annotate FC-modem-family @ 21:69ee60206c53

FC-modem-family article written
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 23 Oct 2019 00:11:37 +0000
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children 1e76655a44bd
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69ee60206c53 FC-modem-family article written
Mychaela Falconia <falcon@freecalypso.org>
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1 I, Mother Mychaela, have a vision for a family of FreeCalypso modem products in
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2 different physical form factors, possibly with variations in offered
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3 functionality, all sharing the same fcmodem firmware build target - meaning that
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4 the same FreeCalypso fw built for the fcmodem target (a proposed successor to
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5 the current fcdev3b build target) will run on all FC modem hw products,
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6 requiring some commonalities across the entire product family but also
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7 supporting some variations. The following design constraints are hereby being
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8 laid down in order to make this common fcmodem firmware possible:
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9
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10 * All FC modem products will be either triband or quadband: triband if the
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11 current Openmoko-based RFFE is kept as is (the safer route with less effort
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12 and design labor cost) or quadband if the customer commissioning a modem
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13 product is more adventurous and willing to pay more and take greater risks in
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14 order to get full quadband capability. Please refer to the Quadband-ideas
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15 article for further details.
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16
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17 * Flash chip type autodetection in both fc-loadtool and firmware FFS driver code
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18 allows different flash chip types to be used depending on business needs,
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19 parts availability and the available physical space on the PCB. Flash
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20 capacity can range from 4 MiB (minimum fw requirement) to 16 MiB (maximum
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21 Calypso addressing capability); if a 16 MiB flash chip with two 8 MiB chip
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22 select banks is used, then the second bank must be on nCS2 (like on FCDEV3B),
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23 not on any other Calypso chip select.
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24
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25 * External RAM (XRAM) capacity can range from 512 KiB (minimum fw requirement)
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26 to 8 MiB (maximum Calypso addressing capability), but it will always be on
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27 nCS1, not on any other Calypso chip select.
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28
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29 * The extra logic voltage level translating buffer IC that was introduced on
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30 FCDEV3B V2 in order to make flash reset timing compatible with S71PL129N flash
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31 may or may not be included. The Mother's preference is to include it when
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32 possible in order to increase the repertoire of usable flash chips, but if a
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33 given design allows no PCB space for this extra little IC, then it will be
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34 acceptable to go back to driving the flash reset line with FDP and require
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35 the use of some flash chip other than S71PL129N - if the same footprint needs
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36 to be kept and/or maximum flash and RAM capacity is desired, S71PL129J can be
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37 used instead.
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38
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39 MCSI
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40 ====
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41
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42 It is envisioned that most FC modem products will provide a digital voice
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43 interface via MCSI as a major feature, but some may not. If a given product
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44 does feature MCSI, pull-down resistors for MCSI_CLK and MCSI_FSYNCH will need
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45 to be included on the modem module itself, so that the user always sees them as
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46 non-floating outputs from the modem. MCSI_RXD will be a pure input without
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47 on-board pull-up or pull-down resistors, i.e., the user will be responsible for
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48 tying it off or pulling it to a stable logic level if it is unused.
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49
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50 However, if a given product does not feature MCSI, then it will be an unwelcome
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51 burden to require extra pull-down resistors on the PCB, hence a different
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52 provision is planned. If and when we ever produce a FreeCalypso modem without
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53 MCSI, we will program a /etc/fc-pinmux file in FFS telling our fcmodem firmware
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54 to switch the MCSI pins into GPIO mode and to drive all 4 of them as dummy
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55 outputs, eliminating floating I/O pins without any effort from the hardware
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56 side.
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57
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58 Why would a FreeCalypso modem product not feature MCSI? Two use cases are
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59 envisioned here:
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60
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61 * The new GTA02-like smartphone motherboard thought experiment - see the
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62 corresponding section at the end of this article;
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63
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64 * If we ever produce a FreeCalypso modem in the form factor of a USB dongle for
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65 laptops, it would be very easy to incorporate an FT2232x adapter presenting
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66 both Calypso UARTs to the USB host, but adding a USB-to-MCSI bridge would
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67 entail much greater complexity, hence it may make sense to omit MCSI for this
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68 product and instead provide an analog headset jack for voice calls.
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69
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70 GPIO
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71 ====
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72
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73 Our firmware configures Calypso GPIO 3 as an input; GPIOs 0-2 and those
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74 multifunction pins which are unused and configured as GPIOs (TSPDI/IO4,
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75 BCLKX/IO6, MCUEN1/IO8 and MCUEN2/IO13) are configured as outputs. Board wiring
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76 must be compatible with these directions: those pins which our fw drives as
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77 outputs can be simply left unconnected, while GPIO 3 needs to be sensibly driven
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78 or tied off as explained below.
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79
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80 Openmoko's modem puts out an application processor wakeup signal (asserted by
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81 the fw when the modem needs to send something to the host but is blocked by CTS
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82 being held high) on GPIO 1. In order to have full feature parity, our FC modems
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83 will provide an equivalent signal as well, but we are moving it to GPIO 0
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84 because in FreeCalypso GPIO 1 is already taken for the loudspeaker control
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85 signal like on TI's D-Sample and Leonardo boards. Our firmware's complete GPIO
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86 usage thus becomes:
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87
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88 GPIO0: application processor wakeup signal
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89 GPIO1: loudspeaker amplifier on/off control
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90 GPIO2: DCD modem control output
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91 GPIO3: DTR modem control input
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92 all others: dummy outputs
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93
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94 Because GPIO3/DTR is an input, it needs to be either sensibly driven or tied
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95 off. On our current FCDEV3B this line has a 100 kOhm pull-down resistor to GND,
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96 but it is not accessible to the user in any way - this design aspect was copied
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97 from TI's Leonardo board before I thought better. Most of our future FC modem
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98 products are planned to have DTR as a user input signal (which the user can tie
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99 off it is not used or needed in a given application), but if there is no DTR
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100 user input signal on a given modem product, then GPIO3 will be grounded inside
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101 the modem to keep the firmware happy.
