How to simulate output delay using next_trigger() in SystemC? - systemc

Question

I have been reading this upvoted answer on Stack Overflow: https://stackoverflow.com/a/26129960/12311164
It says that replacing wait(delay, units); in SC_THREAD to next_trigger(delay, units) in SC_METHOD works.
But when I tried, it does not work. I am trying to build adder module with 2 ns output delay. Instead of having a 2 ns output delay, the adder output is getting updated every 2 ns.
Design:
#include "systemc.h"
#define WIDTH 4
SC_MODULE(adder) {
sc_in<sc_uint<WIDTH> > A, B;
sc_out<sc_uint<WIDTH> > OUT;
void add(){
sc_time t1 = sc_time_stamp();
int current_time = t1.value();
int intermediate = A.read() + B.read();
next_trigger(2, SC_NS);
OUT.write(intermediate);
cout << " SC_METHOD add triggered at "<<sc_time_stamp() <<endl;
}
SC_CTOR(adder){
SC_METHOD(add);
sensitive << A << B;
}
};
I know how to simulate delay using 2 techniques: sc_event and SC_METHOD and the wait statement in SC_THREAD, but I would like to simulate the delay using next_trigger(). I have read the Language Reference Manual, but could not figure how to do it.
Simulated on EDA Playground here: https://edaplayground.com/x/dFzc
I think I need to trigger 2 NS after the inputs change, how to do that?

Answer 1

You will have to track state manually:
sc_uint<WIDTH> intermediate;
void add(){
if (A->event() || B->event() || sc_delta_count() == 0) {
intermediate = A.read() + B.read();
next_trigger(2, SC_NS);
} else {
OUT->write(intermediate);
}
}
The problem is that using next_trigger doesn't magically transform your SC_METHOD into SC_THREAD. In general, I find any usage of next_trigger inconvenient and there are better ways of doing this using sc_event.