Introduction (Buffer and Controlled Buffer Register)

Digital Principles and Computer Organization:

Chapter 7: Sequential Circuits - Registers

Introduction

• We have seen that a flip-flop is nothing but a binary cell capable of storing one bit information and can be connected together to perform counting operations. Such a group of flip-flops is called counter. We have also seen that group of flip-flops can be used to store a word, which is called register.

• A flip-flop can store 1-bit information. So an n-bit register has a group of n flip-flops and is capable of storing any binary information/number containing n-bits.

1. Buffer Register

• Fig. 7.1.1 shows the simplest register constructed with four D flip-flops. This register is also called buffer register. Each D flip-flop is triggered with a common negative edge clock pulse. The input bits set up the flip-flops for loading. Therefore, when the first negative clock edge arrives, the stored binary information becomes,

Q_AQ_BQ_CQ_D = ABCD

• In this register, four D flip-flops are used. So it can store 4-bit binary information. Thus the number of flip-flop stages in a register determines its total storage capacity.

2. Controlled Buffer Register

We can control input and output of the register by connecting tri-state devices at the input and output sides of register as shown in Fig. 7.1.2. So this register is called 'controlled buffer register'.

Here, tri-state switches are used to control the operation. When you want to store data in the register, you have to make  or  signal low to activate the tri-state buffers. When you want the data at the output, you have to make  signal low to activate the buffers. Controlled buffer registers are commonly used for temporary storage of data within a digital system.

As seen above the 4-bit register can store 4-bit binary information. In general, n-bit register can store n-bit binary information.

Example: 1

Determine the number of flip–flops needed to construct a register capable of storing,

i) A 6–bit binary number

ii) Decimal numbers up to 32

iii) Hexadecimal numbers up to F

iv) Octal numbers up to 10.

Solution :

i) A 6–bit binary number requires register with 6 flip–flops.

ii) (32)₁₀ = (100000)₂. The number of bits required to represent 32 in binary are six, therefore, 6 flip–flops are needed to construct a register capable of storing 32 decimal.

iii) (F)₁₆ = (1111)₂. The number of bits required to represent (F)₁₆ in binary are four, therefore four flip–flops are needed to construct a register capable of storing (F)₁₆.

 iv) (10)₈ =(1000)₂. The number of bits required to represent (10)₈ in binary are four, therefore, four flip–flops are needed to construct a register capable of storing (10)₈.

Review Questions

1. What is register?

2. What is buffer register?

3. What do you mean by controlled buffer register?