Hello Giorgio,

I can't see how your circuit works. How do you use RBx pin?

The pin can't be treated as usually in I2C case. Remember, I2C has an
active 0 and a passive 1. See the truth table.
I2C      RBx latch    RBx TRISB
0        0            0
1        x            1

This circuit limits high I2C level to 3V by divider (PIC reads 3V as
high). Zener protects slave from overvoltage while PIC is being
powered up.

                           +5V
       5V                  |                 3.3V
 -------------             R1            -------------
 |    PIC    |             |   SCL (SDA) |           |
 |           |------o------o-------------|  SLAVE    |
 |  MASTER   |      |      |             |           |
 -------------    Zener    R2            -------------
                   3.6V    |
                    |     ===           R2      3
                   ===                ------ = ---
                                      R1 + R2   5

Hope it helps,
 Nikolai

On Sunday, February 06, 2000 Giorgio Alboni wrote:
> I have a I2C master that work with 5.0 volt and a 3.3 volt slave.
> The master is a pic16C84 and the slave is a cmos sensor. I can't use the
> open connector pin RA4 of pic. I use a portB pin.
> How can I resolve the voltage problem?
> Can work this circuit?


> -----                                                    -----
>     |     56K                         I2C DATA LINE      |
> PIC |---/\/\/\-------------------------------------------|CMOS
>     |                        |                           |
> -----                       ---                          -----
>                             \ / Diode
>                              -
>                              |
>                     ?        |           ?
>       (5V)VCC----/\/\/\----------------/\/\/\-------GND
>                             2,8volt

> T.I.A.
> Alboni Giorgio