netlist: prepare road towards trapezoidal integration.

* This is a long term transition goal. Documented in source (see
NL_USE_BACKWARD_EULER).

src/lib/netlist/analog/nld_generic_models.h

@@ -131,7 +131,7 @@ namespace analog
 		nl_fptype m_gmin;
 	};
 
-#if 1
+#if (NL_USE_BACKWARD_EULER)
 	// Constant model for constant capacitor model
 	// Backward Euler
 	// "Circuit simulation", page 274
@@ -166,7 +166,7 @@ namespace analog
 	struct generic_capacitor_const
 	{
 	public:
-		generic_capacitor_const(device_t &dev, const pstring &name)
+		generic_capacitor_const(core_device_t &dev, const pstring &name)
 		: m_gmin(nlconst::zero())
 		, m_vn(0)
 		, m_in(0)
@@ -195,6 +195,10 @@ namespace analog
 			m_trn = h;
 			return { G + m_gmin, -Ieq };
 		}
+		void restore_state() noexcept
+		{
+			// this one has no state
+		}
 		void setparams(nl_fptype gmin) noexcept { m_gmin = gmin; }
 	private:
 		nl_fptype m_gmin;

src/lib/netlist/devices/nld_ne555.cpp

@@ -83,6 +83,7 @@ namespace netlist
 		, m_ff(*this, "m_ff", false)
 		, m_last_reset(*this, "m_last_reset", false)
 		, m_overshoot(*this, "m_overshoot", 0.0)
+		, m_undershoot(*this, "m_undershoot", 0.0)
 		, m_ovlimit(0.0)
 		{
 			register_subalias("GND",  m_R3.N());    // Pin 1
@@ -138,10 +139,13 @@ namespace netlist
 			}
 			else
 			{
-				const auto delta = m_THRES() + m_overshoot - vthresh;
-				const bool bthresh = (delta > 0.0);
+#if (NL_USE_BACKWARD_EULER)
+				const bool bthresh = (m_THRES() + m_overshoot > vthresh);
 				const bool btrig = (m_TRIG() - m_overshoot > vtrig);
-
+#else
+				const bool bthresh = (m_THRES() + m_overshoot > vthresh);
+				const bool btrig = (m_TRIG() - m_undershoot > vtrig);
+#endif
 				if (!btrig)
 					m_ff = true;
 				else if (bthresh)
@@ -154,7 +158,11 @@ namespace netlist
 
 			if (m_last_out && !out)
 			{
-				m_overshoot += ((m_THRES() - vthresh)) * 2;
+#if (NL_USE_BACKWARD_EULER)
+				m_overshoot += ((m_THRES() - vthresh)) * 2.0;
+#else
+				m_overshoot += ((m_THRES() - vthresh));
+#endif
 				m_overshoot = plib::clamp(m_overshoot(), nlconst::zero(), ovlimit);
 				//if (this->name() == "IC6_2")
 				//	printf("%f %s %f %f %f\n", exec().time().as_double(), this->name().c_str(), m_overshoot(), m_R2.P()(), m_THRES());
@@ -166,8 +174,13 @@ namespace netlist
 			}
 			else if (!m_last_out && out)
 			{
-				m_overshoot += (vtrig - m_TRIG()) * 2;
+#if (NL_USE_BACKWARD_EULER)
+				m_overshoot += (vtrig - m_TRIG()) * 2.0;
 				m_overshoot = plib::clamp(m_overshoot(), nlconst::zero(), ovlimit);
+#else
+				m_undershoot += (vtrig - m_TRIG());
+				m_undershoot = plib::clamp(m_undershoot(), nlconst::zero(), ovlimit);
+#endif
 				m_RDIS.change_state([this]()
 					{
 						m_RDIS.set_R(nlconst::magic(R_OFF));
@@ -193,6 +206,7 @@ namespace netlist
 		state_var<bool> m_ff;
 		state_var<bool> m_last_reset;
 		state_var<nl_fptype> m_overshoot;
+		state_var<nl_fptype> m_undershoot;
 		nl_fptype m_ovlimit;
 
 		nl_fptype clamp_hl(const nl_fptype v, const nl_fptype a, const nl_fptype b) noexcept

src/lib/netlist/examples/2N6027.cpp

@@ -28,14 +28,9 @@ NETLIST_EXTERNAL(loc_lib)
 
 NETLIST_START(ex2N6027)
 
-	/* This is a circuit pushing the solvers to the limits
-	 * 50,000 maximum NR loops.
-	 */
 	SOLVER(Solver, 48000)
-	PARAM(Solver.ACCURACY, 1e-5)
 	PARAM(Solver.DYNAMIC_TS, 1)
 	PARAM(Solver.DYNAMIC_MIN_TIMESTEP, 1e-9)
-	PARAM(Solver.NR_LOOPS, 50000)
 	PARAM(Solver.METHOD, "MAT_CR")
 
 	LOCAL_SOURCE(loc_lib)

src/lib/netlist/nl_config.h

@@ -97,6 +97,20 @@
 #define NL_USE_COPY_INSTEAD_OF_REFERENCE (0)
 #endif
 
+/// \brief Use backward Euler integration
+///
+/// This will use backward Euler instead of trapezoidal integration.
+///
+/// FIXME: Longterm this will become a runtime setting. Only the capacitor model
+/// currently has a trapezoidal version and there is no support currently for
+/// variable capacitors.
+/// The change will have impact on timings since trapezoidal improves timing
+/// accuracy.
+
+#ifndef NL_USE_BACKWARD_EULER
+#define NL_USE_BACKWARD_EULER (1)
+#endif
+
 /// \brief  Use the truthtable implementation of 7448 instead of the coded device
 ///
 /// FIXME: Using truthtable is a lot slower than the explicit device

src/lib/netlist/solver/nld_matrix_solver.cpp

@@ -532,7 +532,7 @@ namespace solver
 		if (dynamic_device_count() != 0)
 		{
 			step(timestep_type::FORWARD, delta);
-			auto resched = solve_nr_base();
+			const auto resched = solve_nr_base();
 
 			if (resched)
 				return newton_loops_exceeded(delta);