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+Memristor with threshold
+* Y. V. Pershin, M. Di Ventra: "SPICE model of memristive devices with threshold", 
+* arXiv:1204.2600v1 [physics.comp-ph] 12 Apr 2012, 
+* http://arxiv.org/pdf/1204.2600.pdf
+
+* Parameter selection and plotting by
+* Holger Vogt 2012
+
+.param stime=10n
+.param vmax = 3
+
+* send parameters to the .control section
+.csparam stime={stime}
+.csparam vmax={vmax}
+
+Xmem 1 0 memristor
+* triangular sweep (you have to adapt the parameters to 'alter' command in the .control section)
+*V1 1 0 DC 0 PWL(0 0 '0.25*stime' 'vmax' '0.5*stime' 0 '0.75*stime' '-vmax' 'stime' 0)
+* sinusoidal sweep
+V1 0 1 DC 0 sin(0 'vmax' '1/stime')
+
+* memristor model with limits and threshold
+* "artificial" parameters alpha, beta, and vt. beta and vt adapted to basic programming frequency 
+* just to obtain nice results!
+* You have to care for the physics and set real values! 
+.subckt memristor plus minus PARAMS: Ron=1K Roff=10K Rinit=7.0K alpha=0 beta=20e3/stime Vt=1.6 
+Bx 0 x I='((f1(V(plus)-V(minus))> 0) && (V(x) < Roff)) ? {f1(V(plus)-V(minus))}: ((((f1(V(plus)-V(minus)) < 0) && (V(x)>Ron)) ? {f1(V(plus)-V(minus))}: 0)) '
+Vx x x1 dc 0
+Cx x1 0 1 IC={Rinit} 
+Rmem plus minus r={V(x)} 
+.func f1(y)={beta*y+0.5*(alpha-beta)*(abs(y+Vt)-abs(y-Vt))} 
+.ends
+
+* transient simulation same programming voltage but rising frequencies
+.control
+*** first simulation ***
+* approx. 100 simulation points
+let deltime = stime/100
+tran $&deltime $&stime uic
+* plot i(v1) vs v(1)
+*** you may just stop here ***
+* raise the frequency
+let newfreq = 1.1/stime
+let newstime = stime/1.1
+let deltime = newstime/100
+alter @V1[sin] [ 0 $&vmax $&newfreq ]
+tran $&deltime $&newstime uic
+* raise the frequency even more
+let newfreq = 1.4/stime
+let newstime = stime/1.4
+let deltime = newstime/100
+alter @V1[sin] [ 0 $&vmax $&newfreq ]
+tran $&deltime $&newstime uic
+* the 'programming' currents
+plot tran1.alli tran2.alli alli title 'Memristor with threshold: Internal Programming currents'
+* resistance versus time plot
+settype impedance xmem.x1 tran1.xmem.x1 tran2.xmem.x1
+plot xmem.x1 tran1.xmem.x1 tran2.xmem.x1 title 'Memristor with threshold: resistance'
+* resistance versus voltage (change occurs only above threshold!)
+plot xmem.x1 vs v(1) tran1.xmem.x1 vs tran1.v(1) tran2.xmem.x1 vs tran2.v(1) title 'Memristor with threshold: resistance'
+* current through resistor for all plots versus voltage
+plot i(v1) vs v(1) tran1.i(v1) vs tran1.v(1) tran2.i(v1) vs tran2.v(1) title 'Memristor with threshold: external current loops'
+.endc
+
+.end