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doc/CP2102-EEPROM-format: list maximum lengths of textual strings
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 27 Sep 2023 17:40:04 +0000
parents 2c135bde4dd0
children b36397a56bda
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1 The classic CP2102 single-channel USB-serial chip from Silabs (original CP2102,
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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2 not CP2102N and not CP2105) features an internal 1024-byte EEPROM, readable and
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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3 writable over USB via vendor-specific commands. Note that it is 1024 bytes,
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4 not bits, thus the internal EEPROM of CP2102 is 8 times bigger than that of its
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5 direct competitor FT232R. Compared to FTDI EEPROMs, the internal EEPROM of
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6 CP2102 exhibits these most obvious differences:
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7
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8 * The all-important baud rate remapping table (the signature feature of CP2102)
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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9 is included here;
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10
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11 * All USB descriptors are stored in full in the big EEPROM;
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12
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13 * Each of the 3 string descriptors (manufacturer, product, serial number) has a
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14 fixed area allocated for it, no pointer-to-string scheme like in FTDI's
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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15 EEPROMs;
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16
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17 * The logical structure of CP2102 EEPROM is byte-oriented - no 16-bit words as
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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18 elementary units.
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19
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20 Intel HEX format
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21 ================
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22
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23 Our FreeCalypso tools for working with CP2102 EEPROM are intended as a
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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24 replacement for a Python-language tool from 2014 named cp210x-program-1.0.
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25 This original Python tool exhibits a rather peculiar format for reading and
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26 writing raw EEPROM byte images: it is a variant of Intel HEX format with a
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27 peculiar base address. The size of the EEPROM is 1024 or 0x400 bytes, thus if
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28 one were to represent an image of this EEPROM in Intel HEX with 16 bytes per
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29 data record, the "obvious" address span would be from 0x0000 to 0x03F0.
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30 However, the records that comprise EEPROM images written by the Python tool and
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31 included as examples in the source tarball exhibit a different address range,
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32 spanning from 0x3600 to 0x39F0 - the base address is 0x3600.
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33
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34 As there is no obvious place where this 0x3600 base address could have come
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35 from, it is my (Mother Mychaela's) educated guess that the hex format adopted
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36 by the author of the Python tool could have originated from Silabs' own vendor
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37 tools, which are Windows-only and thus forbidden-ware in Themyscira temples.
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38
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39 Our fc-usbser-tools CP2102 EEPROM tools use the same Intel HEX format for EEPROM
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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40 images, with the same 0x3600 base address - thus we are consistent with the
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41 Python tool which we are directly replacing, and _possibly_ consistent with
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
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42 whatever sight-unseen, untouchable Windows tools might use.
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43
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44 EEPROM image analysis
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45 =====================
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46
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47 An EEPROM image that has been read out of a CP2102 chip that appears to be
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48 pristine (not modified after the chip left Silabs) is captured in
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49 artifacts/CP2102-std-baud, in the variant of Intel HEX described above.
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50
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51 Starting from the notes included in doc/cp210x.txt in the cp210x-program-1.0
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52 package and looking further at the EEPROM image with our own eyes, we get the
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53 following picture of EEPROM structure:
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54
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55 Address 0x3600, 320 or 0x140 bytes:
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56
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57 The baud rate table resides here, 32 entries of 10 bytes each. Each
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58 entry has the following format:
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59
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60 2 bytes: BRG reload value from Silabs AN205, big-endian
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61 2 bytes: timeout reload value from Silabs AN205, big-endian
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62 1 byte: prescaler value from Silabs AN205
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63 1 byte: reserved
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64 4 bytes: intended baud rate, little-endian
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65
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66 It is not clear if the "intended baud rate" field is actually used by
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67 the chip for anything - it may be a sort of comment.
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68
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69 Address 0x3740, 0xBF bytes:
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70
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71 Seems to be an unused area, all 00 bytes.
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72
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73 Address 0x37FF, 1 byte:
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74
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75 Python cp210x package notes indicate that this byte holds the part
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76 number, presumably the one returned by the vendor-specific command that
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77 retrieves it. This aspect remains to be tested at FreeCalypso HQ.
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78
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79 Address 0x3800, 4 bytes:
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80
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81 USB string descriptor 0, as in USB 2.0 spec Table 9-15.
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82
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83 Address 0x3804, 4 bytes:
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84
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85 Seems to be an unused area, all 00 bytes. It is possible that Silabs
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86 may have left extra room here to allow a longer string descriptor 0,
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87 listing more than one language code, but it does not make sense to me
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88 (Mother Mychaela) to list more than one supported language when there
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89 is no mechanism to return different strings in response to different
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90 language requests.
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91
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92 Address 0x3808, 255 or 0xFF bytes:
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93
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94 USB string descriptor for product ID string, in the full format of
83
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95 USB 2.0 spec Table 9-16. The maximum possible length of this string is
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96 126 UCS-2 characters.
82
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97
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98 Address 0x3907, 128 or 0x80 bytes:
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99
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100 USB string descriptor for serial number string, in the full format of
83
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101 USB 2.0 spec Table 9-16. The maximum possible length of this string is
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102 63 UCS-2 characters.
82
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103
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104 Address 0x3987, 1 byte:
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105
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106 Byte with value 0x02, purpose unknown.
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107
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108 Address 0x3988, 18 or 0x12 bytes:
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109
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110 USB device descriptor, as in USB 2.0 spec Table 9-8.
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111
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112 Address 0x399A, 9 bytes:
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113
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114 USB configuration descriptor, as in USB 2.0 spec Table 9-10.
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115
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116 Address 0x39A3, 9 bytes:
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117
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118 USB interface descriptor, as in USB 2.0 spec Table 9-12.
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119
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120 Address 0x39AC, 7 bytes:
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121
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122 USB endpoint descriptor, as in USB 2.0 spec Table 9-13.
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123
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124 Address 0x39B3, 7 bytes:
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125
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Mychaela Falconia <falcon@freecalypso.org>
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126 USB endpoint descriptor, as in USB 2.0 spec Table 9-13.
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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127
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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128 Address 0x39BC, 7 bytes:
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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129
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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130 Seems to be an unused area, all 00 bytes.
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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131
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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132 Address 0x39C3, 60 or 0x3C bytes:
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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133
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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134 USB string descriptor for manufacturer ID string, in the full format of
83
b3989befca98 doc/CP2102-EEPROM-format: list maximum lengths of textual strings
Mychaela Falconia <falcon@freecalypso.org>
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135 USB 2.0 spec Table 9-16. The maximum possible length of this string is
b3989befca98 doc/CP2102-EEPROM-format: list maximum lengths of textual strings
Mychaela Falconia <falcon@freecalypso.org>
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136 29 UCS-2 characters.
82
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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137
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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138 Address 0x39FF, 1 byte:
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
parents:
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139
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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140 0xFF is the value I find here, in the EEPROMs of CP2102-based devices
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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141 I have on hand. Python cp210x package notes say that this byte is lock
2c135bde4dd0 doc/CP2102-EEPROM-format: new article
Mychaela Falconia <falcon@freecalypso.org>
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142 status, with 0xFF meaning unlocked and 0xF0 meaning locked.