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+Mother Mychaela has developed a set of Linux command line tools for manipulating
+configuration EEPROMs that are attached to FT2232x devices and accessed in-band
+via USB.  This document describes these tools.
+
+Supported FTDI chips and EEPROMs
+================================
+
+The present tools work with 93C46, 93C56 and 93C66 EEPROMs attached behind
+FT2232x dual-channel UART/FIFO/MPSSE/etc chips, both FT2232C/D and FT2232H.
+We can read these EEPROMs for examination or backup, and we can program them
+with new bits, either restoring a previously saved backup or creating a new
+from-scratch configuration.  These EEPROM configurations (which we can save,
+restore or create from scratch) set the USB VID:PID and the textual strings
+naming the manufacturer, the product model and an optional serial number,
+select whether each FT2232x channel will come up in the default UART mode or
+one of the other EEPROM-configurable modes (245 FIFO, CPU-style FIFO or fast
+opto-isolated serial), and allow a few other obscure chip settings to be
+tweaked.
+
+Some work has also been done toward the goal of being able to program the
+internal EEPROM in FT232R chips (a very popular single-channel USB to UART
+converter needing no external components), but this work has not been finished
+yet, and the present tools should NOT be used to attempt an EEPROM write on
+that chip - the risking of bricking the chip is too high.
+
+More generally, our fteeprom-read tool should be able to read out the EEPROM
+content from just about any FTDI chip including FT232R, whereas our
+fteeprom-prog tool should be able to program a user-supplied set of bits into
+any FTDI+EEPROM combo where the EEPROM is a separate chip - but it is NOT safe
+to use on FTDI chips like FT232R or FT-X that have their EEPROM built in.
+Furthermore, if the goal is to generate a new EEPROM config from scratch, as
+opposed to restoring a saved backup, we currently have generators only for
+FT2232C/D and for FT2232H.  (We also have a generator program for FT232R, but
+it is of no use for as long as fteeprom-prog is not safe to use on FT232R
+chips.)
+
+libftdi dependency
+==================
+
+We use libftdi (which is in turn layered on libusb) to issue the special USB
+control pipe commands to FTDI chips which are needed to read and write their
+EEPROMs.  We use old-style libftdi-0.x (-lftdi on the link line) as opposed to
+libftdi1 (-lftdi1) because the new versions took away the ability to write to
+the EEPROM directly with ftdi_write_eeprom_location() calls, forcing users to
+go through libftdi1's own EEPROM smarts, which we don't want to do - our tools
+are all about more direct user empowerment at the lowest level.
+
+Selecting the device to operate on
+==================================
+
+Our fteeprom-read, fteeprom-prog and fteeprom-erase tools take a device selector
+argument, selecting the device to operate on.  This required argument is the
+string to be passed to the ftdi_usb_open_string() function in libftdi, allowing
+the device to be operated on to be selected in one of several ways.  Copying
+from libftdi documentation, the available formats are:
+
+d:<devicenode> - path of bus and device-node (e.g. "003/001") within usb device
+tree (usually at /proc/bus/usb/)
+
+i:<vendor>:<product> - first device with given vendor and product id, ids can
+be decimal, octal (preceded by "0") or hex (preceded by "0x")
+
+i:<vendor>:<product>:<index> - as above with index being the number of the
+device (starting with 0) if there are more than one
+
+s:<vendor>:<product>:<serial> - first device with given vendor id, product id
+and serial string
+
+If you have only one FTDI device connected to your PC or laptop at the time of
+your EEPROM manipulation session (generally a good idea to avoid hitting the
+wrong device by mistake) and if that FTDI device has some sensible starting
+USB VID:PID (either from the previous EEPROM config or the chip's sans-EEPROM
+default) that doesn't clash with anything else, then the i: form will probably
+be the most convenient, e.g.:
+
+i:0x0403:0x6001 for single-channel FT232x devices running with the default ID
+i:0x0403:0x6010 for dual-channel FT2232x devices running with the default ID
+i:0x0403:0xPPPP for custom PIDs assigned out of FTDI's VID range
+i:0xVVVV:0xPPPP for totally custom USB IDs
+
+Or if the current device config is totally hosed (the EEPROM has a passing
+checksum, but sets some completely bogus USB ID), then the d: form will
+probably be required for recovery.
+
+Reading the EEPROM
+==================
+
+The basic EEPROM read command is as follows:
+
+fteeprom-read <device-selector>
+
+See the previous section for the device selector argument.  In this default
+form the tool will read the first 64 EEPROM words, which is appropriate for
+93C46 external EEPROMs or for the internal 1024-bit EEPROM in the FT232R chip.
+However, if you are working with an FT2232x board with an external EEPROM and
+that EEPROM is of a larger variety (93C56 or 93C66), this basic form with give
+you an incomplete (truncated) read, and you will need one of the following
+extended forms to read the complete EEPROM:
+
+fteeprom-read -b <device-selector>	-- read 128 EEPROM words (93C56)
+fteeprom-read -B <device-selector>	-- read 256 EEPROM words (93C66)
+
+(If you use one of the extended forms on a smaller EEPROM, you will get 2 or 4
+ copies of the same bits.)
+
+The output of fteeprom-read is in the same format as the input to fteeprom-prog,
+thus you can redirect the output to a file and get a restorable backup copy of
+your EEPROM.
+
+Programming the EEPROM
+======================
+
+In terms of the primitives provided over USB, writing to EEPROMs behind FTDI
+chips is accomplished by writing one 16-bit word at a time: the
+SIO_WRITE_EEPROM_REQUEST command writes a user-supplied word at a user-supplied
+EEPROM address.  However, our fteeprom-prog tool currently supports only writing
+complete EEPROMs (64 or 128 or 256 16-bit words starting at address 0) and we
+do not currently provide any kind of "random access write" utility; the primary
+reason for this design decision is practical usefulness: FTDI's EEPROM structure
+includes a checksum over the first 64 words for 1024-bit EEPROMs or over the
+first 128 words for larger ones, and if this checksum fails to match, the entire
+structure is deemed to be invalid - hence there is no practical use case for
+selectively rewriting individual words.  The only exception may be with 93C66
+EEPROMs: on these giants only the first half would be subject to the checksum,
+and the second half could be used arbitrarily.  However, we have not yet
+encountered any boards out in the wild with such big EEPROMs, and we have no
+plans to use such in any of our own hardware designs either, hence there is no
+business case at the present moment to develop tooling support for them.