Reconfigurable Computing - Designing and Testing John Morris Chung-Ang University The University of Auckland ‘Iolanthe’ at 13 knots on Cockburn Sound, Western Australia

FPGA Architectures  Design Flow  Good engineering practice requires that design exercises should follow a defined procedure  User’s specification  This is your starting point  It may take several forms 1.Informal requirements given to you by your user / client / … 2.Formal written requirements All functional and non-functional requirements are precisely stated Sometimes resulting in a very large (and dull) document! 3.Something in between Your tutorial assignment was in this category Mostly formal, but with some gaps you would need to fill in Using research / further discussion with client / … etc

Step 1 - Analysis  Design Step 1 – Analyze specification  Identify suitable circuit modules by carefully reading specification  Start at top level – identify major components first  In particular, identify interfaces between environment and device that you’re designing Washing machine example: Inputs - User buttons, dials, sensors, … Outputs – Motors, indicator lights, audio, …  From these, specify the top-level entity ENTITY wm IS start, lid, stop : IN std_logic_vector; audio : OUT audio_sample; … END ENTITY wm;

Step 1 - Analysis  Design Step 1 – Analyze specification  Note that you may defer implementation details!  In this case, you don’t want to bother with the details of the audio output stream yet, so just specify a type audio_sample without filling in details for it yet.  Checking your design  A VHDL compiler (usually the simulator’s compiler) should be used to check completeness and consistency of a design from the early stages! ENTITY wm IS start, lid, stop : IN std_logic_vector; audio : OUT audio_sample; … END ENTITY wm;

Step 1a – Analysis : Checking the Design  Design Step 1 – Analyze specification  Checking your design  A VHDL compiler (usually the simulator’s compiler) should be used to check completeness and consistency of a design from the early stages!  Even though many details are missing Initially you may have no architecture blocks at all!  A compiler will warn you of missing, incomplete or inconsistent specifications  Thus although diagrams (and other informal modeling tools – like pencil and eraser!) are very useful, you should be generating formal (checkable) design specifications at a very early stage. These may be abstract High level Missing implementation details ‬but they are vital for detecting design errors!

Step 1a – Analysis : Checking the Design  Design Step 1 – Analyze specification  Checking your design  In this example, you have deliberately deferred a decision about the exact form of audio_sample.  You will need to provide a definition of it for the compiler, but you can substitute anything that the compiler will accept at this stage!  Make a package ENTITY wm IS start, lid, stop : IN std_logic_vector; audio : OUT audio_sample; … END ENTITY wm; PACKAGE audio IS TYPE audio_sample IS integer RANGE 0 TO 255; END PACKAGE audio;

Step 1a – Analysis : Checking the Design  Design Step 1 – Analyze specification  Deferred detail...  Many possibilities exist for the deferred detail … or PACKAGE audio IS TYPE audio_sample IS std_logic_vector(0 TO 7); END PACKAGE audio; PACKAGE audio IS TYPE audio_sample IS std_logic_vector(sample_size); END PACKAGE audio; PACKAGE audio IS TYPE audio_sample IS …; -- Your own idea! END PACKAGE audio; Key idea: Use the compiler to check your design as much as possible! These deferred definitions are necessary to avoid messages like ‘xxx is undefined’ which prevent the compiler from checking the rest of the design!

Step 2 - Refining the design  Once you have a formal VHDL specification for the high level entity for your design  Step 2 is to refine the design wm (washing controller) start lid motor stop valves audio

Step 2 - Refining the design  Step 2 is to refine the design  Determine the internal structure of your component  What entities do we need to implement the requirements? Counters, timers, … State machines, … (overall control) Device controllers eg PWM for motors, display controllers, …  Write formal models (VHDL entities) for each internal component ENTITY count_down_timer IS PORT( clk, reset : IN std_logic; cycles : IN std_logic_vector; complete : OUT std_logic ); END ENTITY count_down_timer; ENTITY speech_synth IS PORT( … ); END ENTITY speech_synth; ENTITY motor_control IS PORT( … ); END ENTITY motor_control;

Step 2 - Refining the design  Step 2 is to refine the design  Determine the internal structure of your component  Write formal models (VHDL entities) for each internal component  Determine how these models are connected wm (washing controller) start lid motorstop valves audio main_fsm speech_synth c_d_timer

Step 3 – Assemble the system  Step 3 – Build a model which includes all the components of the system you are building ie write the ARCHITECTURE block  Generally this will be a structural VHDL model  VHDL models may be mixed, so this is not a hard rule! start lid motor stop valves wm (washing controller) audio main_fsm speech_synth c_d_timer ARCHITECTURE a OF wm IS COMPONENT main_fsm PORT( … ); COMPONENT speech_synth PORT( … ); COMPONENT c_d_timer PORT( … ); SIGNAL … BEGIN fsm: main_fsm PORT MAP( … ); t1: c_d_timer PORT MAP( … ); END ARCHITECTURE a;

Step 3a – Check the design  Use the compiler (simulator or synthesizer) to check the design  Check for  Completeness All required modules are present All required signals are present  Consistency All signals are connected to signals of matching types ‬ eg No std_logic connected to std_logic_vector std_logic_vector sizes are compatible (no 8-bit busses connected to 16-bit ones!) ‬ etc  For example, checking the models sketched in the last few slides would reveal that there was no clock to feed the timer!  So one would be added …  Repeat steps 1 – 5 until no errors are reported  Even though some types ( eg audio_sample ) are not in their final form!

