Serial input - when shift enable input is assertet, the new content of the. Parallel output - the current content of the shift register is sent to the parallel output. Also, it can be easily seen from the VHDL code that signal “Q” is of stdlogicvector. Serial in Serial Out, Serial in Parallel Out, Bi-Directional Shift Registers - Digital Electronics - Duration: 30:11. Physicsanddigitalelectronics 151,536 views 30:11.
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We know that one flip-flop can store one-bit of information. In order to store multiple bits of information, we require multiple flip-flops. The group of flip-flops, which are used to hold (store) the binary data is known as register.
If the register is capable of shifting bits either towards right hand side or towards left hand side is known as shift register. An ‘N’ bit shift register contains ‘N’ flip-flops. Following are the four types of shift registers based on applying inputs and accessing of outputs.
- 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 (SISO) Shift Register
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 is shown in the following figure.
This block diagram consists of three D flip-flops, which are cascaded. That means, output of one D flip-flop is connected as the input of next D flip-flop. All these flip-flops are synchronous with each other since, the same clock signal is applied to each one.
In this shift register, we can send the bits serially from the input of left most D flip-flop. Hence, this input is also called as serial input. For every positive edge triggering of clock signal, the data shifts from one stage to the next. So, we can receive the bits serially from the output of right most D flip-flop. Hence, this output is also called as serial output.
Example
Let us see the working of 3-bit SISO shift register by sending the binary information “011” from LSB to MSB serially at the input.
Assume, initial status of the D flip-flops from leftmost to rightmost is $Q_{2}Q_{1}Q_{0}=000$. We can understand the working of 3-bit SISO shift register from the following table.
No of positive edge of Clock | Serial Input | Q2 | Q1 | Q0 |
---|---|---|---|---|
0 | - | 0 | 0 | 0 |
1 | 1(LSB) | 1 | 0 | 0 |
2 | 1 | 1 | 1 | 0 |
3 | 0(MSB) | 0 | 1 | 1(LSB) |
4 | - | - | 0 | 1 |
5 | - | - | - | 0(MSB) |
The initial status of the D flip-flops in the absence of clock signal is $Q_{2}Q_{1}Q_{0}=000$. Here, the serial output is coming from $Q_{0}$. So, the LSB (1) is received at 3rd positive edge of clock and the MSB (0) is received at 5th positive edge of clock.
Therefore, the 3-bit SISO shift register requires five clock pulses in order to produce the valid output. Similarly, the N-bit SISO shift register requires 2N-1 clock pulses in order to shift ‘N’ bit information.
Serial In - Parallel Out (SIPO) Shift Register
The shift register, which allows serial input and produces parallel output is known as Serial In – Parallel Out (SIPO) shift register. The block diagram of 3-bit SIPO shift register is shown in the following figure.
This circuit consists of three D flip-flops, which are cascaded. That means, output of one D flip-flop is connected as the input of next D flip-flop. All these flip-flops are synchronous with each other since, the same clock signal is applied to each one.
In this shift register, we can send the bits serially from the input of left most D flip-flop. Hence, this input is also called as serial input. For every positive edge triggering of clock signal, the data shifts from one stage to the next. In this case, we can access the outputs of each D flip-flop in parallel. So, we will get parallel outputs from this shift register.
Example
Let us see the working of 3-bit SIPO shift register by sending the binary information “011” from LSB to MSB serially at the input.
Assume, initial status of the D flip-flops from leftmost to rightmost is $Q_{2}Q_{1}Q_{0}=000$. Here, $Q_{2}$ & $Q_{0}$ are MSB & LSB respectively. We can understand the working of 3-bit SIPO shift register from the following table.
No of positive edge of Clock | Serial Input | Q2(MSB) | Q1 | Q0(LSB) |
---|---|---|---|---|
0 | - | 0 | 0 | 0 |
1 | 1(LSB) | 1 | 0 | 0 |
2 | 1 | 1 | 1 | 0 |
3 | 0(MSB) | 0 | 1 | 1 |
The initial status of the D flip-flops in the absence of clock signal is $Q_{2}Q_{1}Q_{0}=000$. The binary information “011” is obtained in parallel at the outputs of D flip-flops for third positive edge of clock.
