annotate doc/High-speed-serial @ 993:d92e4aadeeb3

target-utils/c139explore: a cleaner way of setting the fixed UART base address
author Mychaela Falconia <falcon@ivan.Harhan.ORG>
date Wed, 30 Dec 2015 22:12:35 +0000
parents f4da3071dd61
children
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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. When
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5 working with Motorola C1xx and Openmoko GTA01/02 phones which present a debug
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6 and programming serial interface on a 2.5 mm headset jack, one can make use of
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7 these high serial baud rates by using a USB to headset jack programming cable
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8 based on one of the better USB-serial chips that can support these GSM special
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9 baud rates well above 115200. The two USB-serial chips that are known to work
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10 in this manner are CP2102 and FTDI, although each of the two requires its own
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11 special quirks described below. Other USB to serial cables use chips which
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12 don't support the high baud rates in question, and therefore are limited to
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13 115200 baud max like a "standard" PC serial port.
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14
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15 FreeCalypso tools can use these high serial baud rates in the following ways:
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16
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17 * When you use fc-loadtool to dump and program GSM device flash memory
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18 (flashing firmware images), the transfers get annoyingly slow at 115200 baud
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19 if you have to do it a lot. Switching to 406250 or even better 812500 baud
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20 makes them go considerably faster.
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21
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22 * Some of our target devices have large enough RAM to execute a GSM firmware
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23 image entirely from RAM without flashing - very handy for development and
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24 experimentation. The tool used to run these RAM-based images is fc-xram,
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25 and it also supports the option of using high serial baud rates for the image
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26 transfer for the same reason: repeatedly transferring 1.5 MiB images over
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27 115200 baud gets tiresome.
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28
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29 * If you are building your own GSM firmware (either FC GSM fw or one of our
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30 TCS211-based hacks), you can make it run its RVTMUX interface at 406250 or
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31 812500 baud. We used this trick when we tried to make TCS211 with D-Sample-
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32 targeting UI (176x220 pix LCD, 16 bits per pixel) send its virtual LCD raster
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33 blits out the serial port. Our rvtdump and rvinterf utilities support this
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34 mode of operation by providing options to select different baud rates.
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35
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36 Using CP2102 adapters
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37 =====================
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38
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39 CP2102 chips have a built-in EEPROM that contains (among other things) a
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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40 32-entry table in which the supported serial baud rates are programmed. In
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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41 order to support the special GSM baud rates, these rates need to be added to
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42 that table, displacing some other entries. The convention established by the
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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43 Pirelli DP-L10 phone (has a CP2102 built in and programmed at the factory for
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44 GSM baud rates) is that 203120 baud takes the place of 230400, 406250 takes the
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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45 place of 460800, and 812500 takes the place of 921600.
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46
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47 Because you need a special cable anyway to make the necessary physical
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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48 connection to the debug/programming serial port presented on a 2.5 mm headset
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49 jack, you will probably be buying the requisite cable from a specialized
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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50 professional vendor. In that case it is that vendor's responsibility to sell
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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51 you the cable with the CP2102 chip already programmed with GSM baud rates:
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52 because the physical construction of the cable (2.5 mm headset jack on the
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53 serial end) makes it specific to GSM devices, and all known GSM devices use a
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54 13 MHz clock or some integer multiple thereof, it is pointless for a
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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55 physically-GSM-specific cable to be set up for 230400/460800/921600 baud when
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56 all known GSM devices will need 203125/406250/812500 baud instead.
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57
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58 If you making a CP2102-based serial cable yourself (either for your own personal
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59 use or professionally/commercially), please follow these instructions for baud
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60 rate programming:
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61
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62 http://bb.osmocom.org/trac/wiki/Hardware/CP210xTutorial
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63
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64 If you follow the procedure given on that page, your CP2102 will be programmed
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65 the same way as the one in the Pirelli DP-L10 (Foxconn's original factory
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66 programming).
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67
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68 The serial port handling code in FreeCalypso host tools is written to request
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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69 B230400 from termios when 203125 baud is desired, likewise B460800 for 406250
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70 baud and B921600 for 812500 baud. Therefore, if you have a CP2102-based cable
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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71 with properly programmed EEPROM, everything will Just Work.
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72
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73 Using FTDI adapters
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74 ===================
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75
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76 Unlike CP2102, FTDI adapters don't require any non-volatile EEPROM programming
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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77 for GSM baud rates, but they have a different pain point - arguably a worse one
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78 - that is entirely a software issue. The API which the Linux kernel provides
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79 to userspace applications for opening and configuring serial ports provides no
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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80 clean, sensible way for an application to request a particular baud rate that
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81 is not in the predefined-once-and-for-all list, and to make it unambiguous to
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82 the in-kernel driver exactly what it wants.
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83
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84 The method provided by the ftdi_sio driver in the standard Linux kernel is
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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85 gross, and I (Space Falcon) refuse to use it. The serial port handling code in
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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86 FreeCalypso host tools is written for the clean CP2102 way, and is *not* muddied
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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87 with the muck that would be necessary to get the high GSM baud rates with an
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88 unpatched ftdi_sio driver. Therefore, if you would like to use one of the high
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89 GSM baud rates with FreeCalypso with an FTDI adapter, you will need to dirty
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90 your Linux host system with a hacky kernel patch. The patch provided in
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91 linux-2.6.37.6-ftdi_sio.c.patch (made against Linux 2.6.37.6, which is what I
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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92 use - came with Slackware 13.37 - adapt as necessary for your kernel version)
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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93 makes the ftdi_sio driver behave like a GSM-programmed CP2102: termios B230400
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94 turns into 203125 baud, B460800 turns into 406250 and B921600 turns into 812500.
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95
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96 This patch won't break other software (*cough* osmocom-bb *cough*) that does
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97 use the "standard" ftdi_sio way of requesting high GSM baud rates, i.e., both
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98 ways of selecting these baud rates should still work, but if you have other
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99 (non-GSM) serial devices on the same system which need 230400, 460800 or 921600
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100 baud, those will break.
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101
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102 Using adapters built into phones
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103 ================================
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104
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105 The Calypso chip has no native USB capabilities, thus if a Calypso phone
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106 presents a USB charging+data port to the user, it must have a USB to serial
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Mychaela Falconia <falcon@ivan.Harhan.ORG>
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107 converter built in. The only phone we currently know of that does this is
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108 Pirelli DP-L10, and its built-in USB-serial adapter chip is CP2102. It has
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109 already been programmed with the correct GSM baud rates on Foxconn's original
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110 production line, thus one can always use 812500 baud with FreeCalypso tools on
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111 this phone and it will Just Work.