Ch6_Registers_and_Counters_Kenning

Ch6_Registers_and_Counters_Kenning - Registers A single...

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E&CE 223 Digital Circuits and Systems Page 1 Registers A single flip-flop stores one bit of information. A collection of n flip-flops stores n bits of information – these flip-flops form what is called an n-bit register.
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E&CE 223 Digital Circuits and Systems Page 2 Registers Illustration of a 4-bit register. When clear=0 , all flip-flop outputs are forced to zero (active low reset). When clear=1 , the rising edge of the clock (the active clock edge), results in the 4-bit input transferred to register output. D Q R D Q R D Q R D Q R clock clear I3 I2 I1 I0 A3 A2 A1 A0
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E&CE 223 Digital Circuits and Systems Page 3 Register With Parallel Load We might want to prevent the transfer of data from input to output even though the active clock edge arrives. I.e., we want the register to hold its current value . We can do this by feeding the register outputs back to the inputs and adding some additional logic to control the register operation.
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E&CE 223 Digital Circuits and Systems Page 4 Register With Parallel Load D Q R D Q R D Q R D Q R clock clear I3 I2 I1 I0 A3 A2 A1 A0 load
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E&CE 223 Digital Circuits and Systems Page 5 Register With Parallel Load When load=1 , the data inputs reach the D-input of the flip-flop. When the active clock edge arrives, the data gets transferred, or loaded , to the register output. When load=0 , the data output of each flip-flop is fed back to its D-input. When the active clock edge arrives, the data input gets transferred to the register output, but since the values are the same for all flip-flops, the register holds its current value.
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E&CE 223 Digital Circuits and Systems Page 6 An Aside… Different Register Functions To permit loading or holding of data, we have, in effect simply placed a multiplexer in from each flip-flop input in order to “direct” the correct information to the flip-flop inputs to obtain our desired operation. In our previous example of parallel load, the AND/OR gates are working as a multiplexer – the load signal is the multiplexer control line.
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E&CE 223 Digital Circuits and Systems Page 7 Shift Registers Might want a register that can shift data serially in a direction… This type of register is called a shift register . Illustration of a 4-bit shift register. D Q R D Q R D Q R D Q R clock serial in serial out clear As active clock edges arrive, the data present at the serial input gets transferred towards the serial output – so, data gets shifted to the right one bit at a time as clock edges arrive.
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E&CE 223 Digital Circuits and Systems Page 8 Universal Shift Registers Perhaps we want a more general circuit – e.g., we want to be able to clear the register, load the register, and perform both a shift right and a shift left operation. We can do this by adding addition logic in front of each flip-flop in the register: We need to make sure the correct data is present at the D-input to each flip- flop to perform each operation correctly according to the settings on the register control lines.
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