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annotate doc/High-speed-serial @ 416:30f6d1c32c6f

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doc/Flash-boot-defect article removed (no longer relevant) This article is no longer relevant because the issue in question only affected one (1) defective FCDEV3B board which was not and never will be sold.
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 26 Oct 2018 07:11:08 +0000
parents 00805e7c4c45
children 9edb7c07bb29
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1 The highest baud rate supported by "standard" PC serial ports is 115200 bps,
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2 but Calypso UARTs can go quite a bit faster. Being clocked with 13 MHz (a
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3 standard frequency in the GSM world), these UARTs can produce non-standard
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4 (outside of the GSM world) baud rates of 203125, 406250 and 812500 bps. Even
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5 though these high baud rates aren't supported by "standard" RS-232 serial ports
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6 on PCs, they *are* supported by some of the better USB to serial adapters,
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7 namely CP2102 (USB to single UART) and the FT2232x family (USB to two UARTs).
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8 The Pirelli DP-L10 phone supported by FreeCalypso host tools has a CP2102 built
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9 in, the officially recommended 2.5 mm headset jack USB-serial cables for working
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10 with Motorola C1xx and Openmoko GTA01/02 phones also use CP2102 adapters,
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11 whereas our FreeCalypso development boards (FCDEV3B) are typically used with an
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12 FT2232D or other FT2232x USB to dual UART adapter.
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14 FreeCalypso tools can use these high serial baud rates in the following ways:
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16 * When you use fc-loadtool to dump and program GSM device flash memory
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17 (flashing firmware images), the transfers get annoyingly slow at 115200 baud
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18 if you have to do it a lot. Switching to 406250 or even better 812500 baud
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19 makes them go considerably faster.
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21 * Some of our target devices have large enough RAM to execute a GSM firmware
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22 image entirely from RAM without flashing - very handy for development and
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23 experimentation. The tool used to run these RAM-based images is fc-xram,
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24 and it also supports the option of using high serial baud rates for the image
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25 transfer for the same reason: repeatedly transferring >2 MiB images over
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26 115200 baud gets tiresome.
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28 * If you are building your own FreeCalypso-based or TI-based GSM firmware in a
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29 special non-standard configuration, you can make it run its RVTMUX interface
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30 at 406250 or 812500 baud. We used this trick when we tried to make TCS211
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31 with D-Sample-targeting UI (176x220 pix LCD, 16 bits per pixel) send its
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32 virtual LCD raster blits out the serial port. Our rvtdump and rvinterf
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33 utilities support this mode of operation by providing options to select
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34 different baud rates.
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36 Recent changes for better FTDI adapter support
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37 ==============================================
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38
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39 There is one fundamental difference between the way CP2102 adapters support
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40 non-standard baud rates (like the high GSM baud rates of interest to us) and
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41 the way in which FTDI adapters support them. CP2102 chips have a built-in
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42 EEPROM that contains (among other things) a 32-entry table in which the
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43 supported serial baud rates are programmed, and the programming of this EEPROM
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44 effects a remapping: a Linux userspace process can request B230400, B460800 or
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45 B921600 from termios, but magically get 203125, 406250 or 812500 as the actual
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46 resulting serial baud rate instead. In contrast, FTDI adapters have no such
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47 magic remapping mechanism in hardware, thus in order to get 203125, 406250 or
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48 812500 baud with an FTDI adapter, the userspace process has to explicitly
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49 request these special baud rates from the serial driver in the kernel, and
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50 doing the latter requires foregoing the standard termios API and using Linux-
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51 specific <asm/...> header files and raw ioctl calls instead.
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52
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53 When support for high GSM baud rates was first added to FreeCalypso host tools
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54 back in 2013, there was no need to support the more difficult FTDI adapters as
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55 the easier to work with CP2102 was fully sufficient for our needs, hence our
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56 original FC host tools implementation required "magic" baud rate remapping
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57 somewhere below, usually in form of CP2102 EEPROM programming but also possibly
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58 by way of a hacky patch to the ftdi_sio driver in the Linux kernel to achieve
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59 the same effect with rarely-needed FTDI adapters.
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60
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61 The situation has changed with the introduction of our own FreeCalypso
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62 development boards (currently FCDEV3B, possibly others in the future) which
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63 bring out both Calypso UARTs to the user, rather than just one. The most
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64 convenient serial adapters for working with these dual UARTs are FT2232x (our
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65 current official adapter is FT2232D), thus we now have a strong need to support
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66 the use of these FTDI adapters, including the use of high GSM baud rates, in a
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67 manner which does not fight against the mainline Linux kernel.
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68
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69 In a radical change from fc-host-tools-r6 and earlier, the present version of
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70 FreeCalypso host tools uses new libserial code that differs from the old code
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71 as follows:
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72
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73 * Linux-specific <asm/...> headers are used instead of <termios.h>;
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74
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75 * Linux-specific raw ioctl calls are used instead of tcsetattr() for serial
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76 port setup;
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77
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78 * When the user requests 203125, 406250 or 812500 baud, these are the actual
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79 baud rates requested from the kernel, not 230400/460800/921600 baud.
