--- a/eeproms/dumps/FT232R-notes	Wed Sep 16 23:43:36 2020 +0000
+++ b/eeproms/dumps/FT232R-notes	Thu Sep 17 00:42:17 2020 +0000
@@ -1,31 +1,42 @@
 Unlike FT2232x devices with external EEPROMs, an FT232R device is not expected
 to ever have a blank EEPROM in normal usage: these chips have their EEPROM
-built in, and FTDI ships them with this internal EEPROM already programmed.
-It may be possible to create a "blank" EEPROM by explicitly programming 0xFFFF
-into every word, but it would be an unnatural scenario, and I (Mother Mychaela)
-do not currently have an FT232R device on which I can experiment: I don't have
-an FT232R device which is not valuable and which is not already bricked.
+built in, and FTDI probably ships them with this internal EEPROM already
+programmed.  I said "probably" because I have not yet had an occasion to build
+my own FT232R-containing board where I would be getting completely pristine
+"bare" chips from Digi-Key, thus I have no first-hand verified knowledge.
+
+As an experiment, I have programmed "blank" (0xFFFF in every word) images into
+the two FT232R devices I have available for play at the moment (specifically
+devices which I could afford to brick if things went badly), and FT232R behaves
+the same way as FTDI's earlier chips with external EEPROMs: it runs with a fixed
+default config when the EEPROM is invalid.  But this configuration is NOT
+recommended for production use - you should always have a valid EEPROM config
+in your FT232R chip.
 
-I have read out the EEPROM content from the two specimen I did have available:
-FT232R-specimen1 came from a no-name ebay-sourced FT232RL breakout board;
-FT232R-specimen2 came from George UberWaves' "FTDI Professional" USB-serial
-cable with OsmocomBB branding.  Specimen 2 is probably a genuine FT232RL chip
-(I remember George telling me that he went out of his way to procure genuine
-FTDI chips after having been burned by FTDI's Winblows drivers screwing around
-with close-but-not-perfect clones), but specimen 1 is suspected to be one of
-those less-than-perfect clones: the serial number string was programmed to
-"00000000", whereas FTDI supposedly program true per-unit serial numbers.
+When our FreeCalypso fteeprom tools were first put together in 2019-04, I was
+getting erratic behaviour: when I tried to program my own EEPROM config
+generated with ftee-gen232r, the resulting EEPROM content became a bitwise AND
+between the previous image and the new one, as if the "EEPROM" is not really an
+erasable memory, but one of OTP kind where ones can be turned into zeros, but
+not the other way around.  I was doing this experiment on a no-name FT232RL
+adapter from ebay, thus my first thought was that the FT232RL chip was bad, a
+less-than-perfect clone rather than genuine FTDI.  But then I bought a UB232R
+module from Digi-Key (presumably containing a genuine FT232RQ chip), and it
+behaved the same way.
+
+Further investigation revealed that FT232R EEPROM write operations work
+correctly only if they are preceded by this magic sequence:
 
-The only diffs between FT232R-specimen1 and FT232R-specimen2 are the just-
-mentioned serial number string (specimen 2 has it set to "A9031HG6", which looks
-like a real per-unit serial number), two non-understood "garbage" words after
-the last string, and of course the checksum.
+	ftdi_usb_reset(&ftdi);
+	ftdi_poll_modem_status(&ftdi, &modem_status);
+	ftdi_set_latency_timer(&ftdi, 0x77);
 
-The unit that was specimen 1 (the suspected fake) is now bricked: when I tried
-to program my own EEPROM config generated with ftee-gen232r, the resulting
-EEPROM content became a bitwise AND between the previous image and the new one,
-as if the "EEPROM" is not really an erasable memory, but one of OTP kind where
-ones can be turned into zeros, but not the other way around.  I am not willing
-to experiment on the specimen 2 chip because it is part of a valuable cable
-assembly which I don't want to risk bricking, so I will need to order more
-sacrificial hardware and wait for it to arrive before I can experiment further.
+I can see how FTDI could have reasonably implemented a sort of safety lock on
+their EEPROM write operations, allowing them only if a special unlock sequence
+has been given - but it completely baffles me why they are doing some sort of
+OTP emulation in the absence of the right magic sequence, as opposed to
+disabling EEPROM writes altogether.  It is worth noting that this magic sequence
+is NOT needed for programming external EEPROMs behind FT2232x chips - were FTDI
+folks being deliberately malicious in designing their FT232R chip to simulate
+appearance of being bricked when it is programmed with older (or third-party)
+software tools that don't know the new magic sequence?  Who knows...