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annotate doc/FT232R-notes @ 68:5cbde3c80c24

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fteeprom-{erase,prog}: detach logic: change to detach by default As it turns out, detaching all ttyUSB interfaces of a multichannel device does not require outside knowledge of how many channels there are, as in our previous -d option design that is being removed here - instead we can read the bNumInterfaces constant from the USB device's config descriptor and thus know how many interfaces there are in total. Based on this discovery, change the design of fteeprom-{erase,prog} as follows: * remove -d option; * flip the default to where we detach all interfaces by default; * add -n option to NOT detach any interfaces.
author Mychaela Falconia <falcon@freecalypso.org>
date Wed, 13 Sep 2023 06:37:03 +0000
parents f5fbcf1ff032
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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
f5fbcf1ff032 doc: initial import from freecalypso-hwlab
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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
f5fbcf1ff032 doc: initial import from freecalypso-hwlab
Mychaela Falconia <falcon@freecalypso.org>
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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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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...