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80
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81 This change is expected to have no adverse effect on the existing CP2102 users,
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82 as the cp210x driver in Linux appears to cope fine with the strange baud rate
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83 requests from userspace and the correct CP2102 EEPROM baud rate entry still
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84 gets selected (tested on Slackware 13.37 and Debian 9), but when working with
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85 FTDI adapters such as our FT2232D adapter for the FCDEV3B this change makes the
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86 high GSM baud rates work without needing the dirty kernel patch which the
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87 Mother has been using up until now.
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88
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89 Support for other Unix flavors
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90 ==============================
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91
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92 The serial port handling code for all of FC host tools has been factored out
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93 into a common library called libserial. We have two versions of libserial:
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94
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95 * libserial-orig uses the standard and presumably portable termios API, but
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96 requires "magic" remapping of baud rates by some invisible genie below (like
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97 CP2102 EEPROM programming) in order to get 203125/406250/812500 baud.
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98
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99 * libserial-newlnx uses Linux-specific header files and raw ioctl calls to
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100 request the actual desired baud rates.
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101
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102 If you would like to run FreeCalypso host tools under FreeBSD, illumos or some
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103 other alternative-to-Linux OS, you have two basic choices:
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104
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105 * If you wish to use high GSM baud rates with non-remapping FTDI adapters or
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106 other serial interfaces which support the baud rates in question without
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107 remapping, you will need to figure out how to request non-standard serial
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108 baud rates from the underlying drivers under your OS, and create your own
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109 version of libserial ported to use that method.
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110
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111 * If you don't need high GSM baud rates or need them only with CP2102 adapters
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112 which "magically" remap them, you should be able to use libserial-orig. You
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113 can also completely remove the entries for the high GSM baud rates from
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114 libserial-orig/baudtab.c if you don't need these high baud rates and your
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115 version of termios does not have B230400/B460800/B921600 baud rate constants.
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116
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117 It is assumed that any system on which someone may desire to run our FC host
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118 tools supports at least 115200 baud. The Mother remembers the days when this
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119 baud rate was considered very high and non-standard and even has some of those
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120 lovely old systems still running; fc-loadtool and friends going through the
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121 Calypso boot ROM (not through Compal's bootloader) can be made to work with a
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122 host system whose UARTs max out at 19200 baud, but most Calypso GSM device
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123 firmwares including our own use the 115200 baud rate.
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124
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125 Using CP2102 adapters with Mot C1xx and Openmoko phones
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126 =======================================================
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127
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128 As already mentioned above, CP2102 chips have a built-in EEPROM that contains
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129 (among other things) a 32-entry table in which the supported serial baud rates
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130 are programmed. In order to support the special GSM baud rates, these rates
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131 need to be added to that table, displacing some other entries. The convention
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132 established by the Pirelli DP-L10 phone (has a CP2102 built in and programmed
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133 at the factory for GSM baud rates) is that 203120 baud takes the place of
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134 230400, 406250 takes the place of 460800, and 812500 takes the place of 921600.
0
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135
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136 Because you need a special cable anyway to make the necessary physical
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137 connection to the debug/programming serial port presented on a 2.5 mm headset
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138 jack, you will probably be buying the requisite cable from a specialized
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139 professional vendor. In that case it is that vendor's responsibility to sell
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140 you the cable with the CP2102 chip already programmed with GSM baud rates:
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141 because the physical construction of the cable (2.5 mm headset jack on the
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142 serial end) makes it specific to GSM devices, and all known GSM devices use a
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143 13 MHz clock or some integer multiple thereof, it is pointless for a
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144 physically-GSM-specific cable to be set up for 230400/460800/921600 baud when
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145 all known GSM devices will need 203125/406250/812500 baud instead.
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146
258
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147 If you are making a CP2102-based serial cable yourself (either for your own
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148 personal use or professionally/commercially), please follow these instructions
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149 for baud rate programming:
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150
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151 http://osmocom.org/projects/baseband/wiki/HardwareCP210xTutorial
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152
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153 If you follow the procedure given on that page, your CP2102 will be programmed
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154 the same way as the one in the Pirelli DP-L10 (Foxconn's original factory
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155 programming).
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156
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157 Using adapters built into phones
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158 ================================
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159
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160 The Calypso chip has no native USB capabilities, thus if a Calypso phone
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161 presents a USB charging+data port to the user, it must have a USB to serial
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162 converter built in. The only phone we currently know of that does this is
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163 Pirelli DP-L10, and its built-in USB-serial adapter chip is CP2102. It has
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164 already been programmed with the correct GSM baud rates on Foxconn's original
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165 production line, thus one can always use 812500 baud with FreeCalypso tools on
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166 this phone and it will Just Work.