Add SEClamp support in CoreNEURON (#381)

* Add SEClamp support in CoreNEURON
  - add svclmp.mod mod file from NEURON
  - update registeration callbacks
  - expose at_time in the header file
* Fixed compilation issue for GPU with at_time and updated NMODL commit
* Added definition of at_time in nrnoc_ml.ispc
* Small fix for at_time declaration outside ispc

Co-authored-by: Ioannis Magkanaris <iomagkanaris@gmail.com>

Author: pramodk

coreneuron/mechanism/mech/cfile/cabvars.h

@@ -32,14 +32,14 @@ extern void capacitance_reg(void), _passive_reg(void),
 #if EXTRACELLULAR
     extracell_reg_(void),
 #endif
-    _stim_reg(void), _hh_reg(void), _netstim_reg(void), _expsyn_reg(void), _exp2syn_reg(void);
+    _stim_reg(void), _hh_reg(void), _netstim_reg(void), _expsyn_reg(void), _exp2syn_reg(void), _svclmp_reg(void);
 
 static void (*mechanism[])(void) = {/* type will start at 3 */
                                     capacitance_reg, _passive_reg,
 #if EXTRACELLULAR
                                     /* extracellular requires special handling and must be type 5 */
                                     extracell_reg_,
 #endif
-                                    _stim_reg, _hh_reg, _expsyn_reg, _netstim_reg, _exp2syn_reg, 0};
+                                    _stim_reg, _hh_reg, _expsyn_reg, _netstim_reg, _exp2syn_reg, _svclmp_reg, 0};
 
 }  // namespace coreneuron

coreneuron/mechanism/mech/modfile/svclmp.mod

@@ -0,0 +1,107 @@
+TITLE svclmp.mod
+COMMENT
+Single electrode Voltage clamp with three levels.
+Clamp is on at time 0, and off at time
+dur1+dur2+dur3. When clamp is off the injected current is 0.
+The clamp levels are amp1, amp2, amp3.
+i is the injected current, vc measures the control voltage)
+Do not insert several instances of this model at the same location in order to
+make level changes. That is equivalent to independent clamps and they will
+have incompatible internal state values.
+The electrical circuit for the clamp is exceedingly simple:
+vc ---'\/\/`--- cell
+        rs
+
+Note that since this is an electrode current model v refers to the
+internal potential which is equivalent to the membrane potential v when
+there is no extracellular membrane mechanism present but is v+vext when
+one is present.
+Also since i is an electrode current,
+positive values of i depolarize the cell. (Normally, positive membrane currents
+are outward and thus hyperpolarize the cell)
+ENDCOMMENT
+
+INDEPENDENT {t FROM 0 TO 1 WITH 1 (ms)}
+
+DEFINE NSTEP 3
+
+NEURON {
+	POINT_PROCESS SEClamp
+	ELECTRODE_CURRENT i
+	RANGE dur1, amp1, dur2, amp2, dur3, amp3, rs, vc, i
+}
+
+UNITS {
+	(nA) = (nanoamp)
+	(mV) = (millivolt)
+	(uS) = (microsiemens)
+}
+
+
+PARAMETER {
+	rs = 1 (megohm) <1e-9, 1e9>
+	dur1 (ms) 	  amp1 (mV)
+	dur2 (ms) <0,1e9> amp2 (mV)
+	dur3 (ms) <0,1e9> amp3 (mV)
+}
+
+ASSIGNED {
+	v (mV)	: automatically v + vext when extracellular is present
+	i (nA)
+	vc (mV)
+	tc2 (ms)
+	tc3 (ms)
+	on
+}
+
+INITIAL {
+	tc2 = dur1 + dur2
+	tc3 = tc2 + dur3
+	on = 0
+}
+
+BREAKPOINT {
+	SOLVE icur METHOD after_cvode
+	vstim()
+}
+
+PROCEDURE icur() {
+	if (on) {
+		i = (vc - v)/rs
+	}else{
+		i = 0
+	}
+}
+
+COMMENT
+The SOLVE of icur() in the BREAKPOINT block is necessary to compute
+i=(vc - v(t))/rs instead of i=(vc - v(t-dt))/rs
+This is important for time varying vc because the actual i used in
+the implicit method is equivalent to (vc - v(t)/rs due to the
+calculation of di/dv from the BREAKPOINT block.
+The reason this works is because the SOLVE statement in the BREAKPOINT block
+is executed after the membrane potential is advanced.
+
+It is a shame that vstim has to be called twice but putting the call
+in a SOLVE block would cause playing a Vector into vc to be off by one
+time step.
+ENDCOMMENT
+
+PROCEDURE vstim() {
+	on = 1
+	if (dur1) {at_time(dur1)}
+	if (dur2) {at_time(tc2)}
+	if (dur3) {at_time(tc3)}
+	if (t < dur1) {
+		vc = amp1
+	}else if (t < tc2) {
+		vc = amp2
+	}else if (t < tc3) {
+		vc = amp3
+	}else {
+		vc = 0
+		on = 0
+	}
+	icur()
+}
+

coreneuron/mechanism/membfunc.hpp

@@ -196,3 +196,11 @@ extern void hoc_malchk(void); /* just a stub */
 extern void* hoc_Emalloc(size_t);
 
 }  // namespace coreneuron
+
+/*
+ * at_time function declared outside of coreneuron namespace
+ * because it can be also called from ISPC code. That's also
+ * the reason for extern "C".
+ */
+#pragma acc routine seq
+extern "C" int at_time(coreneuron::NrnThread*, double);

coreneuron/mechanism/nrnoc_ml.ispc

@@ -166,4 +166,7 @@ struct NrnThread {
     void* mapping;
 };
 
-extern uniform double ispc_celsius;
+extern uniform double ispc_celsius;
+
+extern "C"
+int at_time(NrnThread* nt, varying double te);

coreneuron/network/cvodestb.cpp

@@ -105,11 +105,12 @@ void fixed_play_continuous(NrnThread* nt) {
     }
 }
 
-int at_time(NrnThread* nt, double te) {
+}  // namespace coreneuron
+
+extern "C" int at_time(coreneuron::NrnThread* nt, double te) {
     double x = te - 1e-11;
     if (x <= nt->_t && x > (nt->_t - nt->_dt)) {
         return 1;
     }
     return 0;
 }
-}  // namespace coreneuron