FreeCalypso > hg > freecalypso-hwlab
annotate doc/Unbuffered-FT2232x-JTAG @ 73:749e1a14aa24
linux-patch: publishing currently used ftdi_sio driver patch
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Thu, 30 Jul 2020 05:12:44 +0000 |
parents | 96232f00bc9c |
children | 1d38af6d7be8 |
rev | line source |
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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 Among the FOSS community of tinkerers who use OpenOCD to operate on the JTAG |
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5 interfaces of various hardware targets, one of the most common JTAG adapter |
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6 choices (if not the most common) is to use some adapter gadget based on an FTDI |
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7 chip, most commonly one of FT2232x variants. However, a major distinction needs |
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8 to be drawn between specialized purpose-made JTAG adapter products which just |
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9 happen to use an FT2232x chip internally, versus generic FT2232x breakout boards |
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10 which the user wires up for JTAG on his or her own. |
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11 |
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12 In an ideal world, using a purpose-made buffered JTAG adapter (one that has a |
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13 buffer inserted between FT2232x I/O pins and the target connection interface) |
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14 would be strongly preferable for a whole host of reasons. However, to this |
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15 author's disappointment, there are very few community vendors who make such |
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16 adapters, and I was NOT able to find any high-quality buffered JTAG adapter |
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17 which can be bought in the present and which comes with published schematics. |
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18 (There is one very well-known vendor of "community" JTAG adapters who refuses |
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19 to publish schematics for their current model; they have an older model for |
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20 which they did publish schematics, but it is discontinued and they are not |
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21 interested in bringing it back into production or handing the complete design |
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22 over to the community - probably because it would then compete with their |
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23 current sans-schematics product! Selling JTAG adapters to the community while |
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24 keeping their schematics secret is just assinine, and I refuse to give my |
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25 business to such vendors.) |
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26 |
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27 Given the current sorry state of availability of buffered JTAG adapters, I have |
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28 given more thought to the unbuffered option, and I found what appears to be a |
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29 way to make them safe - but my method requires programming the EEPROM on the |
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30 FT2232x board with a special custom configuration, and in this article I am |
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31 going to provide the full details and instructions. |
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32 |
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33 To begin with, an unbuffered JTAG adapter (one in which the target JTAG signals |
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34 are connected directly to FT2232x I/O pins without any buffer in between) can |
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35 work only with targets that operate their JTAG interface at 3.3 V, or perhaps |
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36 a slightly lower but still fully 3.3V-compatible logic voltage level like the |
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37 2.8 V I/O on Calypso GSM baseband processors. An unbuffered adapter CANNOT |
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38 work with, say, a 1.8 V JTAG interface - but as long as your target runs at |
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39 3.3 or 2.8 V, then we can continue. |
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40 |
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41 The next big problem with unbuffered FT2232x adapters is that if you don't put |
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42 a special configuration in the EEPROM (or if your FT2232x board omits the EEPROM |
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43 altogether), the channel which you are going to wire up for JTAG (can only be |
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44 Channel A on FT2232C/D, can be either channel on FT2232H) is going to come up |
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45 in FTDI's default UART mode on power-up, and it is going to stay in that mode |
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46 until and unless you run OpenOCD, which will then switch it into MPSSE mode for |
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47 JTAG. Why is it a problem? Answer: you need to connect the TDO line from the |
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48 target to the FT2232x chip's ADBUS2 pin for JTAG to work via MPSSE, but in the |
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49 power-up default UART mode this ADBUS2 pin is the RTS output. FT2232x RTS |
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50 output fighting with the target's TDO output - not good, and it could even fry |
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51 one or both of the chips. |
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52 |
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53 Unfortunately FTDI's stupid chip design does not allow the desired MPSSE mode |
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54 to be configured in the EEPROM so that it is there right from power-up. But |
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55 there is a workaround: if the EEPROM config is set up to put Channel A (the one |
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56 that will be wired for JTAG) into the rarely-used 245 FIFO mode instead of UART, |
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57 all 8 ADBUS pins (including ADBUS2 where TDO will be connected) will power up |
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58 as inputs with weak internal pull-ups (as long as the ACBUS2 control line is |
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59 left unconnected), which is much safer than what these pins do in the default |
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60 UART mode. |
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61 |
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62 And if we need to program the EEPROM with a special custom config to change |
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63 Channel A from 232 UART to 245 FIFO, we can also assign a different USB VID:PID |
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64 at the same time. FTDI's default FT2232x ID of 0403:6010 works great when both |
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65 channels of the FT2232x device are used as UARTs - the Linux kernel recognizes |
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66 this USB ID, creates a pair of ttyUSB devices (one for each channel), and |
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67 everything Just Works. But what if Channel A is used for JTAG and is therefore |
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68 not a valid UART channel? If the default USB ID is left unchanged, what happens |
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69 is that a pair of ttyUSB devices still gets created, with the first out of the |
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70 pair being completely bogus and non-functional. And when you run OpenOCD, that |
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71 bogus Channel A ttyUSB device disappears, while the Channel B ttyUSB device |
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72 (which will actually work if Channel B is wired as a UART) remains, creating a |
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73 gap in ttyUSB numbers. If you have a ton of ttyUSB devices on your system and |
