Scoreboards

When validating a hardware design, it is important to run a large number of tests through a design. Commonly, the results of these tests are either compared against a known good output or, for more involved designs, matched with the outputs of a (often not synthesizable) model performing the same operations.

In order to enable this testing scheme in ACT, this simulation library contains the sub-namespace sim::scoreboard. It (currently) provides three different scoreboards with different assumptions about the output.

When setting up a testing environment, the scoreboard is the end point of the data pipeline. It consumes the outputs from either an oracle source (providing known good outputs from either a file or sequence) or a model implemented in CHP or external C as well as the design under test (DUT). This setup is similar to what is often seen in UVM/SystemVerilog; however, since there is no border between a asynchronous or message parsing domain and a synchronous register transfer level domain, we do not require an additional driver/monitor combination to connect our DUT to the testing harness. Since ACT supports seamless interconnection of high level description all the way down to analog simulation, the same testing harness can be reused for high level evaluation, all the way down to post-layout verification.

It is important to know about the timing of the processes provided here, as it could influence how your design behaves. There are no input buffers on any of these scoreboards, which means that scoreboards will only proceed to the next test once *all inputs have received a token*. This is by design, so your testing harness can reflect whatever timing requirements your design might have or have to expect in when used in real life by adding a buffer to the input. We do however provide an infinite capacity buffer to decouple your scoreboard completely.

Here's a simple example for a testing harness for a basic adder using a lockstep scoreboard and a sequence source as an oracle. We will go into depth about this type of scoreboard in a second:

import module1;
import sim;
 
defproc test ()
{
    // configuration parameters
    pint D_WIDTH = 8;
    pint NUM_D = 4;
    pint data[NUM_D];
    data = {0, 1, 2, 3};
    pint res[4];
    res = {0, 2, 4, 6};
 
    // instantiations
    module1::adders::serial_adder<D_WIDTH> add;
 
    // this example only uses sources and sinks with locally defined sequences
    sim::source_sequence_multi<D_WIDTH, 2, NUM_D, data, false, 1, false> src_i1;
    sim::source_sequence_multi<D_WIDTH, 2, NUM_D, data, false, 2, false> src_i2;
    sim::source_sequence<D_WIDTH, NUM_D, res, false, 0, false> oracle;
 
    sim::scoreboard::lockstep<D_WIDTH, 2, 1, 0, true> sb;
 
    // connect the DUT
    add.IN1 = src_i1.O[0];
    add.IN2 = src_i2.O[0];
 
    // connect the scoreboard
    sb.IN[0] = src_i1.O[1];
    sb.IN[1] = src_i2.O[1];
    sb.OUT_D[0] = add.OUT;
    sb.OUT_M[0] = oracle.O;
}

We connect use multi-ended sources to both the DUT as well as the scoreboard. If we had a model instead of an oracle, we would also connect the sources to it as well. Due to the simplicity of the design (adds 1 stage of slack), and the use of the oracle, we have omitted buffers on the scoreboard inputs.

We (currently) provide three different scoreboards based on different assumptions about the inputs and outputs of a DUT.

• If your design has a group of inputs and outputs, which always see the same number of tokens in the same order, use a lockstep scoreboard. An example would be an adder which produces as many results as it receives inputs.
• If your design has a group of outputs, which always see the same number of tokens in the same order, use a deterministic scoreboard. An example would be a deterministic merge, where two input channels result in one output channel.
• If you design requires any other more intricate testing condition, can implement your own checks and use a generic scoreboard to have standard output formatting for automatic and distributed testing.

If you need to log your inputs you can additionally use an input logger for any group of inputs which always see the same number of tokens.

The scoreboards in this namespace use the same parameters:

• D_WIDTH: The bit width of the data bus
• IN_CHANNELS: Number of input token channels (only used on lockstep and input_logger)
• OUT_CHANNELS: Number of token output channels on DUT/model (not used on input_logger)
• SB_ID: ID of the scoreboard to be used in log output
• VERBOSE_TESTING: If false, only failed tests will emit a message to the log