The shift register, which allows serial input and produces serial output is known as Serial In – Serial Out (SISO) shift register. The block diagram of 3-bit SISO. Shift register. Shift registers can have both parallel and serial inputs and outputs. These are often configured as 'serial-in, parallel-out' (SIPO) or as 'parallel-in, serial-out' (PISO). There are also types that have both serial and parallel input and types with serial and parallel output.
In electronics, a collection of flip-flops, which are memory elements, is known as a register. Shift registers are special types of registers. Using shift registers we can shift data through a series of flip-flops. In brief, shift registers are sequential logic circuits, where a series of flip-flops are connected together in a daisy chain configuration to shift digital data from one flip-flop to another with every clock cycle.
Shift registers are built using D flip-flops. If we connect four flip-flops in the configuration of a shift register, we get a 4-bit shift register. Generally, 8-bit (1 byte) shift registers are common. However, in this post, we will take a look at the different types of shift registers using only 4-bits or four flip-flops. How do shift registers move data?
We can feed and extract data to and from a shift register in two ways. Serially: Data enters the cascade of flip-flops in a stream. Each bit passes through the cascade in a line. We get the data output at the last flip-flop. The output is in the same order as the input. Parallel: Each flip-flop can have its own input. This particular setting of giving input is known as parallel input.
Similarly, each flip-flop can have its own output too. This is parallel output. So we have two ways in which data can ‘flow’ through a shift register. We can classify shift registers depending on these two data flow methods. Doing that, we get four main configurations. The types of inputs and outputs of these four categories are evident from their names. Serial In Serial Out shift register.
Serial In Parallel Out shift register. Parallel In Parallel Out shift register. Parallel In Serial Out shift register So to quickly summarize a few things. Shift registers are a series of flip-flops connected together through which data shifts. They have four main types.
The difference between these four types lies in the way we input and output data to/from them. Apart from inputs and outputs, shift registers also have a clock input and a reset signal. The reset signal clears the contents of all the flip-flops. Additionally, since the clock input is given to all the flip-flops simultaneously, shift registers are also synchronous circuits. What are the uses of shift registers? Since a shift register comprises of flip-flops, we can use them for the following general purposes.
To shift data – Shift registers can shift data either to the right, to the left or in both directions. In this post, we will look at shift registers where the data moves in the right direction. To store data – The flip-flops shift data on the application of a clock pulse. In the absence of a clock pulse, the shift register holds that data.
To produce a delay – The data can stay inside the shift register or pass through it. Either way, the processes consume some clock cycles. So we can use them to introduce some delay if we need it. To convert between serial-parallel – Since we have both serial and parallel types of inputs and outputs, we can use shift registers to convert serial data to parallel or vice versa. How to design a 4-bit Serial In Serial Out shift register (SISO)? Okay, so we know that we have 4 D flip-flops. All of them have the same clock input.
And all of them have the same reset input. Since we have a serial input, we will take only one input port. This input port is to the first flip-flop in the register. Similarly, since we are taking the output serially. We will have only one output which will be taken at the output pin of the last flip-flop. From the above configuration, our SISO shift register will look like this.
How to design a 4-bit Serial In Parallel Out shift register (SIPO)? Let’s take the four D flip-flops and take outputs from each individual flip-flop.
That covers the parallel out part. Give a single input to the first flip-flop. Similarly, take a single output from the last flip-flop. Connect all the remaining flip-flops’ outputs to their subsequent flip-flops’ input. That settles the serial input part. Finally, apply the clock and the reset signal and voila you have your 4-bit SIPO shift register.
4-bit SIPO shift register How to design a 4-bit Parallel in Parallel Out shift register (PIPO)? Again repeating the same approach we saw earlier, we will go ahead with cues from the title. Let’s straightaway connect the output ports (Q ports) of the D flip-flops to the output pins. The name suggests that we have parallel inputs too. Hence, we will connect the input ports (D ports) of the flip-flops to the input pins.
Eventually, we will connect the clock ports to the clock input and the reset ports to the reset inputs. The resulting logic circuit for the 4-bit PIPO shift register is as follows.
4-bit PIPO shift register How to design a 4-bit Parallel in Serial Out shift register (PISO)? Things get a little bit tricky here. Mostly due to the fact that we need to share ports in order to achieve this configuration.
The title requires us to have parallel inputs, which means that all the D ports will have their own independent input lines. And the title also requires us to have serial outputs. From the preceding designs, we have seen that a serial output means that data has to be taken out from only the final flip-flop. Moreover, the data needs to move through the series of flip-flops.
