1 /*
2  * This file is part of libsidplayfp, a SID player engine.
3  *
4  * Copyright 2011-2016 Leandro Nini <drfiemost@users.sourceforge.net>
5  * Copyright 2007-2010 Antti Lankila
6  * Copyright 2004,2010 Dag Lem <resid@nimrod.no>
7  *
8  * This program is free software; you can redistribute it and/or modify
9  * it under the terms of the GNU General Public License as published by
10  * the Free Software Foundation; either version 2 of the License, or
11  * (at your option) any later version.
12  *
13  * This program is distributed in the hope that it will be useful,
14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
16  * GNU General Public License for more details.
17  *
18  * You should have received a copy of the GNU General Public License
19  * along with this program; if not, write to the Free Software
20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
21  */
22 
23 #include "Dac.h"
24 
25 namespace reSIDfp
26 {
27 
Dac(unsigned int bits)28 Dac::Dac(unsigned int bits) :
29     dac(new double[bits]),
30     dacLength(bits)
31 {}
32 
~Dac()33 Dac::~Dac()
34 {
35     delete [] dac;
36 }
37 
getOutput(unsigned int input) const38 double Dac::getOutput(unsigned int input) const
39 {
40     double dacValue = 0.;
41 
42     for (unsigned int i = 0; i < dacLength; i++)
43     {
44         if ((input & (1 << i)) != 0)
45         {
46             dacValue += dac[i];
47         }
48     }
49 
50     return dacValue;
51 }
52 
kinkedDac(ChipModel chipModel)53 void Dac::kinkedDac(ChipModel chipModel)
54 {
55     const double R_INFINITY = 1e6;
56 
57     // Non-linearity parameter, 8580 DACs are perfectly linear
58     const double _2R_div_R = chipModel == MOS6581 ? 2.20 : 2.00;
59 
60     // 6581 DACs are not terminated by a 2R resistor
61     const bool term = chipModel == MOS8580;
62 
63     // Calculate voltage contribution by each individual bit in the R-2R ladder.
64     for (unsigned int set_bit = 0; set_bit < dacLength; set_bit++)
65     {
66         double Vn = 1.;                   // Normalized bit voltage.
67         double R = 1.;                    // Normalized R
68         const double _2R = _2R_div_R * R; // 2R
69         double Rn = term ?                // Rn = 2R for correct termination,
70                     _2R : R_INFINITY;     // INFINITY for missing termination.
71 
72         unsigned int bit;
73 
74         // Calculate DAC "tail" resistance by repeated parallel substitution.
75         for (bit = 0; bit < set_bit; bit++)
76         {
77             Rn = (Rn == R_INFINITY) ?
78                  R + _2R :
79                  R + (_2R * Rn) / (_2R + Rn); // R + 2R || Rn
80         }
81 
82         // Source transformation for bit voltage.
83         if (Rn == R_INFINITY)
84         {
85             Rn = _2R;
86         }
87         else
88         {
89             Rn = (_2R * Rn) / (_2R + Rn); // 2R || Rn
90             Vn = Vn * Rn / _2R;
91         }
92 
93         // Calculate DAC output voltage by repeated source transformation from
94         // the "tail".
95 
96         for (++bit; bit < dacLength; bit++)
97         {
98             Rn += R;
99             const double I = Vn / Rn;
100             Rn = (_2R * Rn) / (_2R + Rn); // 2R || Rn
101             Vn = Rn * I;
102         }
103 
104         dac[set_bit] = Vn;
105     }
106 
107     // Normalize to integerish behavior
108     double Vsum = 0.;
109 
110     for (unsigned int i = 0; i < dacLength; i++)
111     {
112         Vsum += dac[i];
113     }
114 
115     Vsum /= 1 << dacLength;
116 
117     for (unsigned int i = 0; i < dacLength; i++)
118     {
119         dac[i] /= Vsum;
120     }
121 }
122 
123 } // namespace reSIDfp
124