transition. The third FF will not respond until a time equal to 2 X t pd after the clock transition and so on. In general, we can say that Nth FF cannot change states until a time equal to N X t pd after the clock transition occurs. Assuming, t pd =50 ns, input clock period T=1000ns Output of Flip-flop A toggles after 50 ns of the falling edge of clock, Similarly, output of flip-flop B toggles after 100 ns and output of flip-flop C toggles after 150 ns. In this situation the counter does operate properly in the sense that the FFs do eventually get to their correct states However situation worsens if the input pulses are applied at a much higher frequency. 10

11 Propagation Delay in ripple counter The waveform in fig show what happens if the input pulses occur once every 100ns. The condition C=1, B=A=0 (count of 100) never appears, because the input frequency is too high. Problems such as this can be avoided if the period between input pulses is made longer than the total propagation delay of the counter. That for proper counter operation we need. Where N is the number of FFs. Stated in terms of input-clock frequency, the maximum frequency that can be used is given by- 11

12 Propagation Delay in ripple counter Example: Suppose that a four-bit ripple counter is constructed using 74LS112 J-K flipflop. 74LS112 has low-to-high propagation delay t PLH = 16 ns and high-to-low propagation delay t PHL = 24 ns. Calculate f max. To calculate f max, we will assume the worst case, that is, we will use t pd = t PHL = 24 ns Clearly, as the number of FFs in the counter increases, the total propagation delay increases and f max decreases. Thus asynchronous counters are not useful at very high frequencies, especially for large number of bits. 12

13 Synchronous/Parallel Counter Synchronous/Parallel Counter 13

14 Synchronous/Parallel Counter

15 Synchronous down and up/down Counter

16 Presettable Counters

17 Note: Read detail circuit operation from book. Ref. Book: Digital systems principle and application by Ronald J Tocci 17