How can I check the difference between sc_buffer and sc_signal?

Question

I would like to check the difference between using sc_buffer and sc_signal. I have coded a module which adds two random numbers and then I run two tests in parallel: one using sc_buffer and the other using sc_signal. Nevertheless, when I check with gtkwave I see the same traces for both examples, so I think for this case there should not be any difference. How can I check the difference? or is it that these two different types of channel are intended for different applications?

Answer 1

sc_buffer is probably most useful when modeling at an abstract level.

For example, consider modeling a serial communication channel. The transmitter could send the same character twice in a row. If an sc_signal was used as the channel, the receiver wouldn't detect the second character, but with an sc_buffer, it would.

#include <systemc>
#include <iostream>

using namespace sc_core;
using namespace std;

struct Transmitter : public sc_module {

    sc_out<char> out;

    Transmitter(sc_module_name name) : sc_module(name) {
        SC_THREAD(transmit);
    }

    void transmit() {
        wait(1, SC_NS);
        out.write('x');

        wait(1, SC_NS);
        out.write('x');

        wait(1, SC_NS);
        out.write('y');
    };

    SC_HAS_PROCESS(Transmitter);
};

struct Receiver : public sc_module {

    sc_in<char> in;

    Receiver(sc_module_name name) : sc_module(name) {
        SC_METHOD(receive);
        sensitive << in;
        dont_initialize();
    }

    void receive() {
        cout << sc_time_stamp() << ": " << name() << " received "
             << in.read() << endl;
    }

    SC_HAS_PROCESS(Receiver);
};

int sc_main(int argc, char* argv[])
{
    sc_signal<char> signal;
    sc_buffer<char> buffer;

    Transmitter signal_transmitter("signal_transmitter");
    Receiver signal_receiver("signal_receiver");
    Transmitter buffer_transmitter("buffer_transmitter");
    Receiver buffer_receiver("buffer_receiver");

    signal_transmitter.out(signal);
    signal_receiver.in(signal);

    buffer_transmitter.out(buffer);
    buffer_receiver.in(buffer);

    sc_start();

    return 0;
}

The above example produces this output:

1 ns: signal_receiver received x
1 ns: buffer_receiver received x
2 ns: buffer_receiver received x
3 ns: signal_receiver received y
3 ns: buffer_receiver received y

Notice that signal_receiver didn't detect the character sent at 2 ns.

You won't see any difference in a VCD trace, because the values stored in the sc_buffer and sc_signal channels are identical. The difference is when the receiver is triggered.