Simple I2C question - Electronic Circuits Projects Diagrams Free

speakerguy79 · 29th September 2008, 12:29 AM

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 · 29th September 2008, 12:44 AM

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.

speakerguy79 · 29th September 2008, 12:56 AM

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

speakerguy79 · 29th September 2008, 02:31 AM

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.

http://www.standardics.nxp.com/suppo...df/an97055.pdf

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29th September 2008, 12:29 AM	(permalink)
speakerguy79	Simple I2C question 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.

29th September 2008, 12:44 AM	(permalink)
Pommie	I think you will have problems as the inputs to the I²C module are schmitt triggers and require 0.8Vdd 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. Last edited by Pommie; 29th September 2008 at 12:52 AM. Reason: Changed link.*

29th September 2008, 12:56 AM	(permalink)
speakerguy79	Nuts. I was thinking they would just be TTL/CMOS compatible.

29th September 2008, 02:31 AM	(permalink)
speakerguy79	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. http://www.standardics.nxp.com/suppo...df/an97055.pdf Last edited by speakerguy79; 29th September 2008 at 02:32 AM.