1、Flip-flopsChapter4nCounting is a fundamental function of digital circuits.nA digital counter consists of a collection of flip-flops that change states (set or reset) in a prescribed sequence. nAsynchronous counternThe output of one flip-flop provides the clock for the next flip-flops.nWhen all of th

2、e flip-flop outputs are available, these are called modulo-n counters where n represent the terminal counter.KJQ1Q1CCLKKJQ2Q2C11VCCCLKQ1Q2001001110010KJQ1Q1CCLKKJQ2Q2C11VCCCLKQ1Q2001101100011nThe output of one flip-flop directly connects to the input of another.DQ1Q1CDQ2Q2CDQ3Q3CDQ4Q4CCLKD1Q1Q1CD2Q2

3、Q2CD3Q3Q3CD4Q4Q4CCLK1 0 0 0 nBefore the first clock pulse(Q1,Q2,Q3,Q4)=(0,0,0,0)(D1,D2,D3,D4)=(1,0,0,0)0 0 0 0 D1Q1Q1CD2Q2Q2CD3Q3Q3CD4Q4Q4CCLK1 1 0 0 nAfter the first clock pulse(Q1,Q2,Q3,Q4)=(1,0,0,0)(D1,D2,D3,D4)=(1,1,0,0)1 0 0 0 1100D1Q1Q1CD2Q2Q2CD3Q3Q3CD4Q4Q4CCLKCLKQ1Q3Q4Q2ttnTime t, CLK=1, Q1Q2

4、Q3Q4=1100D1=Q4=1, D2=Q1=1, D3=Q2=1, D4=Q3=0nTime t, CLK=0, Qn+1=D, Q1Q2Q3Q4=111011101 0 0 0 nThe Johnson counter can produce a series of outputs from each flip-flop that is offset by a clock pulse from the preceding flip-flop output.nThe Johnson counter can be used to produce time delay; the output

5、of each flip-flop is delayed by one clock pulse from the previous output.DQ1Q1CDQ2Q2CDQ3Q3CDQ4Q4CCLKnThe output of one flip-flop directly connects to the input of another.nThe ability to load the flip-flops to a particular state permits a repeatable output pattern. nThe counter is initialized to the

6、 appropriate data pattern, using the flip-flops asynchronous inputs.nEach flip-flop has separate preset and clear inputs .nFeeding back the most significant output to the least significant input, Data are shifted from one flip-flop to the next on each negative/positive edge of the clock pulse.nThe preloaded pattern repeats every n clock pulse. n is the length of data pattern.DQ1Q1CDQ2Q2CDQ3Q3CDQ4Q4CCLKCLKQ1Q3Q4Q2ttTime t, CLK=1, Q4