Simple I2C question

[Speakerguy]

I'm interfacing a PIC with a bunch of 3.3V devices. I *might* need to run the PIC off 5V for reasons I'll mention later. If the PIC is running off 5V, can I just use pull up resistors on the SDA and SCL lines to the 3.3V supply since they are open drain/open collector?

The reason I might need to use 5V is because of ICSP programming and USB stuff. I have to check if the 18F2550 can handle flash mem erases with a 3.3V supply with my programmer (melabs U2) and whether or not I need a 5V supply for any of the USB stuff.

 

[Pommie]

I think you will have problems as the inputs to the I²C module are schmitt triggers and require 0.8*Vdd for a valid high. This is only my interpretation of the data sheet and so I could be wrong.

There are various ways to solve this problem. See Microchips Hints & Tips

Mike.

 

[Speakerguy]

Nuts. I was thinking they would just be TTL/CMOS compatible.

 

[Speakerguy]

I just found this, which looks like an awesome and cheap solution to the problem. It just takes two MOSFETS and two additional pull-up resistors, is bidirectional, and offers isolation between the 3.3 and 5V sections when either power supply is powered down.

https://www.electro-tech-online.com/custompdfs/2008/09/an97055.pdf