RE: [Gumstix-users] I2C Bus use with Gumstix....

[Basham, R. R <ric...@bo...>]

Dave,

Thanks. I have read some of these papers and that is why I asked the
question.  I was going to use a level shifting circuit but that message
made it sound like I could make a direct connection. I thought maybe I
had missed something.=20

Rich

-----Original Message-----
From: Dave Hylands [mailto:dhy...@gm...]=20
Sent: Thursday, January 06, 2005 9:47 AM
To: gum...@li...
Subject: Re: [Gumstix-users] I2C Bus use with Gumstix....

Hi Rich,

> Can you connect the waysmall I2C bus to a 5V I2C bus?
> As I understand it the waysmall is running on 3.3V.

Be careful here. The datasheet inidcates that these pins can go above
Vcc (i.e. 3.3v).  This means that the pullups should only pullup to
3.3v. However, you can probably connect tje bus with 3.3 pullups to a 5v
device and it will work. I2C works by using open collector outputs, so
the device driving the bus either pulls the signal to ground, or
doesn't, in which case the pullup pulls it back up to 3.3v. Many 5v
devices (especially today) will recognize a 3.3v signal as a logic high.

I haven't yet connected the i2c bus on the gumstix to a 5v device, but
I've done a bunch of research into level shifting and stuff, and here's
what I've come up with.

In the i2c spec
(http://www.semiconductors.philips.com/markets/mms/protocols/i2c/index.h
tml)
on page 43 they show how you can connect two busses that are running at
different voltages using a couple of MOSFETs.

A much more in-depth tech-note from Philips:
http://www.standardproducts.philips.com/support/appnotes/i2c/pdf/an97055
.pdf

A paper which analyses the MOSFET approach:
http://www.mbari.org/staff/lance/N-MOSFET_bidirectional_level_shifter_an
alysis.pdf

Maxim makes a part designed specifically for doing i2c level
translating:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3253 (just be aware that
some of the parts are only rated to 230kbits, the others can all do 16
Mbits, which exceeds the 400kbits that the gumstix can do). You can get
free samples from Maxim.

TI also makes a part:
http://focus.ti.com/docs/prod/folders/print/sn74lvc2t45.html (which is
available from Digikey as part: 296-16845-1-ND) that should work.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/


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