On 6/5/2014 10:47 AM, [email protected] wrote: > Hi everyone, > > I'm trying to interface a RTC with my BBB using the I2C bus. The reference > manual states that none of the pins in the expansion connectors should be > driven while the system is not powered on. I have isolated other inputs > using tristate buffers and the SYS_RESETn, but the I2C lines, being > bidirectional is a bit trickier. Is it necessary in this case to isolate > the lines during power up? I'm connecting the pull-ups to the +3v3 pin on > the expansion connector. > > If I should isolate the lines, what's the best method? I'm thinking of > using a cmos switch to disconnect the resistors during power up, but maybe > there are better ideas.
A single FET per line will isolate the BeagleBone if power is off, and as a bonus will make the I2C inputs 5V tolerant. The basic circuit and theory of operation is described in section 18 of Version 2.1 of the I2C Specification (it's not in the newer 3.x versions of the spec). Basically, you tie the FET gate to the BBB I/O power rail, the source to the BBB I/O pin, and the drain to the "high-voltage" I2C signal (ie: pulled up to a non-zero value when the BBB is at 0V). -- Charles Steinkuehler [email protected] -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
