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Chapter 10

Chapter 10. State Machine Design. State Machine Definitions. State Machine: A synchronous sequential circuit consisting of a sequential logic section and a combinational logic section.

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Chapter 10

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  1. Chapter 10 State Machine Design

  2. State Machine Definitions • State Machine: A synchronous sequential circuit consisting of a sequential logic section and a combinational logic section. • The outputs and internal flip flops (FF) progress through a predictable sequence of states in response to a clock and other control inputs.

  3. State Machine Types • Moore Machine: A Finite State Machine (FSM) whose outputs are determined only by the Sequential Logic (FF) of the FSM. • Mealy Machine: An FSM whose outputs are determined by both the sequential logic and combinational logic of the FSM.

  4. State Machine Basics • State Variable: The variable held in the SM (FF) that determines its present state. • A basic FSM has a memory section that holds the present state of the machine (stored in FF) and a control section that controls the next state of the machine (by clocks, inputs, and present state).

  5. State Machine Basics

  6. State Machine Basics

  7. FSM Design Techniques • Classical Design: Makes use of state tables, FF excitation tables, and Karnaugh Mapping to find FF input control logic. • VHDL Design: Uses case statements or IF THEN ELSE statements to set the design and the logic synthesis tools to define equations.

  8. Classical Design Approach – 1 • Define the actual problem. • Draw a state diagram (bubble) to implement the problem. • Make a state table. Define all present states and inputs in a binary sequence. Then define the next states and outputs from the state diagram.

  9. Classical Design Approach – 2 • Use FF excitation tables to determine in what states the FF inputs must be to cause a present state to next state transition. • Find the output values for each present state/input combination. • Simplify Boolean logic for each FF input and output equations and design logic.

  10. FSM Design Example 1 • Gray Code Counter that sequences {000, 001, 011, 010, 110, 111, 101, 100, 000}. • From this the state and excitation table is developed for D flip flops (see Table 10.2 in the textbook).

  11. FSM Design Example 1

  12. FSM Design Example 1

  13. FSM Design Example 2 • From the K-Maps for the inputs D0, D1, and D2, the following equations are developed: • Refer to Figure 10.6 in the textbook.

  14. FSM Design Example 2

  15. VHDL FSM Design • Uses an enumerated type to declare state variables. • Enumerated Type: A user-defined type in which all possible values of a named identifier are listed in a type definition. • An FSM uses a CASE statement on the enumerated type state variable.

  16. FSM VHDL Example -- gray_ct1.vhd -- 3-bit Gray code counter -- (state machine with decoded outputs) LIBRARY ieee; USE ieee.std_logic_1164.ALL;

  17. FSM VHDL Entity ENTITY gray_ct1 IS PORT( clk : IN STD_LOGIC; q : OUT STD_LOGIC_VECTOR(2 downto 0)); END gray_ct1;

  18. FSM VHDL Architecture – 1 ARCHITECTURE a OF gray_ct1 IS TYPE STATE_TYPE IS (s0,s1,s2,s3,s4,s5,s6,s7); SIGNAL state :STATE_TYPE; BEGIN PROCESS(clk) BEGIN

  19. FSM VHDL Architecture – 2 IF (clk’EVENT AND clk = ‘1’) THEN CASE state IS WHEN s0 => state <= s1; WHEN s1 => state <= s2; WHEN s2 => state <= s3; WHEN s3 => state <= s4; WHEN s4 => state <= s5;

  20. FSM VHDL Architecture – 3 WHEN s5 => state <= s6; WHEN s6 => state <= s7; WHEN s7 => state <= s0; END CASE; END IF; END PROCESS;

  21. FSM VHDL Architecture – 4 WITH state SELECT q <= “000” WHEN s0, “001” WHEN s1, “011” WHEN s2 “010” WHEN s3, “110” WHEN s4, “111” WHEN s5, “101” WHEN s6, “100” WHEN s7; END a;

  22. VHDL Output Assignment • The output assignment for the following example could have also been in the CASE test statements (in the process). WHEN s0 => state <= s1; q <= “001”; WHEN s1 => state <= s2; q <= “011”;

  23. FSM with Control Inputs • Same design approach used for FSM such as counters. • Uses the control inputs and clock to control the sequencing from state to state. • Inputs can also cause output changes not just FF outputs.

  24. FSM with Control Inputs

  25. SM Diagram Notation – 1 • Bubbles contain the state name and value (StateName/Value), such as Start/000. • Transitions between states are designated with arrows from one bubble to another. • Each transition has an ordered Input/Output, such as in1/out1.

  26. SM Diagram Notation – 2 • For example, if SM is at State = Start and if in1 = 0, it then transitions to State = Continue and out1 = 1, out2 = 0. • The arrow is drawn from start bubble to continue bubble. • On the arrow the value 0/10 is given to represent the in1/out2,out1.

