Shift Registers

Shift registers are sequential logic circuits that are used to store and move data. They accept binary inputs from one serial or parallel source and then shift the data through a chain of flip-flops, one bit at time. There are several types of devices. Serial in/serial out (SISO) registers receive serial data from one input and send serial data from one output. Parallel in/serial out (PISO) registers receive parallel data from several inputs and send serial data from one output. Serial in/parallel out (SIPO) registers receive serial data from one input and send parallel data from several outputs. Parallel in/parallel out (PIPO) registers receive parallel data from several inputs and send parallel data from several outputs. Universal shift registers have selectable inputs that allow users to set the register to operate as a SISO, PISO, SIPO, or PIPO device. Some shift registers move the register data to the right so that each operation successively divides the binary number by two. Other shift registers move the register data to the left so that each operation successively multiplies the binary number by two. Bidirectional or reversible registers that can shift data to either the left or the right are also available.

Selecting shift registers requires an analysis of performance specifications. The number of bits is the maximum number of bits that a register can store at one time. The clock frequency is the highest rate in hertz (Hz) at which the shift register can shift data reliably. Propagation delay is the time delay between the occurrence of a change at the output and the application of a change at the inputs. Operating current and operating temperate are other important considerations. Power dissipation, the device's total power consumption, is generally expressed in watts (W) or milliwatts (mW). Supply voltages range from - 5 V to 5 V and include many intermediate voltages. 

Shift registers vary in terms of output characteristics and optional features. Open-collector outputs are connected internally to the collector for a bipolar transistor. Open-drain outputs are connected internally to the drain for a field effect transistor. Devices with three-state output buffers are also available. In terms of features, some shift registers are radiation tolerant or provide protection from electrostatic discharge (ESD). Others have buffered inputs or synchronous or asynchronous resets. Variable length registers that can be programmed to any number of bits between 1 and a maximum value are commonly available. 

Selecting shift registers requires an analysis of logic families. Transistor-transistor logic (TTL) and related technologies such as Fairchild advanced Schottky TTL (FAST) use transistors as digital switches. By contrast, emitter coupled logic (ECL) uses transistors to steer current through gates that compute logical functions. Another logic family, complementary metal-oxide semiconductor (CMOS), uses a combination of p-type and n-type metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. Other logic families for shift registers include cross-bar switch technology (CBT), gallium arsenide (GaAs), integrated injection logic (I²L), and Gunning with transceiver logic (GTL).

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