FreeCalypso > hg > freecalypso-hwlab
annotate doc/FT232R-notes @ 95:5f826e428641
fc-simtool: readef command implemented
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sun, 24 Jan 2021 20:47:00 +0000 |
parents | 026dd69e4ebb |
children | df4bf4e06221 |
rev | line source |
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1 Unlike FT2232x devices with external EEPROMs, an FT232R device is not expected |
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2 to ever have a blank EEPROM in normal usage: these chips have their EEPROM |
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3 built in, and FTDI probably ships them with this internal EEPROM already |
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4 programmed. I said "probably" because I have not yet had an occasion to build |
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5 my own FT232R-containing board where I would be getting completely pristine |
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6 "bare" chips from Digi-Key, thus I have no first-hand verified knowledge. |
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7 |
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8 As an experiment, I have programmed "blank" (0xFFFF in every word) images into |
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9 the two FT232R devices I have available for play at the moment (specifically |
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10 devices which I could afford to brick if things went badly), and FT232R behaves |
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11 the same way as FTDI's earlier chips with external EEPROMs: it runs with a fixed |
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12 default config when the EEPROM is invalid. But this configuration is NOT |
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13 recommended for production use - you should always have a valid EEPROM config |
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14 in your FT232R chip. |
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15 |
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16 When our FreeCalypso fteeprom tools were first put together in 2019-04, I was |
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17 getting erratic behaviour: when I tried to program my own EEPROM config |
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18 generated with ftee-gen232r, the resulting EEPROM content became a bitwise AND |
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19 between the previous image and the new one, as if the "EEPROM" is not really an |
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20 erasable memory, but one of OTP kind where ones can be turned into zeros, but |
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21 not the other way around. I was doing this experiment on a no-name FT232RL |
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22 adapter from ebay, thus my first thought was that the FT232RL chip was bad, a |
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23 less-than-perfect clone rather than genuine FTDI. But then I bought a UB232R |
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24 module from Digi-Key (presumably containing a genuine FT232RQ chip), and it |
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25 behaved the same way. |
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26 |
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27 Further investigation revealed that FT232R EEPROM write operations work |
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28 correctly only if they are preceded by this magic sequence: |
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29 |
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30 ftdi_usb_reset(&ftdi); |
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31 ftdi_poll_modem_status(&ftdi, &modem_status); |
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32 ftdi_set_latency_timer(&ftdi, 0x77); |
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33 |
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34 I can see how FTDI could have reasonably implemented a sort of safety lock on |
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35 their EEPROM write operations, allowing them only if a special unlock sequence |
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36 has been given - but it completely baffles me why they are doing some sort of |
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37 OTP emulation in the absence of the right magic sequence, as opposed to |
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38 disabling EEPROM writes altogether. It is worth noting that this magic sequence |
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39 is NOT needed for programming external EEPROMs behind FT2232x chips - were FTDI |
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40 folks being deliberately malicious in designing their FT232R chip to simulate |
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41 appearance of being bricked when it is programmed with older (or third-party) |
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42 software tools that don't know the new magic sequence? Who knows... |