1. Simple Sequential Circuits in VHDL

2. Contents Sequential circuit examples: - SR latch in dataflow style - D flip-flop in behavioral style - shift register (8-bit) in behavioral style

3. Definition of an SR latch Using Dataflow library IEEE; use IEEE. std_logic_1164.all; entity SRlatch is port (S,R: in std_logic; - - SR latch control inputs. Q, QN: out std_logic); -- outputs. end SRlatch; -- Dataflow. No processes or components. architecture SRlatch_arch1 of SRlatch is -- recall that all of the following are executed in parallel. begin QN <= S nor Q; -- a change in S or Q executes the command. Q <= R nor QN; -- a change in R or QN executes the command. end SRlatch_arch1; -- terminate execution when there are no more changes.

4. D flip-flop in Behavioral Style (I/II) library IEEE; use IEEE.std_logic_1164.all; entity VposD is port (CLK, CLR, D: in std_logic; -- inputs, D specifies what needs to be stored Q, QN: out std_logic); -- flip-flop contents end VposDff; -- for behavioral style, use a process. architecture VposDff_arch of VposDff is begin process (CLK, CLR) begin -- execution if there is a change in CLK or CLR if CLR='1' then Q<='0'; QN<='1' -- asynchronous CLR. elsif CLK' event and CLK='1' then Q<=D; QN<=not D; -- changes at rising edges end if; end process; end VposDff_arch;

5. Here is a simplified version: process -- without a sensitivity list, it will always be executed -- recall that the following are executed serially (as in "C“) wait until CLK='1'; Q<= D; end process Or for an even simpler implementation, we include the following line in an architecture definition, where everything is executed in parallel: Q<= D when CLK' event and CLK='1' else Q; … where everything has been defined as of type std_logic. D flip-flop in Behavioral Style (II/II)

6. Shift-register in Behavioral Style (Wakerly, p. 742) library IEEE; use IEEE.std_logic_1164.all; -- Use processes for behavioral style -- CLK defines the clock. All changes occur at the rising edges. -- S selects the function to be executed -- D stores the information that we would like to load. -- Q contains the register contents. entity Vshftreg is – define inputs and outputs to the register port (CLK, CLR, RIN, LIN: in STD_LOGIC; -- Clock, clear, shift bits in/out S: in STD_LOGIC_VECTOR (2 downto 0); -- select function D: in STD_LOGIC_VECTOR (7 downto 0); -- load data Q: out STD_LOGIC_VECTOR (7 downto 0)); -- register contents end Vshftreg;

7. Αrchitecture Vshftreg_arch of Vshftreg is signal IQ : STD_LOGIC_VECTOR (7 downto 0) – local signal variables begin process (CLK, CLR, IQ) – execution if there is a change in CLK, CLR, IQ begin -- internally, all commands are executed serially (as in "C"). if (CLR='1') then IQ '0'); -- Asynchronous clear elsif (CLK => 'event and CLK='1') then -- load at the rising edge case CONV_INTEGER(S) is when 0 => null;--Hold when 1 => IQ <= D;--Load when 2 => IQ <= RIN & IQ (7 downto 1)--Shift right when 3 => IQ <= IQ(6 downto 0) & LIN;--Shift left when 4 => IQ <= IQ(0) & IQ(7 downto 1);--Shift circular right when 5 => IQ <= IQ(6 downto 0) & IQ(7);--Shift circular left when 6 => IQ <= IQ(7) & IQ(7 downto 1);--Shift circular right when 7 => IQ <= IQ(6 downto 0) & '0';--Shift circular left when others => null; end case; end if; Q <= IQ; -- read what has been stored. end process; end Vshftreg_arch;