This could lead to uncertain results, but the flip-flop will work normally once an input pulse is applied to either input. This should be avoided in normal operation, but is likely to happen when power is first applied. The indeterminate or uncertain logic state only occurs if the inputs change from 0,0 to 1,1 together. This is normally OK, as the outputs will be at the state remembered from the last input pulse. In row 6 both inputs are at logic 1 and the outputs are shown as ‘indeterminate’, this means that although Q and Q will be at opposite logic states it is not certain whether Q will be 1 or 0, Notice however that in the absence of any input pulses, both inputs are normally at logic 1. However, in row 5 both inputs are 0, which makes both Q and Q = 1, and as they are no longer opposite logic states, although this state is possible, in practical circuits it is ‘not allowed’. For conditions 1 to 4 in Table 5.2.1, Q is the inverse of Q. There are however, some problems with the operation of this most basic of flip-flop circuits. Other, more widely used types of flip-flop are the JK, the D type and T type, which are developments of the SR flip-flop and will be studied in Modules 5.3 and 5.4. it can be made to mimic any of the other standard logic functions, it is also cheaper to construct. This is because, as well as being universal, i.e. Flip-flops (or bi-stables) of different types can be made from logic gates and, as with other combinations of logic gates, the NAND and NOR gates are the most versatile, the NAND being most widely used. The SR flip-flop can be considered as a 1-bit memory, since it stores the input pulse even after it has passed. For this reason the circuit may also be called a Bi-stable Latch. When the circuit is triggered into either one of these states by a suitable input pulse, it will ‘remember’ that state until it is changed by a further input pulse, or until power is removed. Just two inter-connected logic gates make up the basic form of this circuit whose output has two stable output states. The basic building bock that makes computer memories possible, and is also used in many sequential logic circuits is the flip-flop or bi-stable circuit. Use software to simulate SR flip-flops.Recognise alternative forms of SR flip-flops.Construct timing diagrams to explain the operation of SR flip-flops.Compile truth tables for SR flip-flops.Recognize SR flip-flop integrated circuits.Recognize standard circuit symbols for SR flip-flops.Describe typical applications for SR flip-flops.Describe SR flip-flop circuits and can:.After studying this section, you should be able to:.
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