Step 4 – Complete the details  Fill in all architectures  You only need top-level structural architectures to check the design!  Now you need to complete them all!  Decide on all deferred types  Any design decision that you deferred …  Add assertions!  Don’t forget to try to make your design check itself

Step 5 - Simulation  Now we’re going to check the design using the simulator  We built a model of the device we’re going to place in an FPGA (or ASIC or …) wm (washing controller) audio main_fsm speech_synth c_d_timer motor valves start lid pause clock Note that the clock has been added now! (We discovered that it was missing in design check!)

Step 5 - Simulation  Build any additional models needed  Clock model  Models to simulate external devices eg camera, printer, keyboard, …  Special models for generating test data eg random data generators wm (washing controller) audio main_fsm speech_synth c_d_timer motor valves start lid pause clock

Step 5a - Build a test bench  Wrap all the models in a test bench  A test bench is usually an ENTITY with no ports!  The ARCHITECTURE of the test bench normally consists of  a structural part  a process block to generate test data wm (washing controller) audio main_fsm speech_synth c_d_timer motor valves start lid pause clock test_bench ENTITY test_bench IS END test_bench;

Test bench  The test bench ARCHITECTURE normally consists of  a structural part  a process block to generate test data wm (washing controller) audio main_fsm speech_synth c_d_timer motor valves start lid pause clock test_bench ENTITY test_bench IS END test_bench; ARCHITECTURE s OF test_bench IS COMPONENT clock PORT( clk: OUT std_logic ); END COMPONENT; COMPONENT wm PORT( clk, start, lid, pause: IN std_logic; ……); END COMPONENT; SIGNAL clk, start, lid, pause : std_logic_vector; BEGIN c : clock PORT MAP( clk => clk ); f : wm PORT MAP( clk => clk, start => start, lid => lid, … ); p : PROCESS BEGIN … END PROCESS; END ARCHITECTURE s;

Test bench  The test bench ARCHITECTURE normally consists of  a structural part  a process block to generate test data wm (washing controller) audio main_fsm speech_synth c_d_timer motor valves start lid pause clock test_bench ENTITY test_bench IS END test_bench; ARCHITECTURE s OF test_bench IS COMPONENT clock PORT( clk: OUT std_logic ); END COMPONENT; COMPONENT wm PORT( clk, start, lid, pause: IN std_logic; ……); END COMPONENT; SIGNAL clk, start, lid, pause : std_logic_vector; BEGIN c : clock PORT MAP( clk => clk ); f : wm PORT MAP( clk => clk, start => start, lid => lid, … ); p : PROCESS BEGIN … END PROCESS; END ARCHITECTURE s;

Test bench  Driving the signals of the component under test  For a simple component, a data flow model with waveform signal assignments may suffice  For our washing machine, we need to check Starting, stopping, pausing, opening the lid, … So for basic operations, waveforms applied to the control inputs will do: start <= ‘0’, ‘1’ after 1 sec, ‘0’ after 50 ms -- check starting ‘1’ after 10 sec, -- 2 nd start should not do anything ‘0’ after 50 ms, ‘1’ after 1000 sec, -- start another test …… lid <= ‘1’, -- start with lid closed ‘0’ after 5 sec, -- open lid after start ‘1’ after 10 sec, -- close again to resume …

Test bench  Driving the signals of the component under test  For a more complex component  (or more complex tests applied to a simple component!) ‬we’ll generally need to write some loops in PROCESS blocks PROCESS BEGIN FOR j IN 0 TO 9 LOOP – Perform 10 tests start <= ‘0’; lid <= ‘1’; pause <= ‘0’; WAIT FOR 1 sec; -- Now change the control inputs according to some -- table start <= s(J); lid <= l(j); pause <= p(j); WAIT FOR t(J); -- Check result END LOOP; END PROCESS; Assumes that we’ve generated some tables, s(0 TO 9), l(0 TO 9), p(0 TO 9), t(0 TO 9) containing test data

Step 6 - Verifying the Design  You can check the design by examining simulator waveforms start valve motor drain wm (washing controller) audio main_fsm speech_synth c_d_timer motor valve (water in) start lid pause clock drain (water out) Note that for this example, we have specifically listed 3 outputs motor - turns the motor on valve - opens the water inlet drain - opens the water outlet Pulsing the start input should start a washing cycle Pulsing it again should not stop the cycle Cycle starts by opening the inlet valve After it closes, the motor should start The motor should stop and the drain open Drain closes, inlet opens ….

Step 6 - Verifying the Design  You can check the design by examining simulator waveforms start valve motor drain wm (washing controller) audio main_fsm speech_synth c_d_timer motor valve (water in) start lid pause clock drain (water out) We could check everything by visually examining this trace... painful, manual, error prone in a large design... We can do much better!!

