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



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I2C: PIC(5V) / Sensor(3.3V)
PostPosted: Tue Apr 08, 2008 9:11 am     Reply with quote
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. Wink
Matro
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PostPosted: Tue Apr 08, 2008 9:19 am     Reply with quote
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



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PostPosted: Tue Apr 08, 2008 9:23 am     Reply with quote
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
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PostPosted: Tue Apr 08, 2008 9:26 am     Reply with quote
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



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PostPosted: Tue Apr 08, 2008 9:40 am     Reply with quote
Here is the schematic:


(click to view full size)
Matro
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PostPosted: Tue Apr 08, 2008 9:47 am     Reply with quote
It seems OK according to the datasheet.

Matro.
RageOfFury



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PostPosted: Tue Apr 08, 2008 11:05 am     Reply with quote
Is it normal for the PCA9306 to become really hot during operation?
Matro
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PostPosted: Tue Apr 08, 2008 11:09 am     Reply with quote
No, there may be a short circuit on 1 or several lines.
Check impedance between each line and ground.

Matro.
RageOfFury



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PostPosted: Tue Apr 08, 2008 11:16 am     Reply with quote
There is a 2000Ohm impedance between each line and the ground.
Matro
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PostPosted: Tue Apr 08, 2008 11:24 am     Reply with quote
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



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PostPosted: Tue Apr 08, 2008 2:53 pm     Reply with quote
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



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PostPosted: Tue Apr 08, 2008 4:22 pm     Reply with quote
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



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PostPosted: Wed Apr 09, 2008 6:53 am     Reply with quote
Thanks for replying. Smile

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



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PostPosted: Fri Apr 11, 2008 10:59 am     Reply with quote
Why do you have the 5v side powered through a 200k resistor?
RageOfFury



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PostPosted: Wed Apr 16, 2008 9:26 am     Reply with quote
SET wrote:
Why do you have the 5v side powered through a 200k resistor?

Well because that's the way its wired in the datasheet typical application example.

I am still having problems communicating with my sensor through the PCA9306. Is there another method I can use to interface a 5V PIC with a 3.3V I2C sensor?

Really need help here Crying or Very sad
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