I2C between 5v PIC and 3.3v device

[rovtech]

I am considering purchasing a 3.3v pressure sensor to connect to a 5v PIC via I2C.
Is the use of resistors and Schottkey diodes enough protection? The SCL/SDA lines can be driven to about 0.3v above the 3.3v.
I assume that 3.3v replies will be enough to get data back to the PIC.
Yes, the PIC lines will be open drain but could be actively driven high by accident and the $87 sensor is a lot to risk.
Voltage dividers sound complicated because they would have to be from PIC pin to ground but then how would it read back and where would the pullup go? Three resistors?
Running the PIC from 3.3v would really complicate all my ADC inputs and possibly some outputs.
The same problem exists for connecting a Raspberry Pi to a PIC by I2C.
Any suggestions?

[temtronic]

You MUST spend $2 for an I2C logic level interface board... it will 'translate' the 5V PIC output to 3V I2C input AND convert the 3V I2C output to 5V PIC input.
Or you can make your own...
BUT the point is you MUST do PROPER logic level conversion !
while some have had some sucess with SMB option for I2C there is NO guarentee it will workk 100% of the time at any speed for ever over cold to hot tempertures.

edit. update...
I checked online , I can buy the convertor boards for 50 Canadian cents, in qty of 20 pcs.

[PCM programmer]

The level conversion can be done this way:

1. Enable SMBus on the PIC. This option is available on the hardware
i2c pins. The lower Vih level on the SMbus allows it to work with 3.3v i2c
chips. Or, use an i/o port that has TTL levels (instead of CMOS levels).
Usually this is PortB on 16F or 18F PICs. But check the Electrical
Specifications of the PIC's data sheet.

2. Connect the pullup resistors on SDA and SCL to +3.3v.

3. Use smaller resistors than for +5v operation. Use 3.3K pullups for
a standard 1 ma pullup on I2C for slow speed.

This does not include any protection against setting the PIC pins to +5v outputs.

[newguy]

http://www.hobbytronics.co.uk/mosfet-voltage-level-converter

This circuit works well and is bidirectional. Cost will be a few cents if you're doing a volume build, and maybe a dollar for a one-off. 2N7000 or 2N7002 MOSFETs.

[rovtech]

I saw that circuit but not the description of how it works as in newguy's link.
This is the other circuit that I was referring to:
http://www.roboard.com/Files/G144/RM-G144_SCH.pdf
It would permit 3.6v on the I2C lines and that is the max spec for the device so I am not crazy about it. Thanks to all.

[temtronic]

The big benefits to using a pre assembled module are
1) low cost - 50 cents per PIC<>device
2) ANY PIC I/O pins can be used
3) ANY PIC can be used
4) simple installation..NO soldering itty,bitty parts( that I can't see....)

Jay

[rovtech]

Thanks Temtronic. What about the custom chips like the FXMA2102?
There is a board with this plus a 3.3v power supply.
The only two with both level translation and a 3.3v power supply I could find are $9 and $17 Canadian $ approximately. There are lots of translators only for $2.

[temtronic]

I've never checked for an interface with regulator and I2C LLC, though it makes sense. It's probably not a 'big seller' as most designers would add the regulator and caps to the PIC board close to the connection of the I2C remote device. That way they control types and placement of parts.
Even in singles, a TO-92 case 3v3 is only 50 cents, so a LOT cheaper than the premade 'all-in-one- modules you describe.
At least TO-92 pkgs I can still see to older ...

[rovtech]

Many of the cheap level shifters have a 5 pin chip besides the expected pair of FETs. I see s201 marked on it.
Does anyone know what it is or what it does. Many boards do not have it.

[temtronic]

would have to see a picture of one....
if YOU have good eyesight, simple reverse engineer the module...BTDT 3 decades ago...

5 pin... maybe 4 zener diodes for OV protection ??

a schematic might shed light on it...

[rovtech]

http://www.ebay.ca/itm/IIC-I2C-Level-Conversion-translator-shifter-Module-5-3v-For-Arduino-AVR-STM32/141728904474?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649
should show a picture you can zoom on. I am trying to find a schematic of one of these. There are many of this style and many without this chip, just the FETs.
I had thought of adding a PNP from 3.3v to gnd with a voltage divider from 5v to gnd on the base like a power zener set just above the 3.3v. I am concerned about the $87 pressure sensor and would like backup protection, especially if using cheap voltage regulators. I have had good luck with Chinese components but have had some that I think were factory rejects. I had a 7805 that slowly crept from 5v to 7v over ten minutes! and it was bought from a reputable supplier. The AMS1117-3.3 or equivalent seem to be common voltage regulators and some suppliers imply they are switching but they are linear. Why would you use a switcher from 5v to 3.3v?

[PCM programmer]

One of the reviews says:

[temtronic]

re:

[rovtech]

Thanks PCM programmer. I found a picture that shows K833 marked on the chip and although I could not find a data sheet I did find this so it is a voltage regulator. Useful to know which one to buy.
https://arduino.stackexchange.com/questions/8511/how-to-identify-arduino-mini-pro-5v-vs-3-3v/8518
Good point Temtronic, without a spec sheet I have no idea what current it will deliver. It is probably similar to an LM1117.
Perhaps a separate regulator would be safer but a 1amp regulator would be harder to protect from. I have used 78L05 so they would burn open cct when crowbar'd rather than try to blow a fuse. The 7805 I was referring to was new and only running a single PIC.

Edit: Seems it is a KB33 which is a MIC5205 but still looking for the data sheet.
Edit: Found a data sheet which shows that the KB33 marking is on a MIC5205
http://www.farnell.com/datasheets/29605.pdf
Edit: KB33 good for 150 mA, sensor 1.25mA peak.

[asmallri]