Clock to Q Propagation Delay

[Zdenek Prikryl]

Hi,

Recently I used the following Verilog code in my project:

module dff8(
    input  wire CLK,
    input  wire RST,
    input  wire [7:0] D0,
    output reg [7:0] Q0
);
    always @( posedge CLK or negedge RST ) begin
        if ( RST == 1'b0 ) begin
            Q0 <= 8'b0;
        end else begin
            Q0 <= #10 D0;
        end
    end
endmodule

Is there any way how to model #<DELAY> in SystemC. The example of systemc register that I use is below.

SC_MODULE(dff8)
{
    // port declarations
    sc_in<bool> CLK;
    sc_in<bool> RST;
    sc_in<sc_uint<8> > D0;
    sc_out<sc_uint<8> > Q0;

    // process declaration
    void do_dff8()
    {
        if (RST.read() == 0)
        {
            Q0.write(0);
        }
        else
        {
            // HOW TO ADD DELAY HERE?
            Q0.write(D0.read());
        }
    }

    SC_HAS_PROCESS(dff8);
    dff8(sc_module_name inst)
      : sc_module(inst)
    {
        SC_METHOD(do_dff8);
        sensitive << CLK.pos();
        sensitive << RST.neg();
    }
};

Thanks for any help.

 

[AmeyaVS]

Hello,

In SystemC you can use SC_THREAD's to model delays by using wait statements but you will incur performance penalty.

Instead you can use event's and event queues to model delays using in SystemC as mentioned here:

http://workspace.accellera.org/Discussion_Forums/helpforum/archive/msg/msg?list_name=help_forum&monthdir=200803&msg=msg00061.html

You can replace the SC_THREAD example given in the above mentioned link with SC_METHOD removing the infinite while loop.

Here's the modified example listed from the above mentioned link:

template<typename T>
class delay_transport : public sc_module {
public:
  sc_in<T> in;
  sc_out<T> out;
  SC_HAS_PROCESS(delay_transport);
  delay_transport(sc_module_name name_, sc_time tdelay_) :
    sc_module(name_),
    tdelay(tdelay_),
    in("in"), out("out")
  {
    SC_METHOD(mi);
    sensitive << in.default_event();
    SC_METHOD(mo);
    sensitive << eq;
  }
  sc_time tdelay;
  void mi() {
    val = in.read();
    vq.push(val);
    eq.notify(tdelay);
  }
  void mo() {
    val = vq.front();
    out.write(val);
    vq.pop();
  }
  sc_event_queue eq;
  std::queue<T> vq;
  T val;
};

Regards,

Ameya Vikram Singh

 

[MagicLantern]

On 1/26/2017 at 7:04 PM, AmeyaVS said:

Hello,

In SystemC you can use SC_THREAD's to model delays by using wait statements but you will incur performance penalty.

Instead you can use event's and event queues to model delays using in SystemC as mentioned here:

http://workspace.accellera.org/Discussion_Forums/helpforum/archive/msg/msg?list_name=help_forum&monthdir=200803&msg=msg00061.html

You can replace the SC_THREAD example given in the above mentioned link with SC_METHOD removing the infinite while loop.

Here's the modified example listed from the above mentioned link:

template<typename T>
class delay_transport : public sc_module {
public:
  sc_in<T> in;
  sc_out<T> out;
  SC_HAS_PROCESS(delay_transport);
  delay_transport(sc_module_name name_, sc_time tdelay_) :
    sc_module(name_),
    tdelay(tdelay_),
    in("in"), out("out")
  {
    SC_METHOD(mi);
    sensitive << in.default_event();
    SC_METHOD(mo);
    sensitive << eq;
  }
  sc_time tdelay;
  void mi() {
    val = in.read();
    vq.push(val);
    eq.notify(tdelay);
  }
  void mo() {
    val = vq.front();
    out.write(val);
    vq.pop();
  }
  sc_event_queue eq;
  std::queue<T> vq;
  T val;
};

Regards,

Ameya Vikram Singh

 

Hello bro. The link you gave seems to be invalid now. Where can I find it?

[Admin]

The link only works if you are an Accellera member and logged into the Accellera Workspace. Perhaps AmeyaVS can paste the information in this thread.

[David Black]

Actually, there is an easier way to model that delay. Simply use a spawned SC_THREAD.

// The following is just a small code snippet in the context of the original posting

void do_dff8()
{
   if (RST->read() == 0) {
     Q0->write(0);
   }
   else {
     sc_spawn( [&](){
	   wait( 10, SC_NS );
	   Q0->write(D0->read());
     });
   }
}

 

[AmeyaVS]

Hello @MagicLantern,

As far as I remember I have reproduced the content as it is from the resource I had found earlier, with minor changes.

You can take a look at suggestion from @David Black, which is an alternate approach to achieving the same desired behavior.

Just remember to enable dynamic processes using the build compilation configuration when using it with the Open source implementation of the SystemC kernel.

Regards,

Ameya Vikram Singh