Registers

Tom Kelliher, CS 240

Apr. 20, 2012

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Assignment

Read 7-6, 7-9.

From Last Time

VHDL for sequential circuits.

Outline

1. Definitions.

2. Parallel registers.

3. VHDL for registers.

Coming Up

Shift registers.

Definitions

1. Register: A set of flip-flops which have common clock and control signals and load related bits -- a whole word, data on a bus.

   Examples: register file, MDR, MAR.

2. Counter: A register which passes through a pre-determined set of states. Usually they count by one from $0$ to $n - 1$.

   Example: program counter.

Parallel Registers

Use a mux approach:

Gating the clock will save the muxes, but clock gating is usually a bad idea -- clock skew problems.

1. Really bad: clockToFF = load and clock

2. Just as bad: clockToFF = !load or clock

Show waveforms and show what skew looks like and why we care.

VHDL for Registers

Note: The handling of the load signal could be used to implement a synchronous reset.

-- VHDL for 32 bit parallel load register with asynchronous low
-- reset.

library ieee;
use ieee.std_logic_1164.all;

entity parallel_register is
  
  port (
    clock, load, reset_n : in  std_logic;
    d                    : in  std_logic_vector (31 downto 0);
    q                    : out std_logic_vector (31 downto 0));

end parallel_register;

architecture dataflow of parallel_register is

  signal state : std_logic_vector (31 downto 0);

begin  -- dataflow

  q <= state;                           -- Output process.
  
  state_reg : process (clock, reset_n)
  begin  -- process state
    if (reset_n = '0') then             -- asynchronous reset (active low)
      state <= X"00000000";
    elsif (clock'event and clock = '1') then  -- rising clock edge
      if (load = '1') then
        state <= d;
      else
        state <= state;
      end if;
    end if;
  end process state_reg;

end dataflow;

Tom Kelliher