This means that the outputs of flip-flops (Q ports) will also need to be connected to the inputs (D ports) of their subsequent flip-flops. This puts us in a pickle. Dealing with multiple inputs The input ports need their separate input line as well as a connection from the previous flip-flops. How can we give two distinct signals to the same port?
What can we use that would allow us to select from one of the two inputs that need to be given to the flip-flops? The answer is. A 2:1 multiplexer will allow us to choose between two inputs. So moving on to the construction of the PISO shift register. To begin with, let’s connect all the clock inputs and reset signals to their respective inputs. Then we know that we have a serial output so let’s connect the output of the last flip-flop to the output pin. The first flip-flop also does not need a serial input because there is no preceding flip-flop.
So let’s give it its solitary input port. The remaining three flip-flops will have their inputs connected to the outputs of three multiplexers. Let’s go ahead and make that connection. Now, the multiplexers have two inputs. The first is the independent input that we require because we are making a parallel input shift register. Second, the output of the preceding flip-flop since we need a serial shift of data to get a serial output.
The multiplexers also need a select input so that will be one extra input port. From the above configurations, we finally get a logic circuit for the 4-bit PISO shift register that looks like this. Generally shift registers are available in 4000 series and 7000 series ICs. 4000 series.
IC 4006 18 stage Shift register. IC 4014 8-stage shift register.
IC 4015 Dual 4-stage shift register. IC 4021 8-bit static shift register. IC 40104 4 bit bidirectional Parallel-in/Parallel-out PIPO Shift Register.
IC 40195 4-bit universal shift register. 7000 series.
IC 7491 8-bit shift register, serial in, serial out, gated input. IC 7495 4-bit shift register, parallel in, parallel out, serial input. IC 7496 5-bit parallel-In/parallel-out shift register, asynchronous preset. IC 7499 4-bit bidirectional universal shift register. IC 74164 8-bit parallel-out serial shift register with asynchronous. IC 74165 8-bit serial shift register, parallel Load, complementary outputs. IC 74166 parallel-Load 8-bit shift register.
IC 74194 4-bit bidirectional universal shift register. IC 74198 8-bit bidirectional universal shift register. IC 74199 8-bit bidirectional universal shift register with J-Not-K serial inputs. IC 74291 4-bit universal shift register, binary up/down counter, synchronous. IC 74395 4-bit universal shift register with three-state outputs.
IC 74498 8-bit bidirectional shift register with parallel inputs and three-state outputs. IC 74671 4-bit bidirectional shift register. IC 74673 16-bit serial-in serial-out shift register with output storage registers. IC 74674 16-bit parallel-in serial-out shift register with three-state outputs. In these ICs, mostly used are. 74HC595 Serial-In-Parallel-Out shift register. 74HC165 Parallel-In-Serial-Out shift register.
74HC 194 4-bit bidirectional universal shift register. 74HC 198 8-bit bidirectional universal shift register.
A Flip flops can be used to store a single bit of binary data (1or 0). However in order to store multiple bits of data we need multiple flip flops. N flip flops are to be connected in an order to store n bits of data. A Register is a device which is used to store such information. It is a group of flip flops connected in series used to store multiple bits of data.
The information stored within these registers can be transferred with the help of shift registers. Shift Register is a group of flip flops used to store multiple bits of data. The bits stored in such registers can be made to move within the registers and in/out of the registers by applying clock pulses.
An n-bit shift register can be formed by connecting n flip-flops where each flip flop stores a single bit of data. The registers which will shift the bits to left are called “Shift left registers”. The registers which will shift the bits to right are called “Shift right registers”. Shift registers are basically of 4 types. These 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 Serial-In Serial-Out Shift Register (SISO) – The shift register, which allows serial input (one bit after the other through a single data line) and produces a serial output is known as Serial-In Serial-Out shift register.
Since there is only one output, the data leaves the shift register one bit at a time in a serial pattern, thus the name Serial-In Serial-Out Shift Register. The logic circuit given below shows a serial-in serial-out shift register. The circuit consists of four D flip-flops which are connected in a serial manner. All these flip-flops are synchronous with each other since the same clock signal is applied to each flip flop. The above circuit is an example of shift right register, taking the serial data input from the left side of the flip flop. The main use of a SISO is to act as a delay element. Serial-In Parallel-Out shift Register (SIPO) – The shift register, which allows serial input (one bit after the other through a single data line) and produces a parallel output is known as Serial-In Parallel-Out shift register.
