// Frequency divider model that works with SpectreRF
//
// Version 1a, 11 Apr 03
//
// Ken Kundert
//
// Downloaded from The Designer's Guide (www.designers-guide.org).
// Post any questions on www.designers-guide.org/Forum.

`include "disciplines.vams"
`include "constants.vams"

// Divide by N counter
//
// Contains no hidden state, and so it works with SpectreRF
//
// Will fail if N is large, say greater than 1000.  Can tighten tolerances
// to increase the range of N, or you can break N into integer factors and
// implement the divide by N with several counters in series.


module divideByN(pout, nout, pin, nin);
output pout, nout;
input pin, nin;
electrical pin, nin, pout, nout;
electrical vg, chg, hold;
parameter integer n = 2 from [2:inf);	// divide ratio
parameter integer nhi = 1 from [1:n-1];	// number of hi counts per output pulse
parameter integer dir = 1 from [-1:1] exclude 0;
					// +1 for rising edge triggered
					// -1 for falling edge triggered
parameter real tt=0.01;			// output transition time
parameter real vdd=5, vss=0;		// defines output high and low levels
parameter real thresh=(vdd+vss)/2;	// input threshold is at midpoint 
real vmax;
integer out, count;
real g;
real vchg;

    analog begin
	vmax = max(abs(vdd),abs(vss));
	g = 5*ln(n);
	@(cross(V(pin,nin) - thresh, 0));

	// hold capacitor with capacitance tt
	I(vg,hold) <+ tt*ddt(V(vg,hold));

	// clamp hold capacitor during dc analysis
	if (analysis("static"))
	    I(vg,hold) <+ V(vg,hold);

	// ideal op amp
	V(hold): V(vg) == 0;

	// titrating capacitor with capacitance tt
	I(chg) <+ tt*ddt(V(chg));

	// titrating capacitor is precharged when input is high
	// if V(hold) is in range,
	//     precharge to vmax/n volts to increment hold cap one level,
	// else
	//     precharge with equal but opposite the charge on the hold cap
	//     in order to reset hold cap to zero
	if ((V(hold) < vmax*(1.5/n-1)) && (dir*V(pin,nin) < dir*thresh)) begin
	    vchg = V(hold);
	end else begin
	    vchg = vmax/n;
	end

	// determine the count
	count = -n*V(hold)/vmax;

	// charge or discharge when input is high through g Siemen switches
	if (dir*V(pin,nin) > dir*thresh) begin
	    // dump titrating charge onto hold cap
	    I(vg,chg) <+ g*V(vg,chg);
	end else begin
	    // precharge titrating capacitor
	    I(chg) <+ g*(V(chg)-vchg);
	    // use discrete-level feedback to correct for any errors in hold chg
	    I(vg) <+ -n*V(hold)/vmax - count;
	end

	// output pulse when V(in) is high and V(hold) below threshold
	out = count >= (n - nhi);
	V(pout,nout) <+ transition(vss + out*(vdd-vss), 0.0, tt, tt);

    end
endmodule




// used to check the divider above
// connect in to input of divider and reset to output of divider
// set parameter values in the same way as on the divider
// this module has hidden state and so cannot be used with SpectreRF

module counter(reset, in);
input in, reset;
electrical in, reset;
parameter real vdd=5, vss=0;
parameter real thresh=(vdd+vss)/2;
parameter integer dir = 1 from [-1:1] exclude 0;
integer n;

    analog begin
	@(cross(V(in) - thresh, dir))
	    n = n + 1;
	@(cross(V(reset) - thresh, dir)) begin
            $strobe( "count = %d\n", n );
            n = 0;
        end
    end
endmodule