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doc/Linux-DTR-RTS-flaw: point to new location of this article
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 11 Dec 2023 19:37:20 +0000
parents 4f5abad5dd40
children
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1 How to make a safe JTAG adapter out of a generic unbuffered FT2232x board
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2 =========================================================================
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3
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4 [This guide was originally written in 2019; it has been updated slightly for
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5 the move of FTDI EEPROM utilities to fc-usbser-tools, plus other minor updates
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6 for the situation in 2023, but the principal ideas are all from 2019.]
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7
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8 Among the FOSS community of tinkerers who use OpenOCD to operate on the JTAG
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9 interfaces of various hardware targets, one of the most common JTAG adapter
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10 choices (if not the most common) is to use some adapter gadget based on an FTDI
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11 chip, most commonly one of FT2232x variants. However, a major distinction needs
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12 to be drawn between specialized purpose-made JTAG adapter products which just
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13 happen to use an FT2232x chip internally, versus generic FT2232x breakout boards
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14 which the user wires up for JTAG on his or her own.
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15
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16 In an ideal world, using a purpose-made buffered JTAG adapter (one that has a
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17 buffer inserted between FT2232x I/O pins and the target connection interface)
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18 would be strongly preferable for a whole host of reasons. However, to this
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19 author's disappointment, there are very few community vendors who make such
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20 adapters, and when I last looked in 2019, I was NOT able to find any high-
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21 quality buffered JTAG adapter which could be readily bought and which comes
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22 with published schematics.
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23
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24 Given the sorry state of availability of buffered JTAG adapters, I gave more
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25 thought to the unbuffered option, and I found what appears to be a way to make
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26 them safe - but my method requires programming the EEPROM on the FT2232x board
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27 with a special custom configuration, and in this article I am going to provide
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28 the full details and instructions.
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30 To begin with, an unbuffered JTAG adapter (one in which the target JTAG signals
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31 are connected directly to FT2232x I/O pins without any buffer in between) can
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32 work only with targets that operate their JTAG interface at 3.3 V, or perhaps
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33 a slightly lower but still fully 3.3V-compatible logic voltage level like the
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34 2.8 V I/O on Calypso GSM baseband processors. An unbuffered adapter CANNOT
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35 work with, say, a 1.8 V JTAG interface - but as long as your target runs at
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36 3.3 or 2.8 V, then we can continue.
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38 The next big problem with unbuffered FT2232x adapters is that if you don't put
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39 a special configuration in the EEPROM (or if your FT2232x board omits the EEPROM
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40 altogether), the channel which you are going to wire up for JTAG (can only be
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41 Channel A on FT2232C/D, can be either channel on FT2232H) is going to come up
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42 in FTDI's default UART mode on power-up, and it is going to stay in that mode
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43 until and unless you run OpenOCD, which will then switch it into MPSSE mode for
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44 JTAG. Why is it a problem? Answer: you need to connect the TDO line from the
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45 target to the FT2232x chip's ADBUS2 pin for JTAG to work via MPSSE, but in the
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46 power-up default UART mode this ADBUS2 pin is the RTS output. FT2232x RTS
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47 output fighting with the target's TDO output - not good, and it could even fry
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48 one or both of the chips.
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49
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50 Unfortunately FTDI's stupid chip design does not allow the desired MPSSE mode
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51 to be configured in the EEPROM so that it is there right from power-up. But
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52 there is a workaround: if the EEPROM config is set up to put Channel A (the one
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53 that will be wired for JTAG) into the rarely-used 245 FIFO mode instead of UART,
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54 all 8 ADBUS pins (including ADBUS2 where TDO will be connected) will power up
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55 as inputs with weak internal pull-ups (as long as the ACBUS2 control line is
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56 left unconnected), which is much safer than what these pins do in the default
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57 UART mode.
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58
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59 And if we need to program the EEPROM with a special custom config to change
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60 Channel A from 232 UART to 245 FIFO, we can also assign a different USB VID:PID
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61 at the same time. FTDI's default FT2232x ID of 0403:6010 works great when both
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62 channels of the FT2232x device are used as UARTs - the Linux kernel recognizes
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63 this USB ID, creates a pair of ttyUSB devices (one for each channel), and
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64 everything Just Works. But what if Channel A is used for JTAG and is therefore
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65 not a valid UART channel? If the default USB ID is left unchanged, what happens
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66 is that a pair of ttyUSB devices still gets created, with the first out of the
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67 pair being completely bogus and non-functional. And when you run OpenOCD, that
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68 bogus Channel A ttyUSB device disappears, while the Channel B ttyUSB device
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69 (which will actually work if Channel B is wired as a UART) remains, creating a
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70 gap in ttyUSB numbers. If you have a ton of ttyUSB devices on your system and
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71 are struggling to keep track of which is which, this behaviour certainly does
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72 not help.
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73
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74 As it happens, our company Falconia Partners LLC has received a block of 8 PIDs
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75 from FTDI, allocated out of FTDI's VID range - these PIDs have been officially
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76 allocated by FTDI to our company for use in products based on FTDI chips. And
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77 because we can spare one PID for a worthy cause, one of these PIDs (0403:7151)
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78 is hereby being donated to the community for use on generic FT2232x boards in
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79 the unbuffered JTAG adapter configuration.
