I2C to USB

[Ttelmah]

First problem is the I2C input threshold depends on the PIC supply voltage.
Then, it'd probably work on a 3.3v PIC running with 5v pullups, but normally
the I2C pins are not 5v tolerant, so this would then have problems.
For example, the PIC24HJ chips are listed as 5v
tolerant, but the Vih for the I2C connection is listed as 3.3v. So this
tolerance only applies to the normal logic I/O functions, not the I2C.
A few of the more sophisticated PIC's do offer this tolerance on these
pins. So (for example), the dsPIC33FJ32MC204 specifically supports this.

[temtronic]

In the 'keep it simple' theme, honestly , have two 'modules' The Orange one tests 3 volt units, the Green one is for testing 5 volt units. The USBTTL modules I've used for years have both 5 and 3 volt power pins, so program 2 PICs. Choose a PIC that can run at both 3 and 5 volt to make it even simpler. Odds are there's an 8 pin PIC, built in RCOSC,1 I2C port for maybe a buck ?? Now in the PC 'control/test' program the menu will ask you to choose '3v - orange or 5v - green' devices, then will select the proper USB port. After that the program does whatever testing you require.
The older I get, the simpler I want 'things' have to be. I can't see more than $10 for each module and they WILL work. Sure you can maybe figure out some clever logic level conversion to make a single 'universal' test module that might save you $2 in real coins BUT cost you $100s in R&D time....
just more to think about...
Jay

[Ttelmah]

Yes. Remembering you also probably need smaller pull-up resistors when
working off 3.3v, having two configurations does help to ensure everything
is correctly configured.....