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102
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103 Calypso's unused DSR_MODEM/LPG pin was left unconnected in Openmoko's version
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104 but on our FCDEV3B it is tied to GND on the board. Future FC modem family
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105 boards will also need to have this pin tied to GND: our firmware leaves this
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106 pin in its default power-up config of DSR_MODEM input and does not change it to
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107 LPG output, thus leaving it unconnected would cause it to float.
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108
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109 Extreme case: a new GTA02-like smartphone motherboard
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110 =====================================================
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111
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112 A rather extreme thought-experiment test case for the FC modem family idea
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113 presented in this article is this thought: suppose we wanted to produce a new
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114 GTA02-like smartphone motherboard, bringing Openmoko's dream back from the dead.
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115 We could try to make a verbatim clone of GTA02-MB-A6 from OM, but for both
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116 technical and political reasons it would be desirable to make a few changes:
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117
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118 * Making a strictly verbatim clone of GTA02-MB-A6 would mean populating a
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119 Samsung K5A3281CTM memory chip onto OM's PCB footprint which does not really
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120 fit this chip properly. Changing the PCB footprint to fit K5A32xx more
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121 properly would not be easy because there are signal traces in the PCB spots
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122 where the extra mounting balls for K5A32xx would need to go. The approach
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123 that seems most naturally sensible would be to populate a better-fitting
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124 memory chip, such as Spansion S71PL129JB0BAW9Z for which that footprint was
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125 properly made - but then the result would no longer be a verbatim clone of
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126 OM's version, and our gtamodem target configuration would no longer fit. And
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127 at that point it would make the most sense to stop following the gtamodem
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128 config and to make it an fcmodem family product instead.
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129
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130 * It would be politically preferable if the new GTA02-like motherboard would run
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131 fcmodem firmware rather than gtamodem, making a clean break from those parts
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132 of OM's troubled history which caused them to be Closedmoko during certain
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133 years.
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134
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135 Hence the technical challenge: would it be possible to make a GTA02 derivative
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136 with absolutely minimal changes that would run fcmodem firmware instead of
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137 legacy mokoN or FC fw builds for the gtamodem target? The answer is yes, and
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138 here is the recipe - note that *no* new components need to be added to the
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139 extremely tight PCB layout:
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140
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141 * Move the 2nd flash chip select connection from nCS4 to nCS2 on the Calypso;
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142
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143 * Move the AP wakeup interrupt line from GPIO1 to GPIO0 to match fcmodem
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144 firmware GPIO assignments;
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145
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146 * Take the useless INT0 AP-to-BP connection line present in the GTA02 original
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147 and move it to Calypso GPIO3, providing fcmodem firmware with the DTR input
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148 it expects;
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149
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150 * Ground Calypso's unused DSR_MODEM/LPG signal, causing it to not float when
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151 our fcmodem firmware keeps the power-up default of DSR_MODEM function;
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152
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153 * Program /etc/fc-pinmux in FreeCalypso FFS to tell the firmware that there is
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154 no MCSI on this board, so that all 4 MCSI signals will become GPIO dummy
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155 outputs and won't float.
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156
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157 Some additional changes which don't affect the firmware one way or the other:
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158
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159 * Remove R1003 and R1004 0Rs which seem to be the root cause of bug #1024 and
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160 replace them with solid PCB traces;
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161
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162 * Up C1009 from 10 uF to 22 uF as additional guard from bug #1024;
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163
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164 * Ground Calypso's unused input RXIR_IRDA to keep it from floating - this input
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165 cannot be changed to an output under firmware control without breaking other
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166 functionality.
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167
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168 At the same time it would also make a lot of sense to remove the Glamo graphics
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169 decelerator from the AP subsystem, connecting the LCD directly to the Samsung AP
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170 like it was on GTA01 - thus from the AP software perspective the new motherboard
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171 would also become its own new platform just like the modem, very similar to the
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172 GTA02 original, but not strictly identical, featuring some improvements instead.
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173
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174 IP rights and FreeCalypso brand integrity
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175 =========================================
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176
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177 The present vision of having the same fcmodem firmware support all FreeCalypso
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178 modem products applies ONLY to those hardware products which are designed and
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179 physically produced by Mother Mychaela or one of her business entities. We will
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180 NOT provide any support whatsoever for any pirate hardware products that may be
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181 produced by any persons or entities who are making, have made or are planning to
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182 make inappropriate use of any of our published or unpublished board design
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183 files.
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184
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185 In the event that some third party buys a legitimate license to produce their
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186 own derived works based on FreeCalypso hardware designs, we may provide
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187 technical support to that specific customer as per our agreement with them, but
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188 any such support will be customer-specific: our generic, non-customer-specific
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189 public FreeCalypso firmwares will only ever support our own official
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190 Falconia/FreeCalypso modem products, and should not be expected to support any
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191 customized products made by or at the request of particular third parties.