Digital Circuits

Shift Registers

22 questions By Tony R. Kuphaldt

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Question 13 of 22

Explain what a universal shift register is. The 74194 is an example of a TTL universal shift register, so you will find that datasheet very helpful in answering this question.

Reveal answer

A universal shift register has the ability to input data in either serial or parallel form, as well as output data in either serial or parallel form.

Note: this answer is purposely minimal. I expect you to provide a more detailed answer during discussion, based on research of universal shift registers!

Notes:
Regarding the point in the answer about student research, ask your students how a single shift register chip is able to perform all the different types of input/output combinations. Ask your students, for instance, how to make the 74194 act only as a parallel-in/parallel-out register.
Question 14 of 22

An important function in computer circuitry is serial-to-parallel data conversion, where a stream of serial data is “read” one bit at a time, then all bits output at once in parallel form. A shift register circuit is ideal for this application. Shown here is an eight-bit shift register circuit:

Draw any necessary wires and labels showing where serial data would enter the circuit, and where parallel data would exit.

Reveal answer

Follow-up question: if we were to actually use this circuit for serial-to-parallel data conversion, we would have to be careful how fast we clocked the shift register. Explain why.

Notes:
The subject of serial-to-parallel data conversion is much deeper than what is suggested by this disarmingly simple circuit. Talk with your students about the need for clock synchronization (even in “asynchronous” serial data transmission).
Question 15 of 22

Suppose we wished to use a shift register circuit to input several binary bits at once (parallel data transfer), and then output the bits one at a time over a single line (serial data transfer). You should be aware of how shift registers are constructed with D-type flip-flops. Now, describe how we can get parallel data entered into a shift register circuit. Note: there is more than one answer to this question!

Reveal answer

Perhaps the most direct way to provide parallel data entry is to make use of the flip-flops’ asynchronous inputs.

Notes:
During discussion, have your students draw a picture of a parallel-in/serial-out shift register circuit, or at least cite a page number reference in their textbook, so you may be sure they understand what they’re talking about (and not just repeating the given answer).