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doc/High-speed-serial updated for libserial rename
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 11 Mar 2020 17:28:11 +0000
parents 00805e7c4c45
children 2b4c3e0f73fc
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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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27
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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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35
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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-posix 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-linux 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-posix. 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-posix/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.
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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
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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.