L16 – VHDL for State Machines with binary encoding

L16 – VHDL for State Machines with binary encoding

VHDL – binary encoded states Another example

Counter – enumeration states Binary encoded states Gray Code Controlled counters

Ref: text Unit 10, 17, 20

9/2/2012 – ECE 3561 Lect 9

Copyright 2012 - Joanne DeGroat, ECE, OSU 2

Counter example Specification – Write a synthesizable VHDL

description of a 3-bit counter that counts from 0 to 7 in binary, i.e., 000, 001, 010, …, 110, 111.

A 3-bit counter has 8 states, so,s1, …, s8. The state occur sequentially without control.

9/2/2012 – ECE 3561 Lect 9


Start the HDL This is a problem that one should be able to

do without a state diagram to start. The entity

ENTITY cnt3bit IS PORT(clk : IN bit; cnt : OUT bit_vector(2 downto 0)); END cnt3bit;

9/2/2012 – ECE 3561 Lect 9


The ARCHITECTURE ARCHITECTURE one OF cnt3bit IS TYPE state_type IS (s0,s1,s2,s3,s4,s5,s6,s7); SIGNAL state,next_state : state_type; BEGIN --F/F process PROCESS BEGIN WAIT UNTIL clk='1' AND clk'event; state <= next_state; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


The next state process --next_state process PROCESS (state) BEGIN CASE state IS WHEN s0 => next_state <= s1; WHEN s1 => next_state <= s2; WHEN s2 => next_state <= s3; WHEN s3 => next_state <= s4; WHEN s4 => next_state <= s5; WHEN s5 => next_state <= s6; WHEN s6 => next_state <= s7; WHEN s7 => next_state <= s0; END CASE; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


The output process --output process PROCESS (state) BEGIN CASE state IS WHEN s0 => cnt <= "000"; WHEN s1 => cnt <= "001"; WHEN s2 => cnt <= "010"; WHEN s3 => cnt <= "011"; WHEN s4 => cnt <= "100"; WHEN s5 => cnt <= "101"; WHEN s6 => cnt <= "110"; WHEN s7 => cnt <= "111"; END CASE; END PROCESS; END one; --end of the architecture

9/2/2012 – ECE 3561 Lect 9


Notes on code Standard VHDL coding style

An enumeration type for coding of the state Signals for state and state type 3 processes for modeling 1st process – latching of next_state into state 2nd process – generation of next_state

Here simple as no control signals 3rd process – generation of output given the state

– a Moore machine.

9/2/2012 – ECE 3561 Lect 9


The testbench ENTITY is blank

ENTITY testcnt3bit IS END testcnt3bit;

Declarative region of Arch ARCHITECTURE one OF testcnt3bit IS COMPONENT cnt3bit IS PORT (clk : IN bit; cnt : OUT bit_vector(2 downto 0)); END COMPONENT; FOR all : cnt3bit USE ENTITY work.cnt3bit(one); -- declare signal for stimulus and response of the DUT SIGNAL clk : bit; SIGNAL cnt : bit_vector(2 downto 0); BEGIN

9/2/2012 – ECE 3561 Lect 9


In the testbench arch BEGIN -- As tb has clk will never go quiescent -- set up clk 50% duty cycle 10 ns period clk <= not clk AFTER 5 ns; -- instantiate component u1 : cnt3bit PORT MAP (clk,cnt); END one;

Testbench is very simple – apply the clk and instantiate the component.

9/2/2012 – ECE 3561 Lect 9


The VHDL simulation Waveform shows binary counting output

9/2/2012 – ECE 3561 Lect 9


When entered into Quartis Automatically did a one-hot encoding

5 ALUTs – 8 registers – 4 pins 5 total combinational functions

Does a one hot encoding make sense? Class discussion

9/2/2012 – ECE 3561 Lect 9


Compare to a binary encoding Code the binary coding straight into VHDL ENTITY is the same Declarative region of the ARCHITECTURE

Declarations are different ARCHITECTURE two OF cnt3bit IS SIGNAL state,next_state : bit_vector(2 downto 0);

BEGIN

9/2/2012 – ECE 3561 Lect 9


And in the processes -- latching of next_state PROCESS BEGIN WAIT UNTIL clk='1' AND clk'event; state <= next_state; END PROCESS;

No difference here as both state and next_state are of the same type. (just bit_vector now)

9/2/2012 – ECE 3561 Lect 9


Next State Process --next_state process PROCESS (state) BEGIN CASE state IS WHEN “000” => next_state <= “001”; WHEN “001” => next_state <= “010” ; WHEN “010” => next_state <= “011” ; WHEN “011” => next_state <= “100” ; WHEN “100” => next_state <= “101” ; WHEN “101” => next_state <= “110” ; WHEN “110” => next_state <= “111” ; WHEN “111” => next_state <= “000” ; END CASE; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


Output Process For this style the output process is simply

assigning the state to the output as this is a Moore machine. This can be done with a concurrent signal assignment statement.

cnt <= state;

END one;

9/2/2012 – ECE 3561 Lect 9


Modify the testbench In the declarative region add the 2nd arch and cnt for the 2nd arch (note change in instantiation labels)

ARCHITECTURE one OF testcnt3bit IS COMPONENT cnt3bit IS PORT (clk : IN bit; cnt : OUT bit_vector(2 downto 0)); END COMPONENT; FOR u1 : cnt3bit USE ENTITY work.cnt3bit(one); FOR binenc : cnt3bit USE ENTITY work.cnt3bit(two); -- declare signal for stimulus and response of the DUT SIGNAL clk : bit; SIGNAL cnt,cntbin : bit_vector(2 downto 0); BEGIN

9/2/2012 – ECE 3561 Lect 9


After the begin BEGIN -- As tb has clk will never go quiescent -- set up clk 50% duty cycle 10 ns period clk <= not clk AFTER 5 ns; -- instantiate component u1 : cnt3bit PORT MAP (clk,cnt); binenc : cnt3bit PORT MAP (clk,cntbin); END one;

Note instantiation of 2nd architecture which has output cntbin.

9/2/2012 – ECE 3561 Lect 9


Simulation Result from simulation

9/2/2012 – ECE 3561 Lect 9


Results from Quartis Generating the FPGA for cnt3bitbin This time encoding prevented choice of

one-hot methodology. Conclusion:

Tool is not always right!!

9/2/2012 – ECE 3561 Lect 9


A closer look 3 F/Fs

3 LUTs 4 pins

9/2/2012 – ECE 3561 Lect 9


Controlling the binary assignment Being able to control the binary assign results

in more control for the designer and easier design of other counters.

Can result in more efficient use of FPGA resources.

9/2/2012 – ECE 3561 Lect 9


Time for live demo Demo of code in Modelsim Demo of setup in Quartis

Probably want to take note of steps and editing shortcuts.

9/2/2012 – ECE 3561 Lect 9


Lecture summary HDL from code to simulation

For the 101 sequence detector Testbench for the sequence detector

9/2/2012 – ECE 3561 Lect 9


Documents

L16 – VHDL for State Machines with binary encoding