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80
80
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81 Support for this 0403:7151 USB ID has been added to Linux ftdi_sio driver in
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82 2020-09 with this commit:
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83
81
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84 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6cf87e5edd9944e1d3b6efd966ea401effc304ee
80
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85
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86 This commit is included in mainline kernel versions from 5.10 onward, and in
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87 stable kernel versions 4.4.240, 4.9.240, 4.14.202, 4.19.152, 5.4.72, 5.8.16 and
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88 5.9.1. If your Linux kernel version (or rather ftdi_sio driver version if it's
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89 a module) includes this commit, the ftdi_sio driver will create a ttyUSB device
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90 for Channel B, allowing that channel to function as a UART if desired, but
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91 Channel A will be left alone by the kernel driver, reserved for userspace
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92 applications like OpenOCD. If your kernel/driver version (older than late 2020)
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93 does not include the commit in question, both FT2232x channels will be left
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94 alone by the kernel driver, i.e., no ttyUSB devices will be created. If you are
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95 interested only in JTAG and don't need an extra UART on Channel B, it should not
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96 matter whether your ftdi_sio driver knows about the new custom USB ID or not -
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97 you simply configure your OpenOCD in userspace to find your unbuffered and
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98 ad-hoc-wired JTAG adapter at USB ID 0403:7151. If you do need the UART on
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99 Channel B but your Linux kernel version is old, you will need to manually apply
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100 the trivial patch from the commit linked above.
52
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101
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102 Choice of FT2232x breakout board
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103 ================================
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104
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105 In earlier years we made heavy use of generic FT2232D breakout boards made by
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106 PLDkit OU in Estonia. We are not sure if that original company still makes
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107 them or not, but the person behind that company name did eventually sell us
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108 their Gerber files, and we have published them here:
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110 ftp://ftp.freecalypso.org/pub/USB/FTDI/
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111 or
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112 https://www.freecalypso.org/pub/USB/FTDI/
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113
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114 Given that we have a stash of FT2232D chips and given that we still have use
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115 cases for these generic breakout boards, we have a tentative plan to produce
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116 our own Falconia-branded version of the same adapter/breakout board.
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117
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118 These days FT2232H chips and FT2232H breakout boards are much more popular, but
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119 I generally prefer FT2232C/D for classicness and simplicity. Additionally,
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120 FTDI's AN_184 document lists I/O pin behaviour of various FTDI chips including
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121 FT2232D and FT2232H; according to this document FT2232H I/O pins go through a
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122 brief phase of acting as UART signals (including RTS output on ADBUS2) while
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123 the EEPROM is being read, whereas FT2232D I/O pins are tristated during this
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124 time. Thus I strongly recommend using an FT2232D breakout board.
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125
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126 Programming the EEPROM
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127 ======================
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128
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129 The boards we used to buy from PLDkit (FT2232D breakout) have 93C46 EEPROMs on
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130 them, and the boards were shipped with blank EEPROMs. The blank EEPROM state
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131 is perfectly good for operating the board as a dual UART, but our JTAG
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132 application calls for custom EEPROM programming. A number of people in the
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133 FOSS community have produced several different tools for programming FTDI
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134 EEPROMs, and you could even use FTDI's official Winblows tools if you like, but
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135 I am going to describe how to program the EEPROM using the tools which I
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136 developed and which are used in production here at FreeCalypso HQ.
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137
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138 FreeCalypso FTDI EEPROM tools reside in this Hg repository:
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139
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140 https://www.freecalypso.org/hg/fc-usbser-tools/
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141
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142 You will need to compile and install these tools, then continue.
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143
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144 If you have the FT2232D board in its initial blank-EEPROM state plugged into
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145 your system and you don't have any other FT2232x devices with 0403:6010 IDs,
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146 you can program the EEPROM for JTAG as follows (the command is a shell
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147 pipeline):
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148
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149 ftee-gen2232c jtag-unbuf | fteeprom-prog i:0x0403:0x6010
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151 Then unplug and replug the FT2232D board, and it should come back with the new
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152 0403:7151 USB ID. If you wish to bring it back to its original blank-EEPROM
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153 state, you can do so by erasing the EEPROM:
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154
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155 fteeprom-erase i:0x0403:0x7151
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156
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157 Wire connections
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158 ================
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159
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160 The JTAG signal connections to ADBUS0 through ADBUS3 are fixed by FTDI, and if
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161 you go against my advice and use FT2232H rather than FT2232C/D, then ADBUS7 is
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162 also reserved for RTCK. The I/O pins available for reset and other sideband or
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163 GPIO signals are ADBUS4 through ADBUS7 on FT2232C/D adapters, or ADBUS4 through
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164 ADBUS6 and ACBUS5 through ACBUS7 on FT2232H. The other pins should be left
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165 untouched to avoid problems with the 245 FIFO mode which is active in the time
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166 window between power-up (USB plug-in) and running OpenOCD.