Sunday, 26 February 2012


N-bit register is group of flip-flops which can store n-bits of information. Depending on the way data is fed into register and the way the data is read out of register, there are 4 types of registers.
  1. Parallel in parallel out register
  2. Parallel in serial out resister
  3. Serial in parallel out register
  4. Serial in serial out register
Parallel in parallel out register(PIPO):
At the rising edge of ‘clk’, data on I0I1I2I3 is stored in flip-flops. The data is read all bits at a time. Hence it is called in parallel in parallel out register. To load data into n-bit PIPO register it requires single clock pulse.  Data is always available at the output.

Parallel in serial out register(PISO):
 Data is stored into the flip-flop at a time on single clock pulse when load signal is asserted. To read data out from the register load should be ‘0’. At each clock pulse a single bit is read out. For loading data into n-bit PISO register, single clock pulse is required and to read n-bits data from register n-clock pulses are required.

Serial in Parallel out register:
 Data is stored in flip-flop bit by bit. And data is read out from the register at a time. Hence it is called serial in parallel out register. To store n-bits of data into n-bit SIPO register it requires n clock pulses and data is available all the time at the output.

Serial in serial out register:
Data is stored into the register and read out from it bit by bit. So to load n-bits into n-bit register it requires n-clock pulses, and to read n-bits from it n-clock pulses are required.

Universal shift register:
Universal shift register is n-bit register, which is capable of loading n-bits of data at a time and is capable of shifting data left and right one bit at a time.
In the above figure a 4-bit universal shift register is shown. Depending on mode bits the register performs its operation. Below table shows various operations depending on mode bits.
  No. of mode bits requires depends on the no. of operations performed by register but not on size of register.

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