双语课件（第4章） 4. Flip-flops and counters 《数字设计基础（双语教学版）》Barry Wilknson 教学课件.ppt

* 4. Flip-flops and counters 4.1 Sequential circuits 4.2 Memory design using gates 4.3 Flip-flops 4.4 Registers 4.5 Counters 4.1 Sequential circuits 1. Concept of sequential circuit The logic circuits whose outputs depend not only on the present logic input values but also on previous logic input and output values are called sequential circuits. Counter is a common example of sequential circuit. 4.1 Sequential circuits （2）Asynchronous sequential circuit A sequential logic circuit that does not use a synchronizing clock signal is called asynchronous sequential circuit. 2. Classification of sequential circuit （1）Synchronous sequential circuit A sequential logic circuit whose output changes are initiated by an input clock signal and in which the outputs change upon the required clock signal transition is called a synchronous sequential circuit. 4.2 Memory design using gates A logic circuit can maintain a constant output value by the use of feedback whereby the output is connected to the inputs to reinforce the output value. Set-reset memory design ⅰ) R = 0, S = 1, Q = 0; ⅱ) R = 1, S = 0, Q = 1; From the circuit, we can get: Once the output has been forced to a 0 or a 1, the input can return to a 1 and the output will remain unchanged. 4.2 Memory design using gates It is often convenient to have both a true output Q and a complementary output Q which in normal operation of the memory circuit has the opposite logical value to Q. The memory design shown below is called a latch. The latch can store one binary value, but its outputs will change when one of the inputs changes to a 0. Disadvantage of latch: 4.2 Memory design using gates Sometimes we have active high inputs memory design. 4.3 Flip-flops Normally we want the output changes to be synchronized with a clock signal. Such memory designs are called flip-flops. A flip-flop can store a single bit by producing an output of a 0 or a 1 continuously until changed by conditions on the i