Shift Registers

Tom Kelliher, CS 240

Apr. 28, 2010

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Assignment

Read 7-6, 7-11.

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Registers

Outline

1. Shift registers defined.

2. Serial Addition.

3. VHDL

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Shift registers defined

1. Why is a parallel register parallel?

2. So, we would expect a shift (serial) register to look like:
   

   SO = SI four clocks later.

   Using 2-1 muxes, how would you modify this to incorporate a shift control signal?

3. Parallel register with shift:
   

   D and Q are buses.

Serial Addition

1. Suppose you have two serial bit streams, A and B. Design a serial adder using one one bit full adder and one D flip-flop.

   If A and B are $n$ bits, the output can be how many bits?

2. Suppose A and B are shifted in on a single bit line. Is it possible for us to do the addition? (One shift register needed.)

3. What does a left shift by one do to the value of an unsigned number?

   Use this to design a sequential circuit which takes A as serial input and outputs 3A.

VHDL for Serial Registers

Parallel load, shift left or right, hold.

-- Parallel load shift register.  Shift left or right.
-- Mode bits:
--    00: hold
--    01: load
--    10: shift left (toward msb)
--    11: shift right (toward lsb)
--
-- msi: most significant shift in.
-- lsi: least significant shift in.

library ieee;
use ieee.std_logic_1164.all;

entity shift_reg is
  
  port (
    d            : in  std_logic_vector (31 downto 0);
    mode         : in  std_logic_vector (1 downto 0);
    clk, reset_n : in  std_logic;
    msi, lsi     : in  std_logic;
    q            : out std_logic_vector (31 downto 0));

end shift_reg;

architecture behavioral of shift_reg is

  signal state : std_logic_vector (31 downto 0);

begin  -- behavioral

  q <= state;                           -- Update output.
  
  state_register: process (clk, reset_n)
  begin  -- process state_register
    if reset_n = '0' then               -- asynchronous reset (active low)
      state <= X"00000000";
    elsif clk'event and clk = '1' then  -- rising clock edge
      if mode = "00" then               -- Hold.
        state <= state;
      elsif mode = "01" then            -- Load.
        state <= d;
      elsif mode = "10" then            -- Shift left.
        state <= state (30 downto 0) & lsi;
      else                              -- Shift right.
        state <= msi & state (31 downto 1);
      end if;
    end if;
  end process state_register;

end behavioral;

Tom Kelliher