The logic circuit given below shows a serial-in parallel-out shift register. The circuit consists of four D flip-flops which are connected. The clear (CLR) signal is connected in addition to clock signal to all the 4 flip flops in order to RESET them.
The output of the first flip flop is connected to the input of the next flip flop and so on. All these flip-flops are synchronous with each other since the same clock signal is applied to each flip flop. The above circuit is an example of shift right register, taking the serial data input from the left side of the flip flop and producing a parallel output. They are used in communication lines where demultiplexing of a data line into several parallel line is required because the main use of SIPO register is to convert serial data into parallel data. Parallel-In Serial-Out Shift Register (PISO) – The shift register, which allows parallel input (data is given separately to each flip flop and in a simultaneous manner) and produces a serial output is known as Parallel-In Serial-Out shift register. The logic circuit given below shows a parallel-in serial-out shift register. The circuit consists of four D flip-flops which are connected.
The clock input is directly connected to all the flip flops but the input data is connected individually to each flip flop through a multiplexer at input of every flip flop. The output of the previous flip flop and parallel data input are connected to the input of the MUX and the output of MUX is connected to the next flip flop. All these flip-flops are synchronous with each other since the same clock signal is applied to each flip flop. A Parallel in Serial out (PISO) shift register us used to convert parallel data to serial data.
Parallel-In Parallel-Out Shift Register (PIPO) – The shift register, which allows parallel input (data is given separately to each flip flop and in a simultaneous manner) and also produces a parallel output is known as Parallel-In parallel-Out shift register. The logic circuit given below shows a parallel-in parallel-out shift register.
The circuit consists of four D flip-flops which are connected. The clear (CLR) signal and clock signals are connected to all the 4 flip flops.
In this type of register there are no interconnections between the individual flip-flops since no serial shifting of the data is required. Data is given as input separately for each flip flop and in the same way, output also collected individually from each flip flop. A Parallel in Parallel out (PIPO) shift register is used as a temporary storage device and like SISO Shift register it acts as a delay element.
Bidirectional Shift Register – If we shift a binary number to the left by one position, it is equivalent to multiplying the number by 2 and if we shift a binary number to the right by one position, it is equivalent to dividing the number by 2.To perform these operations we need a register which can shift the data in either direction. Bidirectional shift registers are the registers which are capable of shifting the data either right or left depending on the mode selected.
If the mode selected is 1(high), the data will be shifted towards the right direction and if the mode selected is 0(low), the data will be shifted towards the left direction. The logic circuit given below shows a Bidirectional shift register. The circuit consists of four D flip-flops which are connected. The input data is connected at two ends of the circuit and depending on the mode selected only one and gate is in the active state. Shift Register Counter – Shift Register Counters are the shift registers in which the outputs are connected back to the inputs in order to produce particular sequences. These are basically of two types:.
Ring Counter – A ring counter is basically a shift register counter in which the output of the first flip flop is connected to the next flip flop and so on and the output of the last flip flop is again fed back to the input of the first flip flop, thus the name ring counter. The data pattern within the shift register will circulate as long as clock pulses are applied.
The logic circuit given below shows a Ring Counter. The circuit consists of four D flip-flops which are connected. Since the circuit consists of four flip flops the data pattern will repeat after every four clock pulses as shown in the truth table below: A Ring counter is generally used because it is self-decoding.
No extra decoding circuit is needed to determine what state the counter is in. Johnson Counter – A Johnson counter is basically a shift register counter in which the output of the first flip flop is connected to the next flip flop and so on and the inverted output of the last flip flop is again fed back to the input of the first flip flop. They are also known as twisted ring counters. The logic circuit given below shows a Johnson Counter. The circuit consists of four D flip-flops which are connected.
An n-stage Johnson counter yields a count sequence of 2n different states, thus also known as mod-2n counter. Since the circuit consists of four flip flops the data pattern will repeat every eight clock pulses as shown in the truth table below: The main advantage of Johnson counter is that it only needs n number of flip-flops compared to the ring counter to circulate a given data to generate a sequence of 2n states. Applications of shift Registers –. The shift registers are used for temporary data storage. The shift registers are also used for data transfer and data manipulation. The serial-in serial-out and parallel-in parallel-out shift registers are used to produce time delay to digital circuits.
The serial-in parallel-out shift register is used to convert serial data into parallel data thus they are used in communication lines where demultiplexing of a data line into several parallel line is required. A Parallel in Serial out shift register us used to convert parallel data to serial data.