[工学]数字电子技术_英文版_chapter09.ppt

CHAPTER 9 Counters 第8章 Problems: 2，4，8，12，17，20，21，29，34 Synchronous and Asynchronous Counters同步和异步计数器 Flip-flops can be connected together to perform counting operations. Such a group of flip-flops is a counter. Counters are classified into two broad categories according to the way they are clocked: asynchronous and synchronous. The term asynchronous refers to events that do not have a fixed time relationship with each other and, generally, do not occur at the same time. In asynchronous counters, commonly called ripple counters, the first flip-flop is clocked by the external clock pulse and then each successive flip-flop is clocked by the output of the preceding flip-flop. So the flip-flops within an asynchronous counters do not change states at exactly the same time In synchronous counters, the clock input is connected to all of the flip-flops so that they are clocked simultaneously. A 2-Bit Asynchronous Binary Counter This circuit consists of two flip-flops. The CLK is applied to the clock input of the first flip-flop. The second flip-flop is triggered by the output of the first flip-flop. A 2-Bit Asynchronous Binary Counter This circuit is a 2-bit counter because it exhibits four different states (00, 01, 10, 11). In other words, it goes through a binary sequence (00, 01, 10, 11). It actually counts the number of clock pulses up to three, and on the fourth pulse it recycles to its original state. A 3-Bit Asynchronous Binary Counter A 3-bit counter goes through a binary sequence (000, 001, 010, 011, 100, 101, 110, 111). A 3-Bit Asynchronous Binary Counter A 4-Bit Asynchronous Binary Counter An Asynchronous Decade Counter The modulus of a counter is the number of unique states that the counter will sequence through. The maximum possible number of states (maximum modulus) of a counter is 2n, where n is the number of flip-flops in the counter. Counters can also be designed to have a number of states in their sequence that is less than 2n. The resulting seque