Shift Registers

Topics Introduction Buffer Register Classification of Register Shift Registers and their modes Bidirectional Shift Register Universal Shift Register

Introduction Register is an important application of Flip-Flop. Flip- Flop is a 1 bit memory cell which can be used for storing the digital data. To increase the storage capacity in terms of number of bits, we have to use a group of flip-flop. Such a groups of flip-flop is known as a “Register”. Thus register is a group of flip-flops. The “n-bit” register will consist of “n” number of flip flops.

Buffer Register Constructed by using D flip-flop. We can use any type of edge triggering. But here it is a positive triggered D flip-flop. Operation The data is to be stored is B3B2B1B0 = 1 0 1 0. These bits are connected to each D-Flip-flops. Then the clock is applied and corresponding output will be Q3Q2Q1Q0 = B3B2B1B0 = 1 0 1 0

Buffer Register 4 bits input B3 B2 B1 B0 Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 CLK Q3 Q2 Q1 Q0 Outputs

Operation of Buffer Register 4 bits input B3 B2 B1 B0 1 1 Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 FF-2 FF-1 FF-0 CLK 1 1 Q3 Q2 Q1 Q0 Outputs

Classification of Registers : Mode of operation Serial in serial out (SISO) Shift Right Shift Left Serial in Parallel out (SIPO) Parallel in Serial out (PISO) Parallel in Parallel out (PIPO)

Shift Register Comments Mode Illustrative Diagram 1. Sr. No. Mode Illustrative Diagram Comments 1. SISO (Shift Right) Data bits shift from Left to Right by 1 position per clock cycle. 2. SISO (Shift Left) Data bits shift from Right to Left by 1 position per clock cycle. 3. SIPO All o/p bits are made aval. simult. after 4-clcok pulse 4. PISO All i/p bits are applied simult and. After 4-clk pulse the required o/p is available serially. I/P FF3 FF2 FF1 FF0 O/P FF3 FF2 FF1 FF0 O/P I/P FF3 FF2 FF1 FF0 I/P O/P I/P FF3 FF2 FF1 FF0 O/P

Serial input Serial Output (Shift Left Mode) Din Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 FF-2 FF-1 FF-0 Serial Data Output CLK Serial Shift left register Before application of clock let assume all outputs are zero and apply MSB bit of the number to entered to Din. So Din = D0 = 1.

Serial input Serial Output (Shift Left Mode) Apply the clock . On the first falling edge of clock, the FF-0 is SET and the stored data in the register is Q3Q2Q1Q0 = 0 0 0 1 Din 1 1 Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 FF-2 FF-1 FF-0 CLK Serial Data Output

Serial input Serial Output (Shift Left Mode) Apply the NEXT bit to Din . So if Din =1. As soon as the next positive edge of the clock hits. FF- 1 will SET and the stored data changes to, Q3Q2Q1Q0 = 0 0 1 1 Din 1 1 1 Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 FF-2 FF-1 FF-0 CLK Serial Data Output

Serial input Serial Output (Shift Left Mode) Apply the NEXT bit to Din . So if Din =1. As soon as the next positive edge of the clock hits. FF- 2will SET and the stored data changes to, Q3Q2Q1Q0 = 0 1 1 1 Din 1 1 1 1 Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 FF-2 FF-1 FF-0 CLK Serial Data Output

Serial input Serial Output (Shift Left Mode) Apply the NEXT bit to Din . So if Din =1. As soon as the next positive edge of the clock hits. FF- 3will SET and the stored data changes to, Q3Q2Q1Q0 = 1 1 1 1 Din 1 1 1 1 1 Q3 D3 Q2 D2 Q1 D1 Q0 D0 FF-3 FF-2 FF-1 FF-0 CLK Serial Data Output

Summary of shift left operation CLK Q3 Q2=D3 Q1=D2 Q0=D1 Serial input Din = D0 1

Serial input Serial Output (Shift Right Mode) Q0 FF-0 D1 Q1 FF-1 D2 Q2 FF-2 D3 Q3 FF-3 Din CLK Serial Output

Serial input Parallel Output (SIPO) In this operation the data is entered serially and taken out in parallel. That means first the data is loaded bit by bit. The output are disabled as the loading is taking place. Number of clock cycles required to load a four bits data is 4. Hence the speed of operation of SIPO mode is same as that of SISO mode. D3 Q3 FF-3 D2 Q2 FF-2 D1 Q1 FF-1 D0 Q0 FF-0 Din CLK

Parallel input Parallel Output (PIPO) In this operation the data are entered parallel. The 4-bit binary input B0, B1, B2, B3 is applied to data inputs D0, D1, D2 and D3 respectively of the four flip-flops. As soon as a positive clock edge is applied, the input binary bits will be loaded into the flip-flops simultaneously. The loaded bits will appear simultaneously to the output side. ONLY ONE CLOCK IS ESSENTIAL TO LOAD ALL THE BITS. B0 B1 B2 B3 D0 Q0 D1 Q1 D2 Q2 D3 Q3 FF-0 FF-1 FF-2 FF-3 CLK Q2 Q3 Q0 Q1

Parallel input Serial Output (PISO) In this operation the data are entered parallel. Output of pervious FF is connected to the input of the next via a combinational circuit. The binary input data B0, B1, B2, B3 is applied through the same the combinational circuit. There are two modes in which this circuit can work namely shift mode or load mode.

D3 Q3 FF-3 D2 Q2 FF-2 D1 Q1 FF-1 D0 Q0 FF-0 B0 CLK B1 B2 B3 6 5 4 3 2 Shift/ Load 6 5 4 3 2 1 Serial Data O/P

4-bits Bidirectional Shift Register

D0 Q0 FF-0 D1 Q1 FF-1 D2 Q2 FF-2 D3 Q3 FF-3 CLK 8 7 6 5 4 3 2 1 MODE BIT M 8 7 6 5 4 3 2 1 Serial Shift Left Input DL Shift Right Input DR M=1 Shift Right M=0 Shift Left

4-bits Universal Shift Register Mode Control S1 S0 Register Operation No change 1 Shift Right Shift Left Parallel load

Applications of shift Register For Temporary data storage. For multiplication and division As a delay line Ring Counter Parallel to serial converter