So, the 3-bit SIPO shift register requires three clock pulses in order to produce the valid output. Similarly, the N-bit SIPO shift register requires N clock pulses in order to shift ‘N’ bit information.
Parallel In − Serial Out (PISO) Shift Register
The shift register, which allows parallel input and produces serial output is known as Parallel In − Serial Out (PISO) shift register. The block diagram of 3-bit PISO shift register is shown in the following figure.
This circuit consists of three D flip-flops, which are cascaded. That means, output of one D flip-flop is connected as the input of next D flip-flop. All these flip-flops are synchronous with each other since, the same clock signal is applied to each one.
In this shift register, we can apply the parallel inputs to each D flip-flop by making Preset Enable to 1. For every positive edge triggering of clock signal, the data shifts from one stage to the next. So, we will get the serial output from the right most D flip-flop.
Example
Let us see the working of 3-bit PISO shift register by applying the binary information “011” in parallel through preset inputs.
Since the preset inputs are applied before positive edge of Clock, the initial status of the D flip-flops from leftmost to rightmost will be $Q_{2}Q_{1}Q_{0}=011$. We can understand the working of 3-bit PISO shift register from the following table.
No of positive edge of Clock | Q2 | Q1 | Q0 |
---|---|---|---|
0 | 0 | 1 | 1(LSB) |
1 | - | 0 | 1 |
2 | - | - | 0(LSB) |
Here, the serial output is coming from $Q_{0}$. So, the LSB (1) is received before applying positive edge of clock and the MSB (0) is received at 2nd positive edge of clock.
Therefore, the 3-bit PISO shift register requires two clock pulses in order to produce the valid output. Similarly, the N-bit PISO shift register requires N-1 clock pulses in order to shift ‘N’ bit information.
Parallel In - Parallel Out (PIPO) Shift Register
The shift register, which allows parallel input and produces parallel output is known as Parallel In − Parallel Out (PIPO) shift register. The block diagram of 3-bit PIPO shift register is shown in the following figure.
This circuit consists of three D flip-flops, which are cascaded. That means, output of one D flip-flop is connected as the input of next D flip-flop. All these flip-flops are synchronous with each other since, the same clock signal is applied to each one.
In this shift register, we can apply the parallel inputs to each D flip-flop by making Preset Enable to 1. We can apply the parallel inputs through preset or clear. These two are asynchronous inputs. That means, the flip-flops produce the corresponding outputs, based on the values of asynchronous inputs. In this case, the effect of outputs is independent of clock transition. So, we will get the parallel outputs from each D flip-flop.
Example
Let us see the working of 3-bit PIPO shift register by applying the binary information “011” in parallel through preset inputs.
Since the preset inputs are applied before positive edge of Clock, the initial status of the D flip-flops from leftmost to rightmost will be $Q_{2}Q_{1}Q_{0}=011$. So, the binary information “011” is obtained in parallel at the outputs of D flip-flops before applying positive edge of clock.
Therefore, the 3-bit PIPO shift register requires zero clock pulses in order to produce the valid output. Similarly, the N-bit PIPO shift register doesn’t require any clock pulse in order to shift ‘N’ bit information.
In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, in which the output of each flip-flop is connected to the 'data' input of the next flip-flop in the chain, resulting in a circuit that shifts by one position the 'bit array' stored in it, 'shifting in' the data present at its input and 'shifting out' the last bit in the array, at each transition of the clock input.
More generally, a shift register may be multidimensional, such that its 'data in' and stage outputs are themselves bit arrays; this is implemented simply by running several shift registers of the same bit-length in parallel.
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. There are also 'bidirectional' shift registers which allow shifting in both directions: L→R or R→L. The serial input and last output of a shift register can also be connected to create a 'circular shift register'. A PIPO register (parallel in, parallel out) is very fast – an output is given within a single clock pulse.