State machine outline ENTITY wm IS PORT( clk, start: IN std_logic; motor, valve, drain: OUT std_logic; … ); END ENTITY wm; ARCHITECTURE a OF wm IS TYPE wm_states IS (off, w_in1, wash, w_out1, w_in2, rinse,.. ); COMPONENT … SIGNAL s : wm_states; BEGIN PROCESS( clk ) BEGIN IF clk’EVENT AND clk = ‘1’ THEN CASE s IS WHEN off => IF start = ‘1’ THEN valve <= ‘1’; -- Open the valve -- More detail on next slide!! END PROCESS; END ARCHITECTURE wm; Details not relevant to this example omitted!

State machine outline ARCHITECTURE a OF wm IS TYPE wm_states IS (off, w_in1, wash, w_out1, w_in2, rinse,.. ); COMPONENT … SIGNAL s : wm_states; BEGIN PROCESS( clk ) BEGIN IF clk’EVENT AND clk = ‘1’ THEN CASE s IS WHEN off => IF start = ‘1’ THEN valve IF timer_complete THEN valve … -- and a lot more follows! END CASE; END PROCESS; END ARCHITECTURE wm; To keep it simple: A lot of detail is missing from this model!

Verifying the Design  Looking at the waveforms again, what things can you observe that are (should!!) always be true?? start valve motor drain wm (washing controller) audio main_fsm speech_synth c_d_timer motor valve (water in) start lid pause clock drain (water out)

Verifying the Design  Looking at the waveforms again, what things can you observe that are (should!!) always be true?? start valve motor drain wm (washing controller) audio main_fsm speech_synth c_d_timer motor valve (water in) start lid pause clock drain (water out) Only one of valve, motor or drain is ‘1’ at any time! So we can add an ASSERT to the model! ASSERT ((valve OR motor OR drain) = ‘0’) OR (valve=‘1’ AND motor=‘0’ AND drain=‘0’) OR … REPORT “Two outputs ON” SEVERITY warning;

Verifying the Design  These are not the only assertions that can be made about this design!  What are some others? Hint: Look at some of the other signals! start valve motor drain wm (washing controller) audio main_fsm speech_synth c_d_timer motor valve (water in) start lid pause clock drain (water out)

Adding the assertions ARCHITECTURE a OF wm IS TYPE wm_states IS (off, w_in1, wash, w_out1, w_in2, rinse,.. ); COMPONENT … SIGNAL s : wm_states; BEGIN PROCESS( clk ) BEGIN IF clk’EVENT AND clk = ‘1’ THEN CASE s IS WHEN off => IF start = ‘1’ THEN valve … -- and a lot more follows! END CASE; END PROCESS; ASSERT ((motor=‘0’) OR (valve=‘0’) OR (drain=‘0’)) OR … REPORT … SEVERITY …; ASSERT ((lid=‘0’) AND (motor=‘0’)) REPORT … SEVERITY …; END ARCHITECTURE wm; Unfortunately, the compiler will reject this assertion if you write it just like this! motor, valve and drain are declared OUT and strictly cannot be ‘read’ here! A simple `work-around’ solves this problem

Adding the assertions ARCHITECTURE a OF wm IS TYPE wm_states IS (off, w_in1, wash, w_out1, w_in2, rinse,.. ); COMPONENT … SIGNAL s : wm_states; SIGNAL motor_i, valve_i, drain_i : std_logic; BEGIN PROCESS( clk ) BEGIN IF clk’EVENT AND clk = ‘1’ THEN CASE s IS WHEN off => IF start = ‘1’ THEN valve_i … -- and a lot more follows! END CASE; END PROCESS; ASSERT ((motor_i=‘0’) OR (valve_i=‘0’) OR (drain_i=‘0’)) OR … REPORT … SEVERITY …; ASSERT ((lid=‘0’) AND (motor_i=‘0’)) REPORT … SEVERITY …; motor <= motor_i; valve <= valve_i; drain <= drain_i; END ARCHITECTURE wm; Add internal signals, which are set by the state machine … As internal signals, they can be read Assign them to the actual outputs in the body of the architecture!

Adding the assertions  Because of this minor complication, some might prefer to add the ASSERT ’s to the test bench  The outputs of the module under test can be read in the test bench!  I would prefer to add them to the module itself -  This ensures that they are active in ALL test benches  We might write several test benches to test various aspects of a design!  but, this is a personal preference  The important thing is to have them somewhere!!  They make your model self-checking  Automatic checking is more robust than manual checking!

Alternative verification strategies  When the outputs are complex functions of the inputs, assertions may become  either incredibly complex  and thus difficult to code (and get right!)  impossible  because of the complex logic required  PROCESS blocks may always be added to the test bench to check that the outputs are correct  They can contain arbitrarily complex code - and can thus check complex conditions  Automated checking of models  VHDL is just like a programming language  Test benches can write log files, dump data to disc files, etc  These files can be checked mechanically by other programs  Use the test bench to automate testing!!  Use ASSERT or algorithmic code in PROCESS blocks  Automatic checking is more robust than manual checking! You can also write VHDL functions to simplify ASSERT conditions!