You can use the internal pull-up resistors on the BBB if you don't want to put a pull-up resistor on the 'Bone side of the FET.
In normal operation, the far-side (high-voltage) pull-up resistor will pull the BBB side high through the FET until the BBB side reaches the Vgs threashold voltage (around a volt for a typical logic-level FET). So the standard I2C pull-up resistor will pull the signal to around 2.3V, then the internal BBB pull-up only needs to pull the signal up the rest of the way. Since the node is now isolated by the FET from whatever is on the far side, there is minimal capacitance and the weak internal pull-up will work fine. Plus the signal is already above the input high level threshold voltage, so it really only needs to go all the way to 3.3V to provide additional noise immunity. On 6/5/2014 2:06 PM, Francisco Aguerre wrote: > Charles, > > Thanks for your response. > > I read the section you mentioned from the I2C standard and it seems that, > for this to work properly, there has to be pull-up resistors on both sides > of the FET, which doesn't solve my problem. Maybe I didn't understand > something, but if I don't pull-up the I2C pins of the BBB, the lines will > never go high, because the FET will be in its off state. > > I want to avoid pulling the I/O pins high before the BBB is fully powered > on. > > On Thursday, June 5, 2014 12:47:53 PM UTC-3, Francisco Aguerre 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. >> >> Thnaks! -- 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.