- 1Serial-in serial-out (SISO)
Serial-in serial-out (SISO)[edit]
Destructive readout[edit]
Time | Output 2 | Output 4 | ||
---|---|---|---|---|
0 | 0 | 0 | 0 | 0 |
1 | 1 | 0 | 0 | 0 |
2 | 0 | 1 | 0 | 0 |
3 | 1 | 0 | 1 | 0 |
4 | 1 | 1 | 0 | 1 |
5 | 0 | 1 | 1 | 0 |
6 | 0 | 0 | 1 | 1 |
7 | 0 | 0 | 0 | 1 |
8 | 0 | 0 | 0 | 0 |
These are the simplest kind of shift registers. The data string is presented at 'Data In', and is shifted right one stage each time 'Data Advance' is brought high. At each advance, the bit on the far left (i.e. 'Data In') is shifted into the first flip-flop's output. The bit on the far right (i.e. 'Data Out') is shifted out and lost.
The data is stored after each flip-flop on the 'Q' output, so there are four storage 'slots' available in this arrangement, hence it is a 4-bit Register. To give an idea of the shifting pattern, imagine that the register holds 0000 (so all storage slots are empty). As 'Data In' presents 1,0,1,1,0,0,0,0 (in that order, with a pulse at 'Data Advance' each time—this is called clocking or strobing) to the register, this is the result. The right hand column corresponds to the right-most flip-flop's output pin, and so on.
So the serial output of the entire register is 00001011. It can be seen that if data were to be continued to input, it would get exactly what was put in (10110000), but offset by four 'Data Advance' cycles. This arrangement is the hardware equivalent of a queue. Also, at any time, the whole register can be set to zero by bringing the reset (R) pins high.
This arrangement performs destructive readout - each datum is lost once it has been shifted out of the right-most bit.
Serial-in parallel-out (SIPO)[edit]
This configuration allows conversion from serial to parallel format. Data input is serial, as described in the SISO section above. Once the data has been clocked in, it may be either read off at each output simultaneously, or it can be shifted out.
In this configuration, each flip-flop is edge triggered. All flip-flops operate at the given clock frequency. Each input bit makes its way down to the Nth output after N clock cycles, leading to parallel output.
In cases where the parallel outputs should not change during the serial loading process, it is desirable to use a latched or buffered output. In a latched shift register (such as the 74595) the serial data is first loaded into an internal buffer register, then upon receipt of a load signal the state of the buffer register is copied into a set of output registers. In general, the practical application of the serial-in/parallel-out shift register is to convert data from serial format on a single wire to parallel format on multiple wires.
Parallel-in serial-out (PISO)[edit]
This configuration has the data input on lines D1 through D4 in parallel format, D1 being the most significant bit. To write the data to the register, the Write/Shift control line must be held LOW. To shift the data, the W/S control line is brought HIGH and the registers are clocked. The arrangement now acts as a PISO shift register, with D1 as the Data Input. However, as long as the number of clock cycles is not more than the length of the and 'shift left' the data in a register, effectively dividing by two or multiplying by two for each place shifted.
Very large serial-in serial-out shift registers (thousands of bits in size) were used in a similar manner to the earlier delay line memory in some devices built in the early 1970s. Such memories were sometimes called 'circulating memory'. For example, the Datapoint 3300 terminal stored its display of 25 rows of 72 columns of upper-case characters using fifty-four 200-bit shift registers, arranged in six tracks of nine packs each, providing storage for 1800 six-bit characters. The shift register design meant that scrolling the terminal display could be accomplished by simply pausing the display output to skip one line of characters.[1]
History[edit]
One of the first known examples of a shift register was in the Mark 2 Colossus, a code-breaking machine built in 1944. It was a six-stage device built of vacuum tubes and thyratrons.[2] A shift register was also used in the IAS machine, built by John von Neumann and others at the Institute for Advanced Study in the late 1940s.
![Serial Serial](/uploads/1/2/5/8/125884008/543509328.png)
See also[edit]
- Linear feedback shift register (LFSR)
- SerDes (Serializer/Deserializer)
- Shift register lookup table (SRL)
References[edit]
- ^bitsavers.org, DataPoint 3300 Maintenance Manual, December 1976.
- ^Flowers, Thomas H. (1983), 'The Design of Colossus', Annals of the History of Computing, 5 (3): 246, doi:10.1109/MAHC.1983.10079
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