One-Bit Memory Cell

One–Bit Memory Cell

• Fig. 5.2.1 shows the basic bistable element used in latches and flip–flops. The basic bistable element has two outputs Q and . It has two cross–coupled inverters, i.e., the output of the first inverter is connected as an input to the second inverter and the output of second inverter is connected as an input to the first inverter.

• The basic bistable element circuit has two stable states logic 0 and logic 1, hence the name 'bistable'. To illustrate this, assume A = 0. When A = 0, the output of inverter 1 is 1 (Ā), i.e., Q = 1. 

Since the output of inverter 1 is the input to the inverter 2, Ā = B = 1. Consequently, the output of inverter 2, i.e.,  is 0. Since the output of the inverter 2 is connected to the input of the inverter 1,  =  = A = 0. We have assumed same value for A. Thus, the circuit is stable with  = A =  = 0 and Q Ā = B = 1. Using similar explanation it is easy to show that if it is assumed that A =1, the basic bistable element is stable with  = A =  = 1 and Q = Ā = B = 0. This is a second stable condition of the basic bistable element.

• The two stable states of basic bistable elements are used to store two binary elements, 0 and 1. In positive logic system, state Q = 1 is used to store logic 1, and state Q = 0 is used to store logic 0. It is important to note that the two outputs are complementary. That is when Q = 0,  = 1; and when  = 1,  = 0.

• From the above discussion we can note following things about the basic bistable element.

1. The outputs Q and  are always complementary.

2. The circuit has two stable states. The state corresponds to Q = 1 is referred to as 1 state or set state and state corresponds to Q = 0 is referred to as 0 state or Reset state.

3. If the circuit is in the set (1) state, it will remain in the set state and if the circuit is in the reset (0) state, it will remain in the reset state. This property of the circuit shows that it can store 1–bit of digital information. Therefore, the circuit is called a 1–bit memory cell.

4. The 1–bit information stored in the circuit is locked or latched in the circuit. Therefore, this circuit is also referred to as a latch.

Review Questions

1. Explain the operation of one–bit memory cell.

2. Draw the NOR gate latch and write its truth table.