- A Logic Circuit Simulation Library in C++

(libLCS Example) Functional module of a 1-bit fulladder

The aim of this example is the same as that of the example presented here. However, this time we will see how to create a functional realisation of a full-adder module, unlike a gate-level realisation as in that example. The circuit diagram is as follows.


The code which implements the above circuit using functional modules of 1-bit full-adders is as follows. The implementation of the full-adder module here is the same as that of the full-adder class lcs::FullAdder provided in libLCS.

#include <lcs/or.h>
#include <lcs/and.h>
#include <lcs/not.h>
#include <lcs/tester.h>
#include <lcs/simul.h>
#include <lcs/changeMonitor.h>

// All classes of the libLCS are defined in the namespace lcs.
using namespace lcs;

using namespace std;

// Define a class MyFullAdder. The instances of this class will serve as
// 1-bit full-adder modules for our circuit.
class MyFullAdder : public Module
    // The constructor should take single line Bus<1> objects as inputs, one each for
    // the two data lines, carry input, sum output and the carry output.
    MyFullAdder(const Bus<1> &S, const Bus<1> Cout, const InputBus<1> &A,
              const InputBus<1> &B, const InputBus<1> &Cin);

    // Destructor.

    // This function is re-implemented from the class lcs::Module.
    // When state of any of the input bus lines changes, the corresponding bus
    // will call this function to notify the fulladder module of such a
    // change. This function then perform a 1-bit full binary addition of the
    // line states of input busses and propogate the result to the output busses.
    virtual void onStateChange(int portId);

    Bus<1> s, cout;
    InputBus<1> a, b, c;

MyFullAdder::MyFullAdder(const Bus<1> &S, const Bus<1> Cout, const InputBus<1> &A,
                     const InputBus<1> &B, const InputBus<1> &Cin)
         : Module(), s(S), cout(Cout), a(A), b(B), c(Cin)
    // A functional module has to be driven by the data lines of the input busses.
    // In order to be driven, a module has to register itself with each of the input
    // busses using the drive function. If not registered, a change to the state of the data
    // lines of an input bus will not trigger the module to propogate the change and produce
    // the output.
    a.notify(this, LINE_STATE_CHANGE, 0);
    b.notify(this, LINE_STATE_CHANGE, 0);
    c.notify(this, LINE_STATE_CHANGE, 0);

    // After setting up the busses, the line data should be propogated to the output so that
    // the output is the one corresponding to the input after the module is constructed.

    // When a module ceases to exist anylonger, it should notify its input busses to stop triggering
    // it. This is done using the lcs::InputBus::stopNotification function. If not de-registered in this
    // way, the input busses will try to drive a non existant module, resulting in a segmentation fault.
    a.stopNotification(this, LINE_STATE_CHANGE, 0);
    b.stopNotification(this, LINE_STATE_CHANGE, 0);
    c.stopNotification(this, LINE_STATE_CHANGE, 0);

void MyFullAdder::onStateChange(int portId)
    // The sum and carry outputs are calculated using the overloaded bitwise operators
    // rather than through an ensemble of logics gates.
    s = (!a[0] & !b[0] & c[0]) | (!a[0] & b[0] & !c[0]) |
                    (a[0] & !b[0] & !c[0]) | (a[0] & b[0] & c[0]) ,
    cout = (!a[0] & b[0] & c[0]) | (a[0] & !b[0] & c[0]) |
                      (a[0] & b[0] & !c[0]) | (a[0] & b[0] & c[0]) ;

int main(void)
    // Declaring the busses involved in out circuit.
    // Note that the bus c0 (the carry input to the first full-adder)
    // has been initialised with a value of 0 (or lcs::LOW) on its line.
    Bus<> a1, b1, a2, b2, c0(0), S1, C1, S2, C2;

    // Initialising the 1-bit full adder modules.
    MyFullAdder fa1(S1, C1, a1, a2, c0), fa2(S2, C2, b1, b2, C1);

    // Initialising lcs::ChangeMonitor objects which monitor the inputs and the
    // 3 sum bits.
    ChangeMonitor<4> inputMonitor((a1,b1,a2,b2), "Input", DUMP_ON);
    ChangeMonitor<3> outputMonitor((S1,S2,C2), "Sum", DUMP_ON);

    Tester<4> tester((a1,b1,a2,b2));

    Simulation::setStopTime(2000); // Set the time upto which the simulation should run.
    Simulation::start();           // Start the simulation.

    return 0;

The following is the output when the following is compiled and run.

At time: 0,     Input: 0000
At time: 0,     Sum: 000
At time: 200,   Input: 0001
At time: 200,   Sum: 001
At time: 300,   Input: 0010
At time: 300,   Sum: 010
At time: 400,   Input: 0011
At time: 400,   Sum: 011
At time: 500,   Input: 0100
At time: 500,   Sum: 001
At time: 600,   Input: 0101
At time: 600,   Sum: 010
At time: 700,   Input: 0110
At time: 700,   Sum: 011
At time: 800,   Input: 0111
At time: 800,   Sum: 100
At time: 900,   Input: 1000
At time: 900,   Sum: 010
At time: 1000,  Input: 1001
At time: 1000,  Sum: 011
At time: 1100,  Input: 1010
At time: 1100,  Sum: 100
At time: 1200,  Input: 1011
At time: 1200,  Sum: 101
At time: 1300,  Input: 1100
At time: 1300,  Sum: 011
At time: 1400,  Input: 1101
At time: 1400,  Sum: 100
At time: 1500,  Input: 1110
At time: 1500,  Sum: 101
At time: 1600,  Input: 1111
At time: 1600,  Sum: 110

Below is the screenshot of the gtkwave plot of the generated VCD file.


Copyright © 2006, 2007 Siva Chandra