CHAPTER 7. DIGITAL CIRCUITS154for parallel use.ADCLOCKDCQDCQDCQFigure 7.28: A 3-bit shift register constructed with D ﬂip-ﬂops.If A is connected back to D the device is known as a circular shift register or ring counter.A circular shift register can be preloaded with a number and then used to provide a repeatedpattern at Q.7.13.3CountersThere are several diﬀerent ways of categorizing counters:1. binary-coded decimal (BCD) versus binary,2. one direction versus up/down and3. asynchronous ripple-through versus synchronous.Counters are also classiﬁed by their clearing and preloading abilities. The BCD type count isdecimal, and is most often used for displays. In the synchronous counter each clock pulse isfed simultaneously or synchronously to all ﬂip-ﬂops. For the ripple counter, the clock pulseis applied only to the ﬁrst ﬂip-ﬂop in the array and its output is the clock to the secondﬂip-ﬂop, etc.. The clock is said to ripple through the ﬂip-ﬂop array.Shown in ﬁgure 7.29 is a binary, ripple-through, up counter.COUNTCOUNT ENABLEJCKSRQQJCKSRQQJCKSRQQFigure 7.29: A 3-bit ripple counter constructed from JK ﬂip-ﬂops.Because of pulse delays, the counter will show a transient and incorrect result for short timeperiods. If the result is used to drive additional logic elements, these transient states maylead to a spurious pulse. This problem is avoided by the synchronous clocking scheme shownin ﬁgure 7.30. All output signals will change state at essentially the same time.