libftdi Archives

Subject: Accessing EEPROM user area and EEPROM checksum

From: Thilo Schulz <thilo@xxxxxxx>
To: libftdi@xxxxxxxxxxxxxxxxxxxxxxx
Date: Sat, 10 Oct 2015 13:30:22 +0200
Hello,

at this time, libftdi does not seem to feature an explicit mechanism for 
reading the user area (except directly accessing the data buffer in ftdi 
context structure) and more importantly, it has no direct support for writing 
to the user area with a correct EEPROM checksum.

Is there a good reason for this? One could image extending the ftdi_eeprom 
structure (which is opaque to programs using the library anyways) by 
appropriate fields and adding library support functions for reading and setting 
of user space area data, so the API would remain fully backwards compatible.

I'd also be interested to know about the significance of the EEPROM checksum. 
My FTDI chip comes preshipped with the checksum that libftdi would compute, so 
where does this come from?
Does it have any  effect on chip initialisation (e.g. EEPROM checksum invalid, 
chip will start up with some default values) or is it solely to satisfy user-
space programs running on the host-side?

Last but not least I wanted to point something out that might be of interest 
to you.
>From the discussion:
http://developer.intra2net.com/mailarchive/html/libftdi/2015/msg00010.html

> The descriptive strings begin at byte 0x1C (=28) in the EEPROMs I have at
> hand (both written with FTProg and ftdi_eeprom). 

In the EEPROM from my FT232R that is factory-shipped, the descriptive strings 
start at 0x18 (=24). See attached EEPROM.

-- 
Best regards,
Thilo Schulz

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