An FF, or Flip-Flop, is a circuit element used to store data. Standard FFs can be used to store data in the form of 1 or 0, but to store several data bits, many FFs are required. In these cases, a shift register is used to store the data. A shift register is an electronic device used to store and move data in a series of connected FFs. They are considered sequential circuits, mainly used to store data and move it to the output on each clock cycle. In general, there are four types of shift registers: serial in serial out (SISO), serial in parallel out (SIPO), parallel in serial out (PISO), and parallel in parallel out (PIPO).
In a SISO shift register, the device allows a serial input and generates a serial output. Because there is only one output, the data is only able to leave the register one bit at a time. SISO circuits can be built with four FFs, and, once connected, the same clock signal is provided to each of the FFs. In SISO registers, the serial data input can be taken from the left side of the FF. The most common application of SISO registers is to operate as a delay element.
SIPO shift registers allow a serial input and generate a parallel output. Similar to an SISO, SIPOs can be formed with four FFs but also feature a CLR (clear) signal connected to the clock signal and FFs in order to rearrange them. Each FF is connected to the one next to it, allowing all FFs to work in sync with each other. Shift registers of this type can take serial data from the adjacent FF and generate an equivalent output. Applications of SIPO registers include communication lines, as the main function of an SIPO register is to convert serial information into parallel information.
PISO registers allow parallel input and create a serial output. Like the above registers, it is built with four FFs and a clock signal connected to each one. However, input data is connected separately to each FF through the use of a multiplexer at each FF unit. The first FF output, as well as the parallel data input, are connected to the multiplexer input. The multiplexer output is connected to the second FF. Once the clock signal is transmitted to every FF, they are synchronized. The primary use of PISO registers is to convert parallel data into serial data.
The final type of shift register, PIPO, allows parallel input data and produces parallel output. These registers consist of four FFs each connected to clear and clock signals. In these registers, the FFs themselves are not connected, because data serial shifting is not necessary. Instead, the data is given as individual input for each FF, and the output is also received individually. PIPO shift registers are used like a temporary storage device and can perform like a delay element.
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