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DIGITAL 2 : EKT 221 RTL : Microoperations on a Single Register

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Today’s Outline

- Multiplexer-based transfers
- Transforming Block Diagram to Detailed Logic
- Shift Registers
- Shift Registers with Parallel Loads
- Shift Registers with Parallel Loads and Hold

Microoperations on a Single Register

- Implement one or more microoperations with a single register as the destination of all primary results.
- The single register may also serve as a source of an operand for binary and unary operations.
- A simple technique using multiplexers for selection is introduced to allow multiple microoperations on a single register

Multiplexer-Based Transfers

- A register receives data transfers from more than 1 sources.
- A dedicated multiplexer is used to select the wanted input
- Example shows:
- If K1=1, R0 receives data from R1.
- If K1=0, R0 receives data from R2.

K2

K1

R2

n=4

S

R0

0

1

n=4

R1

2:1 MUX

n=4

Multiplexer-Based Transfers

- How do we represent this in RTL form?
- Written in if-then-else:

If (K1=1) then (R0 R1),

else if (K2=1) then (R0 R2).

- Written in RTL:

K2

K1

R2

n=4

S

R0

K1:R0 R1, K1K2:R0 R2

0

1

n=4

R1

2:1 MUX

n=4

Hardware connections from two source registers, R1 and R2, to one common destination register, R0.

Selection between R1 and R2 must be based on the control variables K1 and K2.

Analyse the diagram for input:

*n.c : no change

Multiplexer-Based TransfersK2

K1

R2

n=4

S

R0

0

1

n=4

R1

2:1 MUX

n=4

K1

Load

K2

K1

2 to 1 MUX

Load

D0

D1

D2

D3

Q0

Q1

Q2

Q3

R2

R2

S

Block Diagram

n=4

A0

A1

A2

A3

Load

S

R0

R0

0

Q0

Q1

Q2

Q3

Y0

Y1

Y2

Y3

Load

1

n=4

B0

B1

B2

B3

R1

2:1 MUX

CLK

n=4

Q0

Q1

Q2

Q3

D0

D1

D2

D3

D0

D1

D2

D3

R1

Detailed Logic

Transforming a Block Diagram into Detailed LogicShift Registers move data laterally within the register toward its MSB or LSB position

In the simplest case, the shift register is simply a set of D flip-flops connected in a row like this:

Shift Registers*CP: a common clock pulse input that activates the shift

Data input, In, is called a serial inputor the shift right input.

Data output, Out, is often called the serial output.

The vector (A, B, C, Out) is called the parallel output.

Shift RegistersParallel Output

Serial Output

Serial Input

Shift Registers

- T0 is the register state just before the first clock pulse occurs
- T1 is after the first pulse and before the second.
- Initially unknown states are denoted by “?”
- Complete the last three rows of the table

Shift Registers with Parallel Load

- The shift register shown earlier has no control input, thus data is always shifted on clock pulse.
- How to make the shift registers more controllable?
- E.g. shifts only on select positive clock edges.
- Shift operation can be controlled through D inputs of the FFs, rather than through the clock inputs CP.

By adding a mux between each shift register stage, data can be shifted or loaded

If SHIFT is LOW, A and B are replaced by the data on DA and DB lines, else data shifts right on each clock.

Shift Registers with Parallel LoadSerial Input

2 to 1 MUX

Dn

A0

A1

IN

Selector

SHIFT

Shift Registers with Parallel Loads and Hold

Function Table for the Register of Fig 7-10

- But what if we want to hold to the current data, meaning no shift or no loading of new data?
- The design must have 2 controls:
- For the SHIFT
- For the LOAD

We use an AND gate to disabled the Load input, so we mark with don’t care condition

In Register Transfer Language:

Shift : Q sl Q, Shift Load : Q D

4-bit SHIFT REGISTER WITH PARALLEL LOAD AND HOLD OPERATION

Control

inputs

- AND gates:
- Enables the Shift operation
- Enables the input data
- Restores the contents of reg. when no operation

1

1

1

1

2

2

2

2

3

3

3

3

Figure 7-10

M. Morris Mano

LOGIC AND COMPUTER DESIGN FUNDAMENTALS

Shift Registers with Parallel Loads and Hold

- S = 0, L = 0 :
- AND3 in each stage is enabled
- The output of each FF is applied to its own D input.
- A +ve transition of CLK restores the contents of reg.
- Output Qi is unchanged

1

2

3

Shift Registers with Parallel Loads and Hold

- S = 0, L = 1 :
- AND2 in each stage is enabled
- The input Di is applied to D input of corresponding FF.
- Next +ve transition of CLK transfers the parallel input data into reg.
- Output Qi = Di

1

2

3

Shift Registers with Parallel Loads and Hold

- S = 1 :
- AND1 in each stage is enabled

When +ve edge occurs on CLK:

- Data from serial input SI to be transferred to FF Q0,
- Output Q0 to be transferred to FF Q1,

…and so on down the line.

1

2

3

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