l16 – testbenches for state machines. vhdl language elements more examples hdl coding of class...

L16 – Testbenches for state machines

VHDL Language Elements More examples

HDL coding of class examples Testbench for example

Testing of examples – testbench construction Note trade off and difference in Mealy vs Moore

implementation from simulation results Constructing simple testbenches – general rules

Ref: text Unit 10, 17, 209/2/2012 – ECE 3561 Lect 9

Copyright 2012 - Joanne DeGroat, ECE, OSU 2

More examples Consider the state machine we designed earlier for

detecting an input that ends in the sequence 101. Developed both Mealy and Moore implementations.

9/2/2012 – ECE 3561 Lect 9


Now translate these to VHDL The ENTITY – the same ports

ENTITY mealy101 IS PORT (clk,x : IN bit; z : OUT bit); END mealy101;

ENTITY moore101 IS PORT (clk,x : IN bit; z : OUT bit); END moore101;

9/2/2012 – ECE 3561 Lect 9


Start the architecture The declarative region

ARCHITECTURE one OF mealy101 IS TYPE state_type IS (s0,s1,s2); SIGNAL state,next_state : state_type; BEGIN

ARCHITECTURE one OF moore101 IS TYPE state_type IS (s0,s1,s2,s3); SIGNAL state,next_state : state_type; BEGIN

9/2/2012 – ECE 3561 Lect 9


The F/F process The state elements

--state elements mealy PROCESS BEGIN WAIT UNTIL clk='1' AND clk'event; state <= next_state; END PROCESS;

--state elements moore PROCESS BEGIN WAIT UNTIL clk='1' AND clk'event; state <= next_state; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


The next state processes – Mealy --next state logic mealy PROCESS (state,x) BEGIN CASE state IS WHEN s0 => IF (x='0') THEN next_state <= s0; ELSE next_state <= s1; END IF; WHEN s1 => IF (x='0') THEN next_state <= s2; ELSE next_state <= s1; END IF; WHEN s2 => IF (x='0') THEN next_state <= s0; ELSE next_state <= s1; END IF; END CASE; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


The next state process – Moore PROCESS (state,x) BEGIN CASE state IS WHEN s0 => IF (x='0') THEN next_state <= s0; ELSE next_state <= s1; END IF; WHEN s1 => IF (x='0') THEN next_state <= s2; ELSE next_state <= s1; END IF; WHEN s2 => IF (x='0') THEN next_state <= s0; ELSE next_state <= s3; END IF; WHEN s3 => IF (x='0') THEN next_state <= s2; ELSE next_state <= s1; END IF; END CASE; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


The output logic - Mealy -- output logic - mealy machine PROCESS (state,x) BEGIN CASE state IS WHEN s0 => z<='0'; WHEN s1 => z<='0'; WHEN s2 => IF (x='1') THEN z<='1'; ELSE z<= '0'; END IF; END CASE; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


The output logic - Moore --output logic - Moore machine PROCESS (state) BEGIN CASE state IS WHEN s0 => z <= '0'; WHEN s1 => z <= '0'; WHEN s2 => z <= '0'; WHEN s3 => z <= '1'; END CASE; END PROCESS;

9/2/2012 – ECE 3561 Lect 9


Creating a testbench Start with the ENTITY

As the testbench is the top unit there is no interface. Process within testbench generate stimulus and possibly

check response and generate reports.

ENTITY tb101 IS END tb101;

9/2/2012 – ECE 3561 Lect 9


Declarations The declrative region – declare DUTs

ARCHITECTURE one OF tb101 IS --declare units to be tested COMPONENT mealy101 PORT (clk,x : IN bit; z : OUT bit); END COMPONENT; FOR all : mealy101 USE ENTITY work.mealy101(one); COMPONENT moore101 PORT (clk,x : IN bit; z : OUT bit); END COMPONENT; FOR all : moore101 USE ENTITY work.moore101(one);

9/2/2012 – ECE 3561 Lect 9


Declarative Region Declare signal to connect to DUT

BOTH stimulus and response

-- delcare input signals and input stream SIGNAL xs : BIT_VECTOR (1 to 30) :=

('0','0','0','1','0','1','0','0','0','1','0','1','0','1','0','1', '0','0','0','1','0','1','1','0','1','1','1','0','1','0'); SIGNAL x,z1,z2 : BIT; SIGNAL clk : BIT :='1';

9/2/2012 – ECE 3561 Lect 9


Wire in DUT Instantiate components

BEGIN --Instantiate units ml : mealy101 PORT MAP (clk,x,z1); mo : moore101 PORT MAP (clk,x,z2);

9/2/2012 – ECE 3561 Lect 9


Set up clocks Set up a 50% duty cycle clock

--Set up clock clk <= NOT clk AFTER 5 ns;

50% duty cycle clock is easy as above More complex clocks can be set up

9/2/2012 – ECE 3561 Lect 9


More complex clocks Use a process

PROCESS -- clk starts set high BEGIN clk <= ‘1’; WAIT FOR 5 ns; -- time high clk <= ‘0’; WAIT FOR 15 ns; --time low END PROCESS;

This is a 25% duty cycle clock the is high 25% of the period. Easy to adapt for any duty cycle and clock period.

9/2/2012 – ECE 3561 Lect 9


The stimulus process --Stimulus process PROCESS BEGIN WAIT FOR 1 ns; FOR i IN 1 to 30 LOOP x <= xs(i); WAIT FOR 10 ns; END LOOP; WAIT FOR 10 ns; WAIT; END PROCESS;

Process grabs inputs from the input vector set up in the declarative region. A good option when a simple sequence on a single signal.

More advanced techniques may be needed for complex machines.

9/2/2012 – ECE 3561 Lect 9


The simulation result The waveform – note clock edge versus X valid

time. ('0','0','0','1','0','1','0','0','0','1','0','1','0','1','0','1‘,'0','0','0','1','0','1','1','0','1','1','1','0','1','0');

9/2/2012 – ECE 3561 Lect 9


Simulation result 2 If timing of clock edge versus input X is

shifted output can be different

9/2/2012 – ECE 3561 Lect 9


Demo of simulation A look at a live demonstration of Model Sim

for simulation of this machine. Note that in Moore implementation output

seems to be delayed ~1 clock.

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


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