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View Full Version : [Question(s)] How to modify Devantech USB-I2C for 3.3v i/o?



Kuchta
05-14-2010, 02:19 PM
The Devantech USB-I2C converter comes wired for 5v i/o.

Does anyone know how to modify this for 3.3v I2C operation?

For example, if I were to lift the two 4.7K pull-up resistors on SCL and SDA from the 5v trace and connect them over to pin 16 of the FTDI chip (3.3v output pin), would I be able to interface to 3.3v I2C devices?

Does anyone have a schematic for this converter board?

Sincerely,
Dan Kuchta

Quantum
05-14-2010, 04:39 PM
No, don't pull the resisters on the SDA and SCL this will make the bus almost 0 Volts and it won't work. You need these lines to be at 5V in order for the bus to work. No way around this.

You will have to either use a different power source or regulate it down.

What sensor is it as it may work on 5V?

cdop
05-15-2010, 12:20 PM
Just put a i2c level shifter between the units:
http://www.ti.com/corp/docs/landing/hpadavinci/sp_logic_pca9306.htm

Kuchta
05-17-2010, 01:32 PM
Thanks for the suggestion but I didn't want to get a translator chip.

So I tried the modification and it works.
I've attached a picture of the modified part.

The two 4.7k Ohm pull-up resistors on SCL and SDA are standing vertically on one end (hard to see from this angle in the picture).
The other free ends are soldered to a 30 gauge wire to pin 16 of the FTDI chip.
This pin 16 provides 3.3v.
So now SDA and SCL are pulled up to 3.3v instead of 5v.
The microcontroller that drives SDA and SCL indicates that it has 'weak pullups' to Vcc.
In this case Vcc will still be 5v but 'weak pullup' must mean some resistor value much larger than 4.7K Ohm so the 3.3v pull-up dominates

I attached to my 3.3v I2C device and was able to communicate.

Sincerely,
Dan Kuchta