Shift Registers

This month I'll continue the digital electronics primer. I've received many positive responses to this feature, and I'm glad that so many of you are enjoying it. The topic this time is shift registers. Shift registers are the first digital component that we've looked at that is synchronous; that is, an externa! clocking signal ensures that the inputs and outputs are all synchronized together.

In general, shift registers are used to convert serial data to parallel and vice versa. For example, data sent on the packet network is obviously in parallel. Once inside your computer, it is converted to parallel for use on the computer's parallel bus. Sending from your computer reverses the process, requiring parallel data to be converted to serial.

Parallel Input/Output

Fig 1 is a logic symbol for a shift register. For serial-to-parallel conversion, the serial bits are input on the "serial IN" pin. One bit is input each time the clock pulses. Once four bits have been serially entered, they can be read in parallel from the outputs Q0-Q3. Clearly, there is no net gain or loss in speed. For parallel-to-serial conversion, the Q0-Q3 pins are inputs and the "PE" (parallel enable) pin is brought high to load the four bits. They are then clocked out in serial form with each clock pulse.

Shift registers have many applications, and there are several variations of the basic shift register architecture that I just described. You can also perform the serial-to-parallel and parallel-to-serial conversions in software when using a microprocessor.

0 0

Post a comment