THE SHIFT REGISTER


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What is a "SHIFT REGISTER"?

In many applications it may be necessary to store the status of events that have previously happened. If only one or two events are required to be monitored then it would simply be a matter of using "FLAGS" or "INTERNAL MARKERS".

However it might be desirable to store the status of many events and this is where the Shift Register would be used.

The Shift Register will usually have a minimum of three "controlling" inputs.

  1. DATA.   

                    This input is the DATA that needs to be monitored and can be                 
                    stored as "BIT" information. In some cases it can be stored as       
                    "BYTE or WORD" information.

  1. CLOCK.   

                     This input controls the "DATA" input. When the Shift register                                   
                     "sees" the rising edge of this input (i.e.: the Clock input goes 
                     from status "0" to status "1") it will transfer the status of the
                     Data input into the Shift Register.

  1. RESET.

                     This input will clear all the information inside the Shift Register to 0.


The shift register goes by many names. SFT (ShiFT), BSL (Bit Shift Left), SFR (Shift Forward Register) are some of the
common names.

These registers shift the bits to the left. BSR (Bit Shift Right) and SFRN (Shift Forward Register Not) are some examples of instructions that shift bits to the right.

It should be noted that not all manufacturers have shift registers that shift data to the right but most all do have left shifting registers.