SystemC has its own tracing functions for generating VCDs (sc_create_vcd_trace_file, sc_close_vcd_trace_file and sc_trace). However these only allow tracing of SystemC signals.

Tracing the signals in the underlying Verilator model requires a SystemC module which can drive Verilator's trace functions. In this example, that module is TraceSC.

Tracing must be enabled when the Verilator model is created, by use of the -trace flag. This can be conveniently passed in using the VFLAGS macro with the Makefile. When tracing has been turned on the VM_TRACE macro is defined, so C++ code can be made conditional by using #if VM_TRACE.

Tracing requires that the main model header is included and the SystemPerl VCD tracing header. However the latter is only available if the -trace flag has been used, so its inclusion must be conditional:

#include "Vorpsoc_fpga_top.h"

#if VM_TRACE
#include <SpTraceVcdC.h>
#endif
      

Tracing requires a SystemC method to be woken on each clock edge to generate trace output, a pointer to the Verilator model and a pointer to a the SystemPerl trace file object of type SpTraceVcdFile. This last is only available if the -trace flag has been used, so its definition must be conditional on VM_TRACE.