I2C: PIC(5V) / Sensor(3.3V)

[RageOfFury]

I have a I2C pressure sensor that I wish to read using a PIC18F4685. The sensor runs at 3.3V and the PIC at 5V. I'm using the PCA9306 as a voltage-level translator. Will this setup work?

Thanks.

[Matro]

That's not needed.
If you enter "I2C level shifter" in Google or other, you will find THE classical I²C level shifter that is just 2 MOSFETs and 4 pull-up resistors (2 for each side) and that perfectly works.

Matro

[RageOfFury]

Well I already have the PCA9306.

But looking at the datasheet it would seem that the PIC must be 3.3V and the sensor 5V and not the other way around(the way I have it now).

[Matro]

If your PCA is well mounted so it will works.
But how can we know that since we don't have the schematic?...

Matro.

[RageOfFury]

Here is the schematic:


(click to view full size)

[Matro]

It seems OK according to the datasheet.

Matro.

[RageOfFury]

Is it normal for the PCA9306 to become really hot during operation?

[Matro]

No, there may be a short circuit on 1 or several lines.
Check impedance between each line and ground.

Matro.

[RageOfFury]

There is a 2000Ohm impedance between each line and the ground.

[Matro]

That is a very small value.
You should have at least 10x the value of the pull-up resistors.
Further investigate to know why this impedance is so small.

Matro.

[mskala]

The picture is small, I can't tell, do you have a 200K resistor from 5V to PCA9306 power? Why would you need a resistor there at all? Your pullups of ~348 ohms should work, but unless you have a long cable or high capacitance, in general I would use pullup values between 1K and 5K.

[PCM programmer]

Sample schematics for PCA9306:

Sheet 6 shows 2.2K pullups on each side:
http://c6000.spectrumdigital.com/davincievm/revd/files/DaVinciEVM_Schematic.pdf

Page 5 shows 3.3K on the +3.3v side, and 1.6K on the 5v side:
http://www.atmel.com/dyn/resources/prod_documents/doc32088.pdf

These are more normal pull-up values for i2c. 348 ohms is way low.

[RageOfFury]

Thanks for replying.

Everywhere I look the microcontroller is on the 3.3V side and the sensor on the 5V side. Me it's the other way around. Everything except the sensor is running at 5V, this is why I'm a bit confused about how to wire everything properly.

[SET]

Why do you have the 5v side powered through a 200k resistor?

[RageOfFury]