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annotate doc/FT232R-notes @ 99:2e35070d289f

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fc-simtool: savebin command implemented
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 25 Jan 2021 00:14:19 +0000
parents 026dd69e4ebb
children df4bf4e06221
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af70c59654ed EEPROM dumps moved into eeproms/dumps and properly annotated
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1 Unlike FT2232x devices with external EEPROMs, an FT232R device is not expected
af70c59654ed EEPROM dumps moved into eeproms/dumps and properly annotated
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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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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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16 When our FreeCalypso fteeprom tools were first put together in 2019-04, I was
4e13c90c1405 eeproms/dumps/FT232R-notes: update for current understanding
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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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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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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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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...