FreeCalypso > hg > freecalypso-hwlab
comparison doc/FT232R-notes @ 79:026dd69e4ebb
FT232R-notes article moved to doc directory
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Fri, 18 Sep 2020 02:23:38 +0000 |
parents | eeproms/dumps/FT232R-notes@4e13c90c1405 |
children | df4bf4e06221 |
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1 Unlike FT2232x devices with external EEPROMs, an FT232R device is not expected | |
2 to ever have a blank EEPROM in normal usage: these chips have their EEPROM | |
3 built in, and FTDI probably ships them with this internal EEPROM already | |
4 programmed. I said "probably" because I have not yet had an occasion to build | |
5 my own FT232R-containing board where I would be getting completely pristine | |
6 "bare" chips from Digi-Key, thus I have no first-hand verified knowledge. | |
7 | |
8 As an experiment, I have programmed "blank" (0xFFFF in every word) images into | |
9 the two FT232R devices I have available for play at the moment (specifically | |
10 devices which I could afford to brick if things went badly), and FT232R behaves | |
11 the same way as FTDI's earlier chips with external EEPROMs: it runs with a fixed | |
12 default config when the EEPROM is invalid. But this configuration is NOT | |
13 recommended for production use - you should always have a valid EEPROM config | |
14 in your FT232R chip. | |
15 | |
16 When our FreeCalypso fteeprom tools were first put together in 2019-04, I was | |
17 getting erratic behaviour: when I tried to program my own EEPROM config | |
18 generated with ftee-gen232r, the resulting EEPROM content became a bitwise AND | |
19 between the previous image and the new one, as if the "EEPROM" is not really an | |
20 erasable memory, but one of OTP kind where ones can be turned into zeros, but | |
21 not the other way around. I was doing this experiment on a no-name FT232RL | |
22 adapter from ebay, thus my first thought was that the FT232RL chip was bad, a | |
23 less-than-perfect clone rather than genuine FTDI. But then I bought a UB232R | |
24 module from Digi-Key (presumably containing a genuine FT232RQ chip), and it | |
25 behaved the same way. | |
26 | |
27 Further investigation revealed that FT232R EEPROM write operations work | |
28 correctly only if they are preceded by this magic sequence: | |
29 | |
30 ftdi_usb_reset(&ftdi); | |
31 ftdi_poll_modem_status(&ftdi, &modem_status); | |
32 ftdi_set_latency_timer(&ftdi, 0x77); | |
33 | |
34 I can see how FTDI could have reasonably implemented a sort of safety lock on | |
35 their EEPROM write operations, allowing them only if a special unlock sequence | |
36 has been given - but it completely baffles me why they are doing some sort of | |
37 OTP emulation in the absence of the right magic sequence, as opposed to | |
38 disabling EEPROM writes altogether. It is worth noting that this magic sequence | |
39 is NOT needed for programming external EEPROMs behind FT2232x chips - were FTDI | |
40 folks being deliberately malicious in designing their FT232R chip to simulate | |
41 appearance of being bricked when it is programmed with older (or third-party) | |
42 software tools that don't know the new magic sequence? Who knows... |