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74 are struggling to keep track of which is which, this behaviour certainly does |
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75 not help. |
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76 |
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77 As it happens, our company Falconia Partners LLC has received a block of 8 PIDs |
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78 from FTDI, allocated out of FTDI's VID range - these PIDs have been officially |
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79 allocated by FTDI to our company for use in products based on FTDI chips. And |
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80 because we can spare one PID for a worthy cause, one of these PIDs (0403:7151) |
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81 is hereby being donated to the community for use on generic FT2232x boards in |
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82 the unbuffered JTAG adapter configuration. |
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83 |
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84 As of this writing, this 0403:7151 PID has not been submitted to Linux ftdi_sio |
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85 kernel maintainers yet, thus if you program it into your FT2232x EEPROM |
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86 following the instructions below, the kernel will leave that FT2232x device |
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87 completely alone. If you are interested only in JTAG and don't need an extra |
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88 UART on Channel B, this arrangement should be fully sufficient - you simply |
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89 configure your OpenOCD in userspace to find your unbuffered and ad-hoc-wired |
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90 JTAG adapter at that USB ID. And if you do need the UART on Channel B, you can |
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91 trivially patch your ftdi_sio.c kernel driver, adding the new ID to the table |
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92 with JTAG quirk (&ftdi_jtag_quirk) specified. |
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93 |
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94 Choice of FT2232x breakout board |
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95 ================================ |
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96 |
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97 Here at FreeCalypso HQ we make very extensive use of FT2232C/D breakout boards |
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98 by PLDkit, and I officially recommend and endorse this vendor: |
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99 |
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100 http://pldkit.com/other/ft2232d-module |
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101 |
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102 These modules were originally made with FT2232D chips, then the vendor found a |
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103 stash of old but still good FT2232C chips, so apparently the current ones are |
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104 FT2232C, not D - but this distinction makes no difference for the present |
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105 purpose. |
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106 |
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107 These days FT2232H chips and FT2232H breakout boards are much more popular, but |
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108 I generally prefer FT2232C/D for classicness and simplicity. Additionally, |
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109 FTDI's AN_184 document lists I/O pin behaviour of various FTDI chips including |
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110 FT2232D and FT2232H; according to this document FT2232H I/O pins go through a |
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111 brief phase of acting as UART signals (including RTS output on ADBUS2) while |
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112 the EEPROM is being read, whereas FT2232D I/O pins are tristated during this |
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113 time. Thus I strongly recommend using an FT2232D breakout board. |
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114 |
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115 Programming the EEPROM |
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116 ====================== |
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117 |
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118 The officially recommended FT2232D breakout boards from PLDkit have 93C46 |
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119 EEPROMs on them, and the boards are shipped with blank EEPROMs. The blank |
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120 EEPROM state is perfectly good for operating the board as a dual UART, but our |
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121 JTAG application calls for custom EEPROM programming. A number of people in |
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122 the FOSS community have produced several different tools for programming FTDI |
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123 EEPROMs, and you could even use FTDI's official Winblows tools if you like, but |
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124 I am going to describe how to program the EEPROM using the tools which I |
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125 developed and which are used in production here at Falconia Partners LLC. |
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126 |
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127 To compile my FTDI EEPROM tools, go into the fteeprom directory and run make |
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128 there; you will need to have libftdi (the classic one, not libftdi1) installed |
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129 on your system. If all you seek to do is to program this one EEPROM, you don't |
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130 need to install my tools system-wide - you can just run them from the directory |
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131 where they are compiled. |
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132 |
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133 If you have the FT2232D board in its initial blank-EEPROM state plugged into |
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134 your system and you don't have any other FT2232x devices with 0403:6010 IDs, |
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135 you can program the EEPROM for JTAG as follows - run this pipeline from the top |
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136 directory of this code repository: |
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137 |
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138 fteeprom/ftee-gen2232c eeproms/jtag-unbuf | fteeprom/fteeprom-prog i:0x0403:0x6010 |
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139 |
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140 Then unplug and replug the FT2232D board, and it should come back with the new |
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141 0403:7151 USD ID. If you wish to bring it back to its original blank-EEPROM |
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142 state, you can do so by erasing the EEPROM: |
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143 |
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144 fteeprom-erase i:0x0403:0x7151 |
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145 |
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146 Wire connections |
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147 ================ |
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148 |
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149 The JTAG signal connections to ADBUS0 through ADBUS3 are fixed by FTDI, and if |
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150 you go against my advice and use FT2232H rather than FT2232C/D, then ADBUS7 is |
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151 also reserved for RTCK. The I/O pins available for reset and other sideband or |
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152 GPIO signals are ADBUS4 through ADBUS7 on FT2232C/D adapters, or ADBUS4 through |
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153 ADBUS6 and ACBUS5 through ACBUS7 on FT2232H. The other pins should be left |
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154 untouched to avoid problems with the 245 FIFO mode which is active in the time |
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155 window between power-up (USB plug-in) and running OpenOCD. |