Re: Even easier I2C bus interface :-)
Date: January 09, 2008 10:37PM
> The I2CBus need preferably open collector driver,
> so the need for the transistor.
> Especially the SDA data line need to be
> The external supply is needed since the LPT port
> can't provide a stable 5V.
I am happy that I was able to make this work with external supply, but is there a way to provide power from LPT port? External battery supply is inconvenient. It would be nice to just have a cable with a IC grabber at the end. Each pin of the LPT port can drive 2 mA or so, and the I2C EEPROMs only draw 3 mA during write, so if the program holds all the other data lines high, and they are connected with diodes (just in case) to make +V, there should be plenty of current available for these chips.
The only problem I see is that on my computer's parallel port, it looks like the output pins are +3.3 V, while the computer's internal pull-ups on the data lines are +5 V, so when these two transistors are open, the internal pull-up pulls the SDA and SCL lines above 3.3 V. These lines are not supposed to be higher than VCC + 0.5 V, so maybe this is a problem, though the pull-up resistors prevent much current from flowing.
> It's a low dropout with a very low quiescent
But you *could* replace it with 7805 if that's all you had, right? Just that the other chip is better.
> In the case of EasyI2CBus interface it writes to
> data-out and read back data-in
Is it just pulling up one pin and down the other and checking at 12/13 that the correct inputs are seen?
> If the LPT port is a standard (on board) LPT port
> there should be no problem.
I had to switch on "Enable legacy Plug and Play detection" to get it to work, but then it worked.