Sequential Circuits - Registers: Two Marks Important Questions and Answers

Digital Principles and Computer Organization:

Chapter 7: Sequential Circuits – Registers

Two Marks Questions with Answers

1. Define registers.

 Answer: A register is a group of flip–flops. So an n–bit register has a group of n flip–flops and is capable of storing any binary information / number containing n–bits.

2. Define shift registers.

 Answer: The binary information in a register can be moved from stage to stage within the register or into or out of the register upon application of clock pulses. This type of bit movement or shifting is essential for certain arithmetic and logic operations used in microprocessors. This gives rise to group of registers called shift registers.

3. What are the different types of shift type? or Classify the registers with respect to serial and parallel input output.

 Answer: There are five types. They are :

• Serial in serial out shift register

• Serial in parallel out shift register

• Parallel in serial out shift register

• Parallel in parallel out shift register

• Bidirectional shift register

4. What is the difference between serial and parallel transfer? What type of register is used in each case?

 Answer: When data is transferred one bit at a time, the process of transfer is known as serial transfer. When multiple bits of data are transferred at a time, the process is known as parallel transfer. For parallel transfer, we can use parallel in and parallel out register. For serial transfer we can use left shift or right shift register.

5. If a serial–in–serial–out shift register has N stages and if the clock frequency is f, what will be time delay between input and output?

 Answer: The time delay between input and output is T_D = N / f

6. How many flip–flops are needed to build an 8 bit shift register?

 Answer: 21 ≥  8       n = 3

7. What is parallel shifting ?

 Answer: In a shift register all the data are moved simultaneously and then the technique is called parallel shifting.

8. Define universal shift register.

 Answer: The register which has both shifts (right–shift and left–shift) and parallel load capabilities is referred to as universal shift register.

9. What are the applications of a shift register?

 Answer:

1. Delay line

2. Serial to parallel converter

3. Parallel to serial converter.

4. Pseudo–random binary sequence generator.

5. Sequence detector.

10. Draw the timing diagram of 4–bit ring counter.

11. What is a ring counter?

Fig. 7.6.1 shows the logic diagram for four–bit ring counter. As shown in the bale Fig. 7.6.1, the Q output of each stage is connected to the D input of the next stage and the output of last stage is fed back to the input of first stage. The  followed by  makes the output of first stage to '1' and remaining outputs are zero, i.e. QA is one and QB, QC,QD are zero.

The ring counter can be used for counting the number of pulses. The number of pulses counted is read by noting which flip–flop is in state 1. No decoding circuitry is required. Since there is one pulse at the output for each of the N clock pulses, this circuit is also referred to as a divide–by–N–counter or an N : 1 scalar. Ring counters can be instructed for any desired MOD number, that is MOD–N ring counter requires N flip–flops.

12. What are the uses of a ring counter?

 Answer:

i) Control section of a digital system.

ii) Controlling events, which occur in strict time sequence.

13. What is a cycle counter ?

 Answer: A cycle counter is a counter that outputs a stated number of counts and then stops.

14. Draw a 2–bit ripple counter and convert this into a 2–bit ring counter.

15. Design a 3 bit ring counter and find the mod of the designed counter.

16. What is Johnson counter?

In a Johnson counter, the Q output of each stage of flip–flop is connected to the D input of the next stage. The single exception is that the complement output of the last flip–flop is connected back to the D–input of the first flip–flop as shown in Fig. 7.7.1.