  27. SM Design – 1 • State Table for the State Diagram

  28. SM Design – 2 • The State Excitation Tables for the JK Inputs

  29. SM Design – 3 • The following equation represents the next state and output logic of the state machine.

  30. SM Design – 4 • The pulser SM has two outputs that are not always synchronized to clock. • The pulse out2 is always synched to a change in clock, but out1 could change if in1 changes. • The following slides show a VHDL implementation of the Pulser SM.

  31. SM Design – 4

  32. SM Design – 4

  33. SM Pulser Architecture – 1 • Uses an enumerated type state listing of start and continue. ARCHITECTURE a OF state_x1 IS TYPE PULSER IS (start, continue); SIGNAL sequence : PULSER; BEGIN PROCESS(clk) BEGIN

  34. SM Pulser Architecture – 2 • A portion of the case statement: IF(clk’EVENT AND clk = ‘1’) THEN CASE sequence IS WHEN start => IF in1 = ‘1’ THEN sequence <= start; out1 <= ‘0’; out2 <= ‘0’;

  35. SM Pulser Architecture – 3 • In the VHDL case statement, an IF conditional test statement was used to check the Input Signal (in1) for State = Start. • IF in1 = 1, stay at State = Start; IF in1 = 0, then move to State = Continue (next clk). • If the present state was continue, the next state is always start, so an IF statement is not required.

  36. Switch Debouncer • A digital circuit that is used to remove the mechanical “bounce” from a switch contact. • When a switch is closed, the contacts bounce from open to closed to cause false transitions. • A simple debouncer is a cross-coupled NAND Latch.

  37. Switch Debouncer

  38. Shift Register Debouncer – 1 • An SR can be used to delay the glitches (bounces) with a slow clock (T = 2.6 ms). • The input of the SR is from the switch, and it is clocked through a 4-bit register. • The output of the SR is compared to the input, and if it is equal, the SR is still loaded.

  39. Shift Register Debouncer – 2 • When the switch bounces, the SR input and output are not equal, so the register stops loading to allow the delay to be added. • The comparison is done by an XNOR gate to control the SR Load.

  40. Shift Register Debouncer – 3 • 4-bit LPM SR clocked every 2.6 ms • If pb_in pb_out for 10.4 ms, the output changes. • If not, then the output remains the same. • Change on the input must be stable for 4 clock cycles before the output changes.

  41. Shift Register Debouncer – 3

  42. SR Debounce Entity • Uses an in from SW (pb_in) and a debounced output (pb_out). ENTITY debouncer IS PORT( pb_in, clock : IN STD_LOGIC; pb_out : OUT STD_LOGIC); END debouncer;

  43. SR Debounce Architecture – 1 ARCHITECTURE debounce OF debouncer IS SIGNAL ctr_q :STD_LOGIC_VECTOR (15 downto 0); SIGNAL srg_parallel_in :STD_LOGIC_VECTOR (3 downto 0); SIGNAL srg_serial_in :STD_LOGIC; SIGNAL srg_serila_out :STD_LOGIC; SIGNAL srg_load :STD_LOGIC;

  44. SR Debounce Architecture – 2 BEGIN clock_divider : lpm_counter GENERIC MAP (LPM_WIDTH => 16) PORT MAP (clock => clock, q => ctr_q);

  45. SR Debounce Architecture – 3 -- shift register clocked by MSB of counter 216 delay_ element : lpm_shiftreg GENERIC MAP (LPM_WIDTH => 4) PORT MAP (clock => ctr_q (15), load => srg_load, data => srg_parallel_in, shiftin => srg_serial_in, shiftout => srg_serial_out);

  46. SR Debounce Architecture – 4 srg_load <= (not pb_in) xnor srg_serial_out; srg_parallel_in (3) <= srg_serial_out; srg_parallel_in (2) <= srg_serial_out; srg_parallel_in (1) <= srg_serial_out; srg_parallel_in (0) <= srg_serial_out; srg_serial_in <= not pb_in; pb_out <= srg_serial_out; END debounce;

  47. Unused States – 1 • Some modulus counters, such as MOD-10, have states that are not used in the counter sequence. • The MOD-10 Counter would have 6 unused states (1010, 1011….1111) based on 4-bits.

  48. Unused States – 2 • An FSM can also have unused states, such as an SM, with only 5 bubbles in the state diagram (5-states). This FSM still requires 3 bits to represent these states so there will be 3 unused states. • These unused states can be treated as don’t cares (X) or assigned to a specific initial state.

  49. Unused States – 2

  50. Unused States Example Slide – 1 • A five-variable state diagram is shown in the previous slide, with unused states assigned to the initial state = Start. • The normal state sequence is StartWait1  Wait2  Pulse1  Pulse2 and then back to Start. • Any other states cause a